The extended common object file format (XCOFF) is the object file format for the operating system. XCOFF combines the standard common object file format (COFF) with the TOC module format concept, which provides for dynamic linking and replacement of units within an object file. A variation of XCOFF is used for 64-bit object files and executable files.
XCOFF is the formal definition of machine-image object and executable files. These object files are produced by language processors (assemblers and compilers) and the binder (or link editor), and are used primarily by the binder and the system loader.
The default name for an XCOFF executable file is a.out.
Read the following information to learn more about XCOFF object files:
Selecting XCOFF32 Declarations
To select the XCOFF32 definitions, an application merely needs to include the appropriate header files. Only XCOFF32 structures, fields, and preprocessor defines will be included.
Selecting XCOFF64 Declarations
To select the XCOFF64 definitions, an application should define the preprocessor macro __XCOFF64__. When XCOFF header files are included, the structures, fields, and preprocessor defines for XCOFF64 will be included. Where possible, the structure names and field names are identical to the XCOFF32 names, but field sizes and offsets may differ.
Selecting Both XCOFF32 and XCOFF64 Declarations
To select structure definitions for both XCOFF32 and XCOFF64, an application should define both the preprocessor macros __XCOFF32__ and __XCOFF64__. This will define structures for both kinds of XCOFF files. Structures and typedef names for XCOFF64 files will have the suffix "_64" added to them. (Consult the header files for details.)
Selecting Hybrid XCOFF Declarations
An application may choose to select single structures that contain field definitions for both XCOFF32 and XCOFF64 files. For fields that have the same size and offset in both XCOFF32 and XCOFF64 definitions, the field names are retained. For fields whose size or offset differ between XCOFF32 and XCOFF64 definitions, the XCOFF32 fields have a "32" suffix, while the XCOFF64 fields have a "64" suffix. To select hybrid structure definitions, an application should define the preprocessor macro __XCOFF_HYBRID__. For example, the symbol table definition (in /usr/include/syms.h) will have the name n_offset32 used for the n_offset field for XCOFF32, and the name n_offset64 used for the n_offset field for XCOFF64.
Assemblers and compilers produce XCOFF object files as output. The binder combines individual object files into an XCOFF executable file. The system loader reads an XCOFF executable file to create an executable memory image of a program. The symbolic debugger reads an XCOFF executable file to provide symbolic access to functions and variables of an executable memory image.
An XCOFF file contains the following parts:
Not every XCOFF file contains every part. A minimal XCOFF file contains only the file header.
Object and Executable Files
XCOFF object files and executable files are similar in structure. An XCOFF executable file (or "module") must contain an auxiliary header, a loader section header, and a loader section.
The loader raw-data section contains information needed to dynamically load a module into memory for execution. Loading an XCOFF executable file into memory creates the following logical segments:
The XCOFF file Organization illustrates the structure of the XCOFF object file.
XCOFF Header Files
The xcoff.h file defines the structure of the XCOFF file. The xcoff.h file includes the following files:
Item | Description |
---|---|
filehdr.h | Defines the file header. |
aouthdr.h | Defines the auxiliary header. |
scnhdr.h | Defines the section headers. |
loader.h | Defines the format of raw data in the .loader section. |
typchk.h | Defines the format of raw data in the .typchk section. |
exceptab.h | Defines the format of raw data in the .except section. |
dbug.h | Defines the format of raw data in the .debug section. |
reloc.h | Defines the relocation information. |
linenum.h | Defines the line number information. |
syms.h | Defines the symbol table format. |
storclass.h | Defines ordinary storage classes. |
dbxstclass.h | Defines storage classes used by the symbolic debuggers. |
The a.out.h file includes the xcoff.h file. All of the XCOFF include files include the xcoff32_64.h file.
For more information on sections of the XCOFF object file, see "Sections and Section Headers." For more information on the symbol table, see "Symbol Table Information." For more information on the string table, see "String Table." For more information on the Debug section, see "Debug Section."
The following sections describe the XCOFF composite file header components:
File Header (filehdr.h)
The filehdr.h file defines the file header of an XCOFF file. The file header is 20 bytes long in XCOFF32 and 24 bytes long in XCOFF64. The structure contains the fields shown in the following table.
Field Name and Description | XCOFF32 | XCOFF64 |
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Field Definitions
Item | Description |
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f_magic | Specifies an integer known as the magic number, which specifies the target machine and environment of the object file. For XCOFF32, the only valid value is 0x01DF (0737 Octal). For XCOFF64 on AIX® 4.3, the only valid value is 0x01EF (0757 Octal). For XCOFF64 on AIX 5.1 and later, the only valid value is 0x01F7 (0767 Octal). Symbolic names for these values are found in the file, /usr/include/filehdr.h. |
f_nscns | Specifies the number of section headers contained in the file. The first section header is section header number one; all references to a section are one-based. |
f_timdat | Specifies when the file was created (number of elapsed seconds since 00:00:00 Universal Coordinated Time (UCT), January 1, 1970). This field should specify either the actual time or be set to a value of 0. |
f_symptr | Specifies a file pointer (byte offset from the beginning of the file) to the start of the symbol table. If the value of the f_nsyms field is 0, then f_nsyms is undefined. |
f_nsyms | Specifies the number of entries in the symbol table. Each symbol table entry is 18 bytes long. |
f_opthdr | Specifies the length, in bytes, of the auxiliary header. For an XCOFF file to be executable, the auxiliary header must exist and be _AOUTHSZ_EXEC bytes long. (_AOUTHSZ_EXEC is defined in aouthdr.h.) |
f_flags | Specifies a bit mask of flags that describe the type of the
object file. The following information defines the flags:
|
Auxiliary Header (aouthdr.h)
The auxiliary header contains system-dependent and implementation-dependent information, which is used for loading and executing a module. Information in the auxiliary header minimizes how much of the file must be processed by the system loader at execution time.
The binder generates an auxiliary header for use by the system loader. Auxiliary headers are not required for an object file that is not to be loaded. When auxiliary headers are generated by compilers and assemblers, the headers are ignored by the binder.
The C language structure for the auxiliary header is defined in the aouthdr.h file. The auxiliary header contains the fields shown in the following table.
Field Name and Description | XCOFF32 | XCOFF64 |
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Field Definitions
The following information defines the auxiliary header fields. For entries with two labels, the label in parentheses is the alternate original COFF a.out file format name.
Item | Description |
---|---|
o_mflags (magic) | If the value of the o_vstamp field is greater than 1, the o_mflags field is reserved for future use and it should contain 0. Otherwise, this field is not used. |
o_vstamp (vstamp) | Specifies the format version for this auxiliary header. The valid values are 1 and 2. When the o_vstamp field is 2 in an XCOFF32 file, the new interpretation of the n_type field in the symbol table entry is used. |
o_tsize (tsize) | Specifies the size (in bytes) of the raw data for the .text section. The .text section typically contains the read-only part of the program. This is the same value as contained in the s_size field of the section header for the .text section. |
o_dsize (dsize) | Specifies the size (in bytes) of the raw data for the .data section. The .data section contains the initialized data of the program and is writable. This is the same value as contained in the s_size field of the section header for the .data section. |
o_bsize (bsize) | Specifies the size (in bytes) of .bss area, which is used for uninitialized variables during execution and is writable. No raw data exists in the file for the .bss section. This is the same value as contained in the s_size field of the section header for the .bss section. |
o_entry (entry) | Specifies the virtual address of the entry point. (See the
definition of the o_snentry field.) For application programs,
this virtual address is the address of the function descriptor. The
function descriptor contains the addresses of both the entry point
itself and its TOC anchor. The offset of the entry point function
descriptor from the beginning of its containing section can be calculated
as follows:
where s_paddr is the virtual address contained in the specified section header. |
o_text_start (text_start) | Specifies the virtual address of the .text section. This is the address assigned to (that is, used for) the first byte of the .text raw-data section. This is the same value as contained in the s_paddr field of the section header for the .text section. |
o_data_start (data_start) | Specifies the virtual address of the .data section.
