ctrace(1)

ctrace -- C program debugger

Synopsis

   ctrace [options] [file]

Description

The ctrace command allows the user to monitor the sequential execution of a C program as each program statement executes. The effect is similar to executing a shell procedure with the -x option. ctrace reads the C program in file (or from standard input if the user does not specify file), inserts statements to print the text of each executable statement and the values of all variables referenced or modified, and writes the modified program to the standard output. The output of ctrace must be placed into a temporary file because the cc(1) command does not allow the use of a pipe. This file can then be compiled and executed.

As each statement in the program executes, it will be listed at the terminal, followed by the name and value of any variables referenced or modified in the statement; these variable names and values will be followed by any output from the statement. Loops in the trace output are detected and tracing is stopped until the loop is exited or a different sequence of statements within the loop is executed. A warning message is printed after each 1000 loop cycles to help the user detect infinite loops. The trace output goes to the standard output so the user can put it into a file for examination with an editor or the bfs(1) or tail(1) commands.

The options commonly used are:

-f functions: Trace only these functions.
-v functions: Trace all but these functions.

The user may want to add to the default formats for printing variables. Long and pointer variables are always printed as signed integers. Pointers to character arrays are also printed as strings if appropriate. char, short, and int variables are also printed as signed integers and, if appropriate, as characters. float, double, and long double variables are printed as floating point numbers in scientific notation. The user can request that variables be printed in additional formats, if appropriate, with these options:

-o: Octal
-x: Hexadecimal
-u: Unsigned
-e: Floating point

These options are used only in special circumstances:

-l n: Check n consecutively executed statements for looping trace output, instead of the default of 20. Use 0 to get all the trace output from loops.
-s: Suppress redundant trace output from simple assignment statements and string copy function calls. This option can hide a bug caused by use of the = operator in place of the == operator.
-t n: Trace n variables per statement instead of the default of 10 (the maximum number is 20). The diagnostics section explains when to use this option.
-P: Preprocess the input before tracing it. The user can also use the -D, -I, and -U cc(1) options.
-p string: Change the trace print function from the default of printf. For example, fprintf(stderr, would send the trace to the standard error output.
-r f: Use file f in place of the runtime.c trace function package. This replacement lets the user change the entire print function, instead of just the name and leading arguments (see the -p option).
-V: Prints version information on the standard error.
-Qarg: If arg is ``y'', identification information about ctrace will be added to the output files. This can be useful for software administration. Giving ``n'' for arg explicitly asks for no such information, which is the default behavior.

Examples

If the file lc.c contains this C program:

    1 #include <stdio.h>
    2 main()	/ count lines in input /
    3 {
    4 	int c, nl;
    5
    6 	nl = 0;
    7 	while ((c = getchar()) != EOF)
    8 		if (c = '\n')
    9 			++nl;
   10 	printf("%d\n", nl);
   11 }

these commands and test data are entered:

cc lc.c
a.out
1
<Ctrl>-d

the program will be compiled and executed. The output of the program will be the number 2, which is incorrect because there is only one line in the test data. The error in this program is common, but subtle. If the user invokes ctrace with these commands:

ctrace lc.c >temp.c
cc temp.c
a.out

the output will be:

    2 main()
    6 	nl = 0;
       	/ nl == 0 /
    7 	while ((c = getchar()) != EOF)

The program is now waiting for input. If the user enters the same test data as before, the output will be:

      	/ c == 49 or '1' /
     8             if (c = '\n')
                   /* c  == 10 or '\n' */
     9                     ++nl;
                           /* nl == 1 */
     7     while ((c = getchar()) != EOF)
           /* c  == 10 or '\n' */
     8             if (c = '\n')
                   /* c  == 10 or '\n' */
     9                     ++nl;
                           /* nl == 2 */
       /* repeating */

Once the end-of-file character (<Ctrl>-d) is entered, the final output will be:

       /* repeated < 1 time */
     7     while ((c = getchar()) != EOF)
           /* c  == -1 */
    10     printf("%d\n", nl);
           /* nl == 2 */2
   
       /* return */

Note the information printed out at the end of the trace line for the nl variable following line 10. Also note the return comment added by ctrace at the end of the trace output. This shows the implicit return at the terminating brace in the function.

The trace output shows that variable c is assigned the value 1 in line 7, but in line 8 it has the value ``\n''. Once user attention is drawn to this if statement, he or she will probably realize that the assignment operator (=) was used in place of the equality operator (==). This error can easily be missed during code reading.

Execution-time trace control

The default operation for ctrace is to trace the entire program file, unless the -f or -v options are used to trace specific functions. The default operation does not give the user statement-by-statement control of the tracing, nor does it let the user turn the tracing off and on when executing the traced program.

The user can do both of these by adding ctroff() and ctron() function calls to the program to turn the tracing off and on, respectively, at execution time. Thus, complex criteria can be arbitrarily coded for trace control with if statements, and this code can even be conditionally included because ctrace defines the CTRACE preprocessor variable. For example:

   #ifdef CTRACE
   	if (c == '!' && i > 1000)
   		ctron();
   #endif

These functions can also be called from debug(1) if they are compiled with the -g option. For example, to trace all but lines 7 to 10 in the main function, enter:

   debug a.out
   debug> stop lc.c @7  {set ctroff();}
   EVENT [1] assigned
   debug> stop lc.c @11  {set ctron();}
   EVENT [2] assigned
   debug> run

The trace can be turned off and on by setting static variable tr_ct_ to 0 and 1, respectively. This on/off option is useful if a user is using a debugger that can not call these functions directly.

Files

/usr/ccs/lib/ctrace/runtime.c run-time trace package

References

cc(1), ctype(3C), debug(1), fclose(3S), fprintf(3S)

Diagnostics

This section contains diagnostic messages from both ctrace and cc(1), since the traced code often gets some cc warning messages. The user can get cc error messages in some rare cases, all of which can be avoided.

ctrace diagnostics

warning: some variables are not traced in this statement: Only 10 variables are traced in a statement to prevent the C compiler "out of tree space; simplify expression" error. Use the -t option to increase this number.
warning: statement too long to trace: This statement is over 400 characters long. Make sure that tabs are used to indent the code, not spaces.
cannot handle preprocessor code, use -P option: This is usually caused by #ifdef/#endif preprocessor statements in the middle of a C statement, or by a semicolon at the end of a #define preprocessor statement.
'if . . . else if' sequence too long: Split the sequence by removing an else from the middle.
possible syntax error, try -P option: Use the -P option to preprocess the ctrace input, along with any appropriate -D, -I, and -U preprocessor options.

Notices

Defining a function with the same name as a system function may cause a syntax error if the number of arguments is changed. Just use a different name.

ctrace assumes that BADMAG is a preprocessor macro, and that EOF and NULL are #defined constants. Declaring any of these to be variables, for example,
"int EOF;", will cause a syntax error.

Pointer values are always treated as pointers to character strings.

ctrace does not know about the components of aggregates like structures, unions, and arrays. It cannot choose a format to print all the components of an aggregate when an assignment is made to the entire aggregate. ctrace may choose to print the address of an aggregate or use the wrong format (for example, 3.149050e-311 for a structure with two integer members) when printing the value of an aggregate.

The loop trace output elimination is done separately for each file of a multi-file program. Separate output elimination can result in functions called from a loop still being traced, or the elimination of trace output from one function in a file until another in the same file is called.