This is the address assigned to (that is, used for) the first byte
of the .data raw-data section. This is the same value as
contained in the s_paddr field of the section header for
the .data section. For addressing purposes, the .bss section is considered to follow the .data section. |
o_toc | Specifies the virtual address of the TOC anchor (see the definition of the o_sntoc field). |
o_snentry | Specifies the number of the file section containing the entry-point. (This field contains a file section header sequence number.) The entry point must be in the .text or .data section. |
o_sntext | Specifies the number of the file .text section. (This field contains a file section header sequence number.) |
o_sndata | Specifies the number of the file .data section. (This field contains a file section header sequence number.) |
o_sntoc | Specifies the number of the file section containing the TOC. (This field contains a file section header sequence number.) |
o_snloader | Specifies the number of the file section containing the system loader information. (This field contains a file section header sequence number.) |
o_snbss | Specifies the number of the file .bss section. (This field contains a file section header sequence number.) |
o_algntext | Specifies the log (base 2) of the maximum alignment needed for any csect in the .text section. |
o_algndata | Specifies the log (base 2) of the maximum alignment needed for any csect in the .data and .bss sections. |
o_modtype | Specifies a module type. The value is an ASCII character string.
The following module type is recognized by the system loader:
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o_cpuflag | Bit flags - cputypes of objects. |
o_cputype | Reserved. This byte must be set to 0. |
o_maxstack | Specifies the maximum stack size (in bytes) allowed for this executable. If the value is 0, the system default maximum stack size is used. |
o_maxdata | Specifies the maximum data size (in bytes) allowed for this executable. If the value is 0, the system default maximum data size is used. |
o_debugger | This field should contain 0. When a loaded program is being debugged, the memory image of this field may be modified by a debugger to insert a trap instruction. |
o_sntdata | Specifies the number of the .tdata file section. (This field contains a file section header sequence number.) |
o_sntbss | Specifies the number of the .tbss file section. (This field contains a file section header sequence number.) |
o_flags | Consists of four 1-bit flags and a 4-bit .tdata alignment:
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o_textpsize | Specifies the size of pages for the exec text. The default value is 0 (system-selected page size). |
o_datapsize | Specifies the size of pages for the exec data. The default value is 0 (system-selected page size). The value of o_datapsize overrides the large page data request (by setting the F_LPDATA bit in the XCOFF file). |
o_stackpsize | Specifies the size of pages for the stack. The default value is 0 (system-selected page size). |
o_x64flags | Consists of 16 flag bits. This field only exists
in the XCOFF64 definition.
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In general, an object file might contain multiple sections of a given type, but in a loadable module, there must be exactly one .text, .data, .bss, and .loader section. A loadable object might also have one .tdata section and one.tbss section.
Section Headers (scnhdr.h)
Each section of an XCOFF file has a corresponding section header, although some section headers may not have a corresponding raw-data section. A section header provides identification and file-accessing information for each section contained within an XCOFF file. Each section header in an XCOFF32 file is 40 bytes long, while XCOFF64 section headers are 72 bytes long. The C language structure for a section header can be found in the scnhdr.h file. A section header contains the fields shown in the following table.
Field Name and Description | XCOFF32 | XCOFF64 |
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Field Definitions
The following information defines the section header fields:
Item | Description |
---|---|
s_name | Specifies an 8-byte, null-padded section name. An 8-byte section name will not have a terminating null character. Use the s_flags field instead of the s_name field to determine a section type. Two sections of the same type may have different names, allowing certain applications to distinguish between them. |
s_paddr | Specifies the physical address of the section. This is the address assigned and used by the compilers and the binder for the first byte of the section. This field should contain 0 for all sections except the .text , .data , and .bss sections. |
s_vaddr | Specifies the virtual address of the section. This field has the same value as the s_paddr field. |
s_size | Specifies the size (in bytes) of this section. |
s_scnptr | Specifies a file pointer (byte offset from the beginning of the file) to this section's raw data. If this field contains 0, this section has no raw data. Otherwise, the size of the raw data must be contained in the s_size field. |
s_relptr | Specifies a file pointer (byte offset from the beginning of the file) to the relocation entries for this section. If this section has no relocation entries, this field must contain 0. |
s_lnnoptr | Specifies a file pointer (byte offset from the beginning of the file) to the line number entries for this section. If this section has no line number entries, this field must contain 0. |
s_nreloc | Specifies the number of relocation entries for this section. In an XCOFF32 file, if more than 65,534 relocation entries are required, the field value will be 65535, and an STYP_OVRFLO section header will contain the actual count of relocation entries in the s_paddr field. Refer to the discussion of overflow headers in "Sections and Section Headers" . If this field is set to 65535, the s_nlnno field must also be set to 65535. |
s_nlnno | Specifies the number of line number entries for this section. In an XCOFF32 file, if more than 65,534 line number entries are required, the field value will be 65535, and an STYP_OVRFLO section header will contain the actual number of line number entries in the s_vaddr field. Refer to the discussion of overflow headers in "Sections and Section Headers" . If this field is set to 65535, the s_nreloc field must also be set to 65535. |
s_flags | Specifies flags defining
the section type. A section type identifies the contents of a section
and specifies how the section is to be processed by the binder of
the system loader. The s_flags field consists of two 16-bit
fields. The low-order 16 bits specify the primary section type. Only
a single bit should be set in the low-order 16 bits. The high-order
16-bits specify a section subtype. For a primary section type without
subtypes, the high-order 16 bits should be 0. Note: The strip command logically deletes a section header by setting the s_flags field to -1.
Valid bit values are:
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Item | Description |
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s_flags continued | Valid bit values are:
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Section headers are defined to provide a variety of information about the contents of an XCOFF file. Programs that process XCOFF files will recognize only some of the valid sections.
See the following information to learn more about XCOFF file sections:
Current applications do not use the s_name field to determine the section type. Nevertheless, conventional names are used by system tools, as shown in the following table.
Description | Multiple Allowed? | s_flag (and its conventional name) |
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Text section | Yes | STYP_TEXT (.text) |
Data section | Yes | STYP_DATA (.data) |
BSS section | Yes | STYP_BSS (.bss) |
Pad section | Yes | STYP_PAD (.pad) |
Loader section | No | STYP_LOADER (.loader) |
Debug section | No | STYP_DEBUG (.debug) |
Type-check section | Yes | STYP_TYPCHK (.typchk) |
Exception section | No | STYP_EXCEPT (.except) |
Overflow section | Yes (one per .text or .data section) | STYP_OVRFLO (.ovrflo) |
Comment section | Yes | STYP_INFO (.info) |
Tdata section | Yes | STYP_TDATA (.tdata) |
TBSS section | Yes | STSTYP_TBSS (.tbss) STYP_TBSS (.tbss)YP_TBSS (.tbss) |
DWARF section | Yes | STYP_DWARF |
SSUBTYPE_DWINFO (.dwinfo) | ||
SSUBTYPE_DWLINE (.dwline) | ||
SSUBTYPE_DWPBNMS (.dwpbnms) | ||
SSUBTYPE_DWBTYP (.dwpbtyp) | ||
SSUBTYPE_DWARNGE (.dwarnge) | ||
SSUBTYPE_DWABREV (.dwabrev) | ||
SSUBTYPE_DWSTR (.dwstr) | ||
SSUBTYPE_DWRNGES (.dwrnges) |
Some fields of a section header may not always be used, or may have special usage. This pertains to the following fields:
Item | Description |
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s_name | On input, ignored by the binder and system loader. On output, the conventional names (shown in the "Conventional Header Names" table) are used. |
s_scnptr | Ignored for .bss sections. |
s_relptr | Recognized for the .text , .data , .tdata, and STYP_DWARF sections only. |
s_lnnoptr | Recognized for the .text section only. Otherwise, it must be 0. |
s_nreloc , s_nlnno | Handles relocation or line-number field overflows in an XCOFF32
file. (XCOFF64 files may not have overflow section headers.) If a
section has more than 65,534 relocation entries or line number entries,
both of these fields are set to a value of 65535. In this
case, an overflow section header with the s_flags field equal
to STYP_OVRFLO is used to contain the relocation and line-number
count information. The fields in the overflow section header are defined
as follows:
The s_size and s_scnptr fields have a value of 0 in an overflow section header. The s_relptr and s_lnnoptr fields must have the same values as in the corresponding primary section header. |
An XCOFF file provides special meaning to the following sections:
For more information on XCOFF file sections, see "Loader Section (loader.h)," "Debug Section," "Type-Check Section," "Exception Section," and "Comment Section."
The loader section contains information required by the system loader to load and relocate an executable XCOFF object. The loader section is generated by the binder. The loader section has an s_flags section type flag of STYP_LOADER in the XCOFF section header. By convention, .loader is the loader section name. The data in this section is not referenced by entries in the XCOFF symbol table.
The loader section consists of the following parts:
The C language structure for the loader section can be found in the loader.h file.
Loader Header Field Definitions
The following table describes the loader section's header field definitions.
Field Name and Description | XCOFF32 | XCOFF64 |
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The following information defines the loader section's header fields:
Item | Description |
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l_version | Specifies the loader section version number. This value must be 1 for XCOFF32, 2 for XCOFF64. |
l_nsyms | Specifies the number of symbol table entries in the loader section. This value is the actual count of symbol table entries contained in the loader section and does not include the three implicit entries for the .text, .data, and .bss symbol entries. |
l_nreloc | Specifies the number of relocation table entries in the loader section. |
l_istlen | Specifies the byte length of the import file ID string table in the loader section. |
l_nimpid | Specifies the number of import file IDs in the import file ID string table. |
l_impoff | Specifies the byte offset from beginning of the loader section to the first import file ID. |
l_stlen | Specifies the length of the loader section string table. |
l_stoff | Specifies the byte offset from beginning of the loader section to the first entry in the string table. |
l_symoff | Specifies the byte offset from beginning of the loader section to the start of the loader symbol table (in XCOFF64 only). |
l_rldoff | Specifies the byte offset from beginning of the loader section to the start of the loader section relocation entries (in XCOFF64 only). |
Loader Symbol Table Field Definitions
The loader section symbol table contains the symbol table entries that the system loader needs for its import and export symbol processing and dynamic relocation processing.
The loader.h file defines the symbol table fields. Each entry is 24 bytes long.
There are five implicit external symbols, one each for the .text, .data, .bss, .tdata, and .tbss sections. These symbols are referenced from the relocation table entries using symbol table index values 0, 1, 2, -1, and -2, respectively.
Field Name and Description | XCOFF32 | XCOFF64 |
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The symbol table fields are:
Item | Description |
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l_name | (XCOFF32 only) Specifies an 8-byte, null-padded symbol name
if it is 8 bytes or less in length. Otherwise, the field is treated
as the following two 4-byte integers for accessing the symbol name:
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l_offset | (XCOFF64 only) This field has the same use as the l_offset field in XCOFF32. |
l_value | Specifies the virtual address of the symbol |
l_scnum | Specifies the number of the XCOFF section that contains the symbol. If the symbol is undefined or imported, the section number is 0. Otherwise, the section number refers to the .text, .data, or .bss section. Section headers are numbered beginning with 1. |
l_smtype | Specifies the symbol type, import flag, export flag, and entry
flag. Bits 0-4 are flag bits defined as follows:
Bits 5-7 constitute a 3-bit symbol type field with the following definitions:
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Specifies the storage mapping class of the symbol, as defined in syms.h for the x_smclas field of the csect auxiliary symbol table entry. Values have the symbolic form XMC_xx, where xx is PR, RO, GL, XO, SV, SV64, SV3264, RW, TC, TD, DS, UA, BS, UC, TL, or UL. See "csect Auxiliary Entry for the C_EXT, C WEAKEXT, and C_HIDEXT Symbols" for more information. |
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Specifies the import file ID string. This integer is the ordinal value of the position of the import file ID string in the import file ID name string table of the loader section. For an imported symbol, the value of 0 in this field identifies the symbol as a deferred import to the system loader. A deferred import is a symbol whose address can remain unresolved following the processing of the loader. If the symbol was not imported, this field must have a value of 0. |
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Specifies the offset to the parameter type-check string. The byte offset is from the beginning of the loader section string table. The byte offset points to the first byte of the parameter type-check string (not to its length field). For more information on the parameter type-check string, see "Type-Check Section" . A value of 0 in the l_parm field indicates that the parameter type-checking string is not present for this symbol, and the symbol will be treated as having a universal hash. |
Loader Relocation Table Field Definitions
The Loader Section Relocation Table Structure contains all of the relocation information that the system loader needs to properly relocate an executable XCOFF file when it is loaded. The loader.h file defines the relocation table fields. Each entry in the loader section relocation table is 12 bytes long in XCOFF32, and 16 bytes long in XCOFF64. The l_vaddr, l_symndx, andl_rtype fields have the same meaning as the corresponding fields of the regular relocation entries, which are defined in the reloc.h file. For more information about relocation entries, see Relocation Information for XCOFF File (reloc.h).
Field Name and Description | XCOFF32 | XCOFF64 |
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The loader.h file defines the following fields:
Name | Description |
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l_vaddr | Specifies the virtual address of the relocatable reference. |
l_symndx | Specifies the loader section symbol table index (n-th
entry) of the symbol that is being referenced. Values 0, 1, and 2
are implicit references to .text, .data, and .bss sections, respectively. Symbol index 3 is the index for
the first symbol actually contained in the loader section symbol table.
Note: A reference to an exported symbol can be made using the symbol's
section number (symbol number 0, 1, or 2) or using the actual number
of the exported symbol.
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l_rtype | Specifies the relocation size and type. (This field has the same interpretation as the r_type field in the reloc.h file.) For more information about relocation entries, see Relocation Information for XCOFF File (reloc.h). |
l_rsecnm | Specifies the section number of the sections being relocated. This is a one-based index into the section headers. |
Loader Import File ID Name Table Definition
The loader section import file ID name strings of a module provide a list of dependent modules that the system loader must load in order for the module to load successfully. However, this list does not contain the names of modules that the named modules depend on.
Offset | Length in Bytes | Name and Description |
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0 | n1 | l_impidpath Import file ID path string, null-delimited |
n1 + 1 | n2 | l_impidbase Import file ID base string, null-delimited |
n1 + n2 + 2 | n3 | l_impidmem Import file ID member string, null-delimited |
Fields repeat for each string. |
Each import file ID name consists of three null-delimited strings.
The first import file ID is a default LIBPATH value to be used by the system loader. The LIBPATH information consists of file paths separated by colons. There is no base name or archive member name, so the file path is followed by three null bytes.
Each entry in the import file ID name table consists of:
For example:
/usr/lib\0mylib.a\0shr.o\0
Loader String Table Definition
The loader section string table contains the parameter type-checking strings, all symbols names for an XCOFF64 file, and the names of symbols longer than 8 bytes for an XCOFF32 file. Each string consists of a 2-byte length field followed by the string.
Offset | Length in Bytes | Description |
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0 | 2 | Length of string. |
2 | n | Symbol name string (null-delimited) or parameter type string (not null-delimited). |
Fields repeat for each string. |
Symbol names are null-terminated. The value in the length-field includes the length of the string plus the length of the null terminator but does not include the length of the length field itself.
The parameter type-checking strings contain binary values and are not null-terminated. The value in the length field includes the length of the string only but does not include the length of the length field itself.
The symbol table entries of the loader section contain a byte offset value that points to the first byte of the string instead of to the length field.
Loader Section Header Contents
The contents of the section header fields for the loader section are:
Name | Contents |
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s_name | .loader |
s_paddr | 0 |
s_vaddr | 0 |
s_size | The size (in bytes) of the loader section |
s_scnptr | Offset from the beginning of the XCOFF file to the first byte of the loader section data |
s_relptr | 0 |
s_lnnoptr | 0 |
s_nreloc | 0 |
s_nlnno | 0 |
s_flags | STYP_LOADER |
For general information on the XCOFF file format, see "XCOFF Object File Format."
For more information on XCOFF file sections, see "Sections and Section Headers," "Debug Section," "Type-Check Section," "Exception Section," and "Comment Section."
The debug section contains the symbolic debugger stabstrings (symbol table strings). It is generated by the compilers and assemblers. It provides symbol attribute information for use by the symbolic debugger. The debug section has a section type flag of STYP_DEBUG in the XCOFF section header. By convention, .debug is the debug section name. The data in this section is referenced from entries in the XCOFF symbol table. A stabstring is a null-terminated character string. Each string is preceded by a 2-byte length field in XCOFF32 or a 4-byte length field in XCOFF64.
Field Definitions
The following two fields are repeated for each symbolic debugger stabstring:
Refer to discussion of symbolic debugger stabstring grammar for the specific format of the stabstrings.
Debug Section Header Contents
The contents of the section header fields for the debug section are:
Name | Contents |
---|---|
s_name | .debug |
s_paddr | 0 |
s_vaddr | 0 |
s_size | The size (in bytes) of the debug section |
s_scnptr | Offset from the beginning of the XCOFF file to the first byte of the debug section data |
s_relptr | 0 |
s_lnnoptr | 0 |
s_nreloc | 0 |
s_nlnno | 0 |
s_flags | STYP_DEBUG |
For general information on the XCOFF file format, see "XCOFF Object File Format."
For more information on XCOFF file sections, see "Sections and Section Headers," "Debug Section," "Type-Check Section,", "Exception Section," and "Comment Section."
The DWARF sections contain debugging information for the symbolic debugger. DWARF is an alternate debugging format used instead of debugger stabstrings. The contents of the various DWARF sections is described in the "DWARF Debugging Information Format" which can be obtained from www.dwarfstd.org.
All DWARF sections have a primary section type flag of STYP_DWARF in the XCOFF section header. Conventional sections names are listed in "Conventional Header Names".
DWARF Section Header Contents
The contents of the section header fields for the debug section are:
Name | Contents |
---|---|
s_name | Depends on s_flags |
s_paddr | 0 |
s_vaddr | 0 |
s_size | The size (in bytes) of the section. |
s_scnptr | Offset from the beginning of the XCOFF file to the first byte of the section data. |
s_relptr | Offset from the beginning of the XCOFF file to the relocation entries for the section. |
s_lnnoptr | 0 |
s_nreloc | Number of relocation entries. |
s_nlnno | 0 |
s_flags | STYP_DWARF ored with a DWARF subtype value. |
The type-check section contains the type-checking hash strings and is produced by compilers and assemblers. It is used by the binder to detect variable mismatches and argument interface errors when linking separately compiled object files. (The type-checking hash strings in the loader section are used to detect these errors prior to running a program.) The type-check section has a section type flag of STYP_TYPCHK in the XCOFF section header. By convention, .typchk is the type-check section name. The strings in this section are referenced from entries in the XCOFF symbol table.
Field Definitions
The following two fields are repeated for each parameter type-checking string:
Type Encoding and Checking Format for Data
The type-checking hash strings are used to detect errors prior to execution of a program. Information about all external symbols (data and functions) is encoded by the compilers and then checked for consistency at bind time and load time. The type-checking strings are designed to enforce the maximum checking required by the semantics of each particular language supported, as well as provide protection to applications written in more than one language.
The type encoding and checking mechanism features 4-part hash encoding that provides some flexibility in checking. The mechanism also uses a unique value, UNIVERSAL, that matches any code. The UNIVERSAL hash can be used as an escape mechanism for assembly programs or for programs in which type information or subroutine interfaces might not be known. The UNIVERSAL hash is four blank ASCII characters (0x20202020) or four null characters (0x00000000).
The following fields are associated with the type encoding and checking mechanism:
Item | Description |
---|---|
code length | A 2-byte field containing the length of the hash. This field has a value of 10. |
language identifier | A 2-byte code representing each language. These codes are the same as those defined for the e_lang field in the "Exception Section" information . |
general hash | A 4-byte field representing the most general form by which a data symbol or function can be described. This form is the most common to languages supported by . If the information is incomplete or unavailable, a universal hash should be generated. The general hash is language-independent and must match for the binding to succeed. |
language hash | A 4-byte field containing a more detailed, language-specific representation of what is in the general hash. It allows for the strictest type-checking required by a given language. This part is used in intra-language binding and is not checked unless both symbols have the same language identifier. |
Section Header Contents
The contents of the section header fields for the type-check section are:
Name | Contents |
---|---|
s_name | .typchk |
s_paddr | 0 |
s_vaddr | 0 |
s_size | The size (in bytes) of the type-check section |
s_scnptr | Offset from the beginning of the XCOFF file to the first byte of the type-check section data |
s_relptr | 0 |
s_lnnoptr | 0 |
s_nreloc | 0 |
s_nlnno | 0 |
s_flags | STYP_TYPCHK. |
For general information on the XCOFF file format, see "XCOFF Object File Format."
For more information on XCOFF file sections, see "Sections and Section Headers," "Debug Section," "Type-Check Section," "Exception Section," and "Comment Section."
The exception section contains addresses of trap instructions, source language identification codes, and trap reason codes. This section is produced by compilers and assemblers, and used during or after run time to identify the reason that a specific trap or exception occurred. The exception section has a section type flag of STYP_EXCEPT in the XCOFF section header. By convention, .except is the exception section name. Data in the exception section is referenced from entries in the XCOFF symbol table.
An exception table entry with a value of 0 in the e_reason field contains the symbol table index to a function's C_EXT, C_WEAKEXT, or C_HIDEXT symbol table entry. Reference from the symbol table to an entry in the exception table is via the function auxiliary symbol table entry. For more information on this entry, see "csect Auxiliary Entry for C_EXT, C_WEAKEXT and C_HIDEXT Symbols."
The C language structure for the exception section entries can be found in the exceptab.h file.
The exception section entries contain the fields shown in the following tables.
Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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Field Definitions
The following defines the fields listed of the exception section:
Item | Description |
---|---|
e_symndx | Contains an integer (overlays the e_paddr field). When the e_reason field is 0, this field is the symbol table index of the function. |
e_paddr | Contains a virtual address (overlays the e_symndx field). When the e_reason field is nonzero, this field is the virtual address of the trap instruction. |
e_lang | Specifies the source language. The following list defines the
possible values of the e_lang field.
|
e_reason | Specifies an 8-bit, compiler-dependent trap exception reason code. Zero is not a valid trap exception reason code because it indicates the start of exception table entries for a new function. |
Section Header Contents
The following fields are the contents of the section header fields for the exception section.
Name | Contents |
---|---|
s_name | .except |
s_paddr | 0 |
s_vaddr | 0 |
s_size | The size (in bytes) of the exception section |
s_scnptr | Offset from the beginning of the XCOFF file to the first byte of the exception section data |
s_relptr | 0 |
s_lnnoptr | 0 |
s_nreloc | 0 |
s_nlnno | 0 |
s_flags | STYP_EXCEPT |
For general information on the XCOFF file format, see "XCOFF Object File Format."
For more information on XCOFF file sections, see "Sections and Section Headers," "Debug Section," "Type-Check Section," "Exception Section," and "Comment Section."
The comment section contains information of special processing significance to an application. This section can be produced by compilers and assemblers and used during or after run time to fulfill a special processing need of an application. The comment section has a section type flag of STYP_INFO in the XCOFF section header. By convention, .info is the comment section name. Data in the comment section is referenced from C_INFO entries in the XCOFF symbol table.
The contents of a comment section consists of repeated instances of a 4-byte length field followed by a string of bytes (containing any binary value). The length of each string is stored in its preceding 4-byte length field. The string of bytes need not be terminated by a null character nor by any other special character. The specified length does not include the length of the length field itself. A length of 0 is allowed. The format of the string of bytes is not specified.
A comment section string is referenced from an entry in the XCOFF symbol table. The storage class of the symbol making a reference is C_INFO. See "Symbol Table Field Contents by Storage Class" for more information.
A C_INFO symbol is associated with the nearest C_FILE, C_EXT, C_WEAKEXT, or C_HIDEXT symbol preceding it.
Section Header Contents
The following fields are the contents of the section header fields for the comment section.
Name | Contents |
---|---|
s_name | .info |
s_paddr | 0 |
s_vaddr | 0 |
s_size | The size (in bytes) of the comment section |
s_scnptr | Offset from the beginning of the XCOFF file to the first byte of the comment section data |
s_relptr | 0 |
s_lnnoptr | 0 |
s_nreloc | 0 |
s_nlnno | 0 |
s_flags | STYP_INFO |
For general information on the XCOFF file format, see "XCOFF Object File Format."
For more information on XCOFF file sections, see "Sections and Section Headers," "Debug Section," "Type-Check Section," "Exception Section," and "Comment Section."
Some sections have relocation information. The s_relptr field in the section header specifies the file offset to the relocation entries for the section. The binder uses the relocation information to modify address constants and other relocatable values when individual XCOFF object files are linked to create an XCOFF executable file.
Compilers and assemblers generate the relocation entries for the information for sections. The binder generates relocation information contained in the .loader section, as required by the system loader.
Each relocation entry is 10 bytes long (14 for XCOFF64). (A relocation entry in the .loader section is 12 bytes long (16 for XCOFF64) and is explained in the loader section description in this document. See "Relocation Table Field Definitions" for more information.) You can find the C language structure for a relocation entry in the reloc.h file. A relocation entry contains the fields shown in the following table.
Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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The relocation entries for a section must be in ascending address order.
(The loader section contains a single set of relocation entries used by the system loader; so a section number is required within each relocation entry to identify the section that needs to be modified.)
Field Definitions
Item | Description |
---|---|
r_vaddr | Specifies the virtual
address of the value that requires modification by the binder. The
byte offset value to the data that requires modification from the
beginning of the section that contains the data can be calculated
as follows: offset_in_section = r_vaddr - s_paddr |
r_symndx | Specifies a zero-based index into the XCOFF symbol table for locating the referenced symbol. The symbol table entry contains an address used to calculate a modification value to be applied at the r_vaddr relocation address. |
r_rsize | Specifies the relocation
size and sign. Its contents are detailed in the following list:
|
r_rtype | Specifies an 8-bit relocation
type field that indicates to the binder which relocation algorithm
to use for calculating the modification value. This value is applied
at the relocatable reference location specified by the r_vaddr field. The following relocation types are defined:
|
r_rtype continued |
|
r_rtype continued |
|
Additional Relocation Features
Standard practice is to retain relocation information only for unresolved references or references between distinct sections. Once a reference is resolved, the relocation information is discarded. This is sufficient for an incremental bind and a fixed address space model. To provide the capability for rebinding and handling a relocatable address space model, the relocation information is not discarded from an XCOFF file.
For general information on the XCOFF file format, see "XCOFF Object File Format."
For more information on relocation field table definitions, see "Relocation Table Field Definitions" in the loader section.
Line number entries are used by the symbolic debugger to debug code at the source level. When present, there is a single line number entry for every source line that can have a symbolic debugger breakpoint. The line numbers are grouped by function. The beginning of each function is identified by the l_lnno field containing a value of 0. The first field, l_symndx , is the symbol table index to the C_EXT, C_WEAKEXT, or C_HIDEXT symbol table entry for the function.
Each line number entry is six bytes long. The C language structure for a line number entry can be found in the linenum.h file. A line number entry contains the fields shown in the following tables.
Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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Field Definitions
The following list defines the line number entries:
Item | Description |
---|---|
l_symndx | Specifies the symbol table index to the function name (overlays the l_paddr field). When the l_lnno field is 0, this interpretation of the field is used. |
l_paddr | Specifies the virtual address of the first instruction of the code associated with the line number (overlays the l_symndx field). When the l_lnno field is not 0, this interpretation of the field is used. |
l_lnno | Specifies either the line number relative to the start of a function or 0 to indicate the beginning of a function. |
For general information on the XCOFF file format, see "XCOFF Object File Format."
For information on debugging, see "Debug Section."
One composite symbol table is defined for an XCOFF file. The symbol table contains information required by both the binder (external symbols) and the symbolic debugger (function definitions and internal and external symbols).
The symbol table consists of a list of 18-byte, fixed-length entries. Each symbol represented in the symbol table consists of at least one fixed-length entry, and some are followed by auxiliary entries of the same size.
See the following information to learn more about the symbol table:
For each external symbol, one or more auxiliary entries are required that provide additional information concerning the external symbol. There are three major types of external symbols of interest to the binder, performing the following functions:
For symbols defining a replaceable unit (csect), a csect auxiliary entry defines the length and storage-mapping class of the csect. For symbols defining external names for functions within a csect, the csect auxiliary entry points to the containing csect, the parameter type-checking information, and the symbolic debugger information for the function. For symbols referencing the name of an external function, a csect auxiliary entry identifies the symbol as an external reference and points to parameter type-checking information.
Symbol Table Contents
An XCOFF symbol table has the following general contents and ordering:
The ordering of the symbol table must be arranged by the compilers and assemblers both to accommodate the symbolic debugger requirements and to permit effective management by the binder of the different sections of the object file as a result of such binder actions as garbage collection, incremental binding, and rebinding. This ordering is required by the binder so that if a csect is deleted or replaced, all the symbol table information associated with the csect can also be deleted or replaced. Likewise, if all the csects associated with a source file are deleted or replaced, all the symbol table and related information associated with the file can also be deleted or replaced.
Symbol Table Layout
The following example shows the general ordering of the symbol table.
un_external Undefined global symbols
.file Prolog --defines stabstring compaction level
.file Source file 1
.info Comment section reference symbol with file scope
stab Global Debug symbols of a file
dwarf DWARF Information of a file
csect Replaceable unit definition (code)
.info Comment section reference symbol with csect scope
function Local/External function
stab Debug and local symbols of function
function Local/External function
stab Debug and local symbols of function
..............
csect Replaceable unit definition (local statics)
stab Debug and local statics of file
..............
csect Relocatable unit definition (global data)
external Defined global symbol
stab Debug info for global symbol
..............
.file Source file 2
stab Global Debug symbols of a file
dwarf DWARF Information of a file
csect Replaceable unit definition (code)
function Local/External function
stab Debug and local symbols of function
..............
csect Replaceable unit definition (local statics)
stab Debug and Local statics of file
..............
csect Replaceable unit definition (global data)
external Defined global symbol
stab Debug info for global symbol
.file Source file
..............
Symbol Table Entry (syms.h)
Each symbol, regardless of storage class and type, has a fixed-format entry in the symbol table. In addition, some symbol types may have additional (auxiliary) symbol table entries immediately following the fixed-format entry. Each entry in the symbol table is 18 bytes long. The C language structure for a symbol table entry can be found in the syms.h file. The index for the first entry in the symbol table is 0. The following table shows the structure of the fixed-format part of each symbol in the symbol table.
Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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Field Definitions
The following defines the symbol table entry fields:
Item | Description |
---|---|
n_name | Used by XCOFF32 only. Specifies an 8-byte, null-padded symbol
name or symbolic debugger stabstring. The storage class field is used
to determine if the field is a symbol name or symbolic debugger stabstring.
By convention, a storage class value with the high-order bit on indicates
that this field is a symbolic debugger stabstring. If the XCOFF32 symbol name is longer than 8 bytes, the field is interpreted as the following two fields:
|
n_offset | For XCOFF64: Specifies the byte offset to the symbol name in the string table or .debug section. The byte offset is relative to the start of the string table or .debug section. A byte offset value of 0 is a null or zero-length symbol name. (For XCOFF32 only, used in conjunction with n_zeroes. See entry immediately above.) |
n_value | Specifies the symbol value. The contents of the symbol value
field is storage class-dependent, as shown in the following definitions:
|
n_scnum | Specifies a section number associated with one of the following
symbols:
|
n_type | The use of this field depends on the storage class of the symbol.
For C_FILE symbols, see "File Auxiliary
Entry for the C_FILE Symbol" . For the C_EXT, C_HIDEXT, and C_WEAKEXT symbols, the n_type field has two interpretations in XCOFF32, and a single interpretation in XCOFF64. The old interpretation is used in XCOFF32 if the value of the o_vstamp field in the auxiliary header is 1. In the old XCOFF32 interpretation, bit 10 (0x0020) can be set if the symbol is a function. Otherwise, bit 10 should be 0. The remaining bits are defined in COFF file to represent type information and are no longer used. In XCOFF64 and the new XCOFF32 interpretation, the n_type field is used for the symbol type and visibility as follows:
Symbol visibility is described in the ld command. Note: For all
other storage classes, the n_type field is reserved for future
use and should contain 0.
|
n_sclass | Specifies the storage class of the symbol. The storclass.h and dbxstclass.h files contain the definitions of the storage classes. See "Symbol Table Field Contents by Storage Class" for more information. |
n_numaux | Specifies the number of auxiliary entries for the symbol. In XCOFF64, auxiliary symbols have an identifying type field, but in XCOFF32, there is no type field. Therefore, if more than one auxiliary entry is required for a symbol, the order of the auxiliary entries is determined by convention. |
For general information on the XCOFF file format, see "XCOFF Object File Format."
The symbol table contains auxiliary entries to provide supplemental information for a symbol. The auxiliary entries for a symbol follow its symbol table entry. The length of each auxiliary entry is the same as a symbol table entry (18 bytes). The format and quantity of auxiliary entries depend on the storage class (n_sclass) and type (n_type) of the symbol table entry.
In XCOFF32, symbols having a storage class of C_EXT, C_WEAKEXT or C_HIDEXT and more than one auxiliary entry must have the csect auxiliary entry as the last auxiliary entry. In XCOFF64, the x_auxtype field of each auxiliary symbol table entry differentiates the symbols, but the convention is to generate the csect auxiliary symbol table entry last.
File Auxiliary Entry for C_FILE Symbols
The file auxiliary symbol table entry is defined to contain the source file name and compiler-related strings. A file auxiliary entry is optional and is used with a symbol table entry that has a storage-class value of C_FILE. The C language structure for a file auxiliary entry can be found in the x_file structure in the syms.h file.
The C_FILE symbol provides source file-name information, source-language ID and CPU-version ID information, and, optionally, compiler-version and time-stamp information.
The n_type field of the symbol table entry identifies the source language of the source file and the CPU version ID of the compiled object file. The field information is as follows:
Item | Description |
---|---|
Source Language ID | Overlays the high-order byte of the n_type field.
This field contains the source-language identifier. The values for
this field are defined in the e_lang field in "Exception Section" . This field
can be used by the symbolic debuggers to determine the source language.
The optional values for this field are 248 (TB_OBJECT) for symbols from object files with no C_FILE symbol table entry; or 249 (TB_FRONT) or 250 (TB_BACK) for generated entries used to provide debugging information. If the source language is TB_FRONT or TB_BACK, the 8-character name field begins with ' ' (blank) , '\0'(NULLl). If the source language is TB_FRONT, the third byte is the stabstring compaction level for the object file, and the n_offset field contains the symbol table index of the TB_BACK symbol table entry, if it exists, or 0 otherwise. |
CPU Version ID | Defined as the low-order byte of the n_type field.
Decribes the kind of instructions generated for the file. The following
values are defined:
If both fields are 0, no information is provided about the source language.
|
Field Definitions
The following defines the fields listed above:
Item | Description |
---|---|
x_fname | Specifies the source file name or compiler-related string.
If the file name or string is longer than 8 bytes, the field is interpreted as the following two fields:
|
x_ftype | Specifies the source-file string type.
|
(no name) | Reserved. This field must contain 2 bytes of 0. |
x_auxtype | (XCOFF64 only) Specifies the type of auxiliary entry. Contains _AUX_FILE for this auxiliary entry. |
If the file auxiliary entry is not used, the symbol name is the name of the source file. If the file auxiliary entry is used, then the symbol name should be .file, and the first file auxiliary entry (by convention) contains the source file name. More than one file auxiliary entry is permitted for a given symbol table entry. The n_numaux field contains the number of file auxiliary entries.
csect Auxiliary Entry for C_EXT, C_WEAKEXT, and C_HIDEXT Symbols
The csect auxiliary entry identifies csects (section definitions), entry points (label definitions), and external references (label declarations). A csect auxiliary entry is required for each symbol table entry that has a storage class value of C_EXT, C_WEAKEXT, or C_HIDEXT. See "Symbol Table Entry (syms.h)" for more information. By convention, the csect auxiliary entry in an XCOFF32 file must be the last auxiliary entry for any external symbol that has more than one auxiliary entry. The C language structure for a csect auxiliary entry can be found in the x_csect structure in the syms.h file.
Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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Field Definitions
Item | Description |
---|---|
x_scnlen | Specifies a meaning dependent
on x_smtyp as follows:
|
x_parmhash | Specifies the byte offset of the parameter type-check string in the .typchk section. The byte offset is from the beginning of the .typchk section in an XCOFF file. The byte offset points to the first byte of the parameter type-check string (not to its length field). See "Type-Check Section" for more information. A value of 0 in the x_parmhash field indicates that the parameter type-checking string is not present for this symbol, and the symbol will be treated as having a universal hash. The value should be 0 for C_HIDEXT symbols. |
x_snhash | Specifies the .typchk section number. The XCOFF section number containing the parameter type-checking strings. The section numbers are one-based. For compatibility with object files generated by some compilers, if x_parmhash is not equal to 0 but x_snhash does equal 0, then the first .typchk section in the file is used. The value should be 0 for C_HIDEXT symbols. |
x_smtyp | Specifies symbol alignment
and type:
|
x_smclas | Specifies the csect storage-mapping
class. This field permits the binder to arrange csects by their storage-mapping
class. The x_smclas field is used only when the value of
bits 5-7 of the x_smtyp field is either XTY_SD or XTY_CM. The following storage-mapping classes are read-only and normally mapped to the .text section:
The following storage-mapping classes are read/write and normally mapped to the .data or .bss section:
|
x_smclas continued |
|
x_smclas continued |
|
x_smclas continued |
The following storage mapping class is read-write and is mapped to the .tdata section:
The following storage mapping class is read-write and is mapped to the .tbss section:
|
x_stab | Reserved (Unused for 64-bit). |
x_snstab | Reserved (Unused for 64-bit). |
Auxiliary Entries for the C_EXT, C_WEAKEXT, and C_HIDEXT Symbols
Auxiliary symbol table entries are defined in XCOFF to contain reference and size information associated with a defined function. These auxiliary entries are produced by compilers and assembler for use by the symbolic debuggers. In XCOFF32, a function auxiliary symbol table entry contains the required information. In XCOFF64, both a function auxiliary entry and an exeption auxiliary entry may be needed. When both auxiliary entries are generated for a single C_EXT, C_WEAKEXT, or C_HIDEXT symbol, the x_size and x_endndx fields must have the same values.
The function auxiliary symbol table entry is defined in the following table.
Field Name and Description | XCOFF32 | XCOFF64 |
---|---|---|
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Field Definitions
The following defines the fields listed in the Function Auxiliary Entry Format table:
Item | Description |
---|---|
x_exptr | (XCOFF32 only) This field is a file pointer to an exception table entry. The value is the byte offset from the beginning of the XCOFF object file. In an XCOFF64 file, the exception table offsets are in an exception auxiliary symbol table entry. |
x_fsize | Specifies the size of the function in bytes. |
x_lnnoptr | Specifies a file pointer to the line number. The value is the byte offset from the beginning of the XCOFF object file. |
x_endndx | Specifies the symbol table index of the next entry beyond this function. |
The exception auxiliary symbol table entry, defined in XCOFF64 only, is shown in the following table.
Offset | Length | Name and Description |
---|---|---|
0 | 8 |
|
8 | 4 |
|
12 | 4 |
|
16 | 1 |
|
17 | 1 |
|
Field Definitions
The following defines the fields listed in the Exception Auxiliary Entry Format table:
Item | Description |
---|---|
x_exptr | This field is a file pointer to an exception table entry. The value is the byte offset from the beginning of the XCOFF object file. |
x_fsize | Specifies the size of the function in bytes. |
x_endndx | Specifies the symbol table index of the next entry beyond this function. |
Block Auxiliary Entry for the C_BLOCK and C_FCN Symbols
The symbol auxiliary symbol table entry is defined in XCOFF to provide information associated with the begin and end blocks of functions. The symbol auxiliary symbol table entry is produced by compilers for use by the symbolic debuggers.
Field Name and Description | XCOFF32 | XCOFF64 |
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Field Definitions
The following defines the fields above:
Item | Description |
---|---|
(no name) | Reserved. |
x_lnnohi | For XCOFF32, specifies the high-order 16 bits of a source file line number. |
x_lnno | Specifies the line number of a source file. The maximum value of this field is 65535 for XCOFF64 and 232 for XCOFF64. |
Section Auxiliary Entry for the C_STAT Symbol
The section auxiliary symbol table entry ID is defined in XCOFF32 to provide information in the symbol table concerning the size of sections produced by a compiler or assembler. The generation of this information by a compiler is optional, and is ignored and removed by the binder.
Offset | Length in Bytes | Name and Description |
---|---|---|
0 | 4 |
|
4 | 2 |
|
6 | 2 |
|
8 | 10 |
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Field Definitions
The following list defines the fields:
Item | Description |
---|---|
x_scnlen | Specifies section length in bytes. |
x_nreloc | Specifies the number of relocation entries. The maximum value of this field is 65535. |
x_nlinno | Specifies the number of line numbers. The maximum value of this field is 65535. |
(no name) | Reserved. |
For general information on the XCOFF file format, see "XCOFF Object File Format." For more information on the symbol table, see "Symbol Table Information."
For information on debugging, see "Debug Section."
The SECT auxiliary symbol table entry is defined to provide information in the symbol table concerning the size of the portion of the section represented by the C_DWARF symbol.
Field Name and Description | XCOFF32 | XCOFF64 |
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Field Definitions
The following list defines the fields:
Item | Description |
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(no name) | Reserved |
x_scnlen | Size of portion of section represented by this symbol. |
x_nreloc | Number of relocation entries for this symbol. The binder sets this field to 0. |
This section defines the symbol table field contents for each of the defined storage classes (n_sclass ) that are used in XCOFF. The following table lists storage class entries in alphabetic order. See "Symbol Table Entry (syms.h)" for more information.
Class Definition | Field Contents |
---|---|
C_BCOMM 135 Beginning of common block |
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C_BINCL 108 Beginning of include file |
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C_BLOCK 100 Beginning or end of inner block |
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C_BSTAT 143 Beginning of static block |
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C_DECL 140 Declaration of object (type) |
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C_DWARF 112 DWARF symbol |
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C_ECOML 136 Local member of common block |
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C_ECOMM 137 End of common block |
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C_EINCL 109 End of include file |
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C_ENTRY 141 Alternate entry |
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C_ESTAT 144 End of static block |
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C_EXT 2 External symbol (defining external symbols for binder processing) |
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C_FCN 101 Beginning or end of function |
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C_FILE 103 Source file name and compiler information |
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C_FUN 142 Function or procedure |
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C_GSYM 128 Global variable |
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C_GTLS 145 Global thread-local variable |
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C_HIDEXT 107 Unnamed external symbol |
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C_INFO 100 Comment section reference |
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C_LSYM 129 Automatic variable allocated on stack |
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C_NULL 0 Symbol table entry marked for deletion. |
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C_PSYM 130 Argument to subroutine allocated on stack |
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C_RPSYM 132 Argument to function or procedure stored in register |
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C_RSYM 131 Register variable |
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C_STAT 3 Static symbol (Unknown. Some compilers generate these symbols in the symbol table to identify size of the .text , .data , and .bss sections. Not used or preserved by binder.) |
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C_STSYM 133 Statically allocated symbol |
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C_STTLS 146 Static thread-local variable |
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C_TCSYM 134 Reserved |
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C_WEAKEXT 111 Weak external symbol (defining weak external symbols for binder processing) |
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Storage Classes by Usage and Symbol Value Classification
Following are the storage classes used and relocated by the binder. The symbol values (n_value ) are addresses.
Class | Description |
---|---|
C_EXT | Specifies an external or global symbol |
C_WEAKEXT | Specifies an external or global symbol with weak binding |
C_HIDEXT | Specifies an internal symbol |
C_BLOCK | Specifies the beginning or end of an inner block (.bb or .eb ) |
C_FCN | Specifies the beginning or end of a function (.bf or .ef only) |
C_STAT | Specifies a static symbol (contained in statics csect) |
Following are storage classes used by the binder and symbolic debugger or by other utilities for file scoping and accessing purposes:
Class | Description |
---|---|
C_FILE | Specifies the source file name. The n_value field holds the symbol index of the next file entry. The n_name field is the name of the file. |
C_BINCL | Specifies the beginning of include header file. The n_value field is the line number byte offset in the object file to the first line number from the include file. |
C_EINCL | Specifies the end of include header file. The n_value field is the line number byte offset in the object file to last line number from the include file. |
C_INFO | Specifies the location of a string in the comment section. The n_value field is the offset to a string of bytes in the specified STYP_INFO section. The string is preceded by a 4-byte length field. The n_name field is preserved by the binder. An application-defined unique name in this field can be used to filter access to only those comment section strings intended for the application. |
C_DWARF | Specifies the portion of the DWARF section that applies to the current C_FILE symbol. The n_value field contains the offset with the section to the portion of the section represented by this symbol. The n_scnlen field in the SECT auxiliary entry contains the length of the portion of the section represented by this symbol. |
Following are the storage classes that exist only for symbolic debugging purposes:
Class | Description |
---|---|
C_BCOMM | Specifies the beginning of a common block. The n_value field is meaningless; the name is the name of the common block. |
C_ECOML | Specifies a local member of a common block. The n_value field is byte-offset within the common block. |
C_ECOMM | Specifies the end of a common block. The n_value field is meaningless. |
C_BSTAT | Specifies the beginning of a static block. The n_value field is the symbol table index of the csect containing static symbols; the name is .bs. |
C_ESTAT | Specifies the end of a static block. The n_value field is meaningless; the name is .es. |
C_DECL | Specifies a declaration of object (type declarations). The n_value field is undefined. |
C_ENTRY | Specifies an alternate entry (FORTRAN) and has a corresponding C_EXT or C_WEAKEXT symbol. The n_value field is undefined. |
C_FUN | Specifies a function or procedure. May have a corresponding C_EXT or C_WEAKEXT symbol. The n_value field is byte-offset within the containing csect. |
C_GSYM | Specifies a global variable and has a corresponding C_EXT or C_WEAKEXT symbol. The n_value field is undefined. |
C_LSYM | Specifies an automatic variable allocated on the stack. The n_value field is byte offset relative to the stack frame (platform dependent). |
C_PSYM | Specifies an argument to a subroutine allocated on the stack. The n_value field is byte-offset relative to the stack frame (platform dependent). |
C_RSYM | Specifies a register variable. The n_value field is the register number. |
C_RPSYM | Specifies an argument to a function or procedure stored in a register. The n_value field is the register number where argument is stored. |
C_STSYM | Specifies a statically allocated symbol. The n_value field is byte-offset within csect pointed to by containing C_BSTAT entry. |
C_GTLS | Specifies a global thread-local variable and follows a C_EXT or C_WEAKEXT symbol with the same name. The n_value field is undefined. |
C_STTLS | Specifies a static thread-local variable and follows a C_HIDEXT symbol with the same name. The n_value field is undefined. |
For general information on the XCOFF file format, see "XCOFF Object File Format." For more information on the symbol table, see "Symbol Table Information."
For information on debugging, see "Debug Section."
IN XCOFF32, the string table contains the names of symbols that are longer than 8 bytes. In XCOFF64, the string table contains the names of all symbols. If the string table is present, the first 4 bytes contain the length (in bytes) of the string table, including the length of this length field. The remainder of the table is a sequence of null-terminated ASCII strings. If the n_zeroes field in the symbol table entry is 0, then the n_offset field gives the byte offset into the string table of the name of the symbol.
If a string table is not used, it may be omitted entirely, or a string table consisting of only the length field (containing a value of 0 or 4) may be used. A value of 4 is preferable. The following table shows string table organization.
Offset | Length in Bytes | Description |
---|---|---|
0 | 4 | Length of string table. |
4 | n | Symbol name string, null-terminated. |
Field repeats for each symbol name. |
For general information on the XCOFF file format, see "XCOFF Object File Format."
The debug section contains the symbolic debugger stabstrings (symbol table strings). It is generated by the compilers and assemblers. It provides symbol attribute information for use by the symbolic debugger.
See "Debug Section" for a general discussion.
Stabstring Terminal Symbols
Symbol | Regular Expression |
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NAME | [^ ; : ' "] (A name consists of any non-empty set of characters, excluding ; : ' or ".) |
STRING | '.*' | ".*", where \", \', or \\ can be used inside the string Within a string, the \ (backslash character) may have a special meaning. If the character following the \ is another \, one of the backslashes is ignored. If the next character is the quote character used for the current string, the string is interpreted as containing an embedded quote. Otherwise, the \ is interpreted literally. However, if the closing quote is the last character in the stabstring, and a \ occurs immediately before the quote, the \ is interpreted literally. This use is not recommended. The \ must be quoted only in the following instances:
An escaped quote is required only when a single string contains both a single quote and a double quote. Otherwise, the string should be quoted with the quote character not contained in the strings. A string can contain embedded null characters, so utilities that process stabstrings must use the length field to determine the length of a stabstring. |
INTEGER | (-)[0-9]+ |
HEXINTEGER | [0-9A-F]+ The hexadecimal digits A-F must be uppercase. |
REAL | [H|D|DD]([+-][0-9]+(.)[0-9]*([eEqQ](+-)[0-9]+) | (+-)INF | QNAN | SNAN) |
Stabstring Grammar
REALs may be preceded by white space, and STRINGs may contain any characters, including null and blank characters. Otherwise, there are no null or blank characters in a stabstring.
Long stabstrings can be split across multiple symbol table entries for easier handling. In the stabstring grammar, a # (pound sign) indicates a point at which a stabstring may be continued. A continuation is indicated by using either the ? (question mark) or \ as the last character in the string. The next part of the stabstring is in the name of the next symbol table entry. If an alternative for a production is empty, the grammar shows the keyword /*EMPTY*/.
The following list contains the stabstring grammar:
For i types, ModuleName refers to the Modula-2 module from which it is imported.
INTEGER.
TypeID , PassBy ; #
Gn NumBits FieldList ;
Gc NumBits CondFieldList ;
AccessSpec
/*EMPTY*/
/*EMPTY*/
/*EMPTY*/
NAME : TypeID , BitOffset , NumBits ; #
NAME : INTEGER = q ConditionType , ValueList ; #
ConditionPrimitive , KanjiChar
INTEGER