write(2) System Calls write(2)NAME
write, pwrite, writev - write on a file
SYNOPSIS
#include <unistd.h>
ssize_t write(int fildes, const void *buf, size_t nbyte);
ssize_t pwrite(int fildes, const void *buf, size_t nbyte,
off_t offset);
#include <sys/uio.h>
ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION
The write() function attempts to write nbyte bytes from the buffer
pointed to by buf to the file associated with the open file descriptor,
fildes.
If nbyte is 0, write() will return 0 and have no other results if the
file is a regular file; otherwise, the results are unspecified.
On a regular file or other file capable of seeking, the actual writing
of data proceeds from the position in the file indicated by the file
offset associated with fildes. Before successful return from write(),
the file offset is incremented by the number of bytes actually written.
On a regular file, if this incremented file offset is greater than the
length of the file, the length of the file will be set to this file
offset.
If the O_SYNC bit has been set, write I/O operations on the file
descriptor complete as defined by synchronized I/O file integrity com‐
pletion.
If fildes refers to a socket, write() is equivalent to send(3SOCKET)
with no flags set.
On a file not capable of seeking, writing always takes place starting
at the current position. The value of a file offset associated with
such a device is undefined.
If the O_APPEND flag of the file status flags is set, the file offset
will be set to the end of the file prior to each write and no interven‐
ing file modification operation will occur between changing the file
offset and the write operation.
For regular files, no data transfer will occur past the offset maximum
established in the open file description with fildes.
A write() to a regular file is blocked if mandatory file/record locking
is set (see chmod(2)), and there is a record lock owned by another
process on the segment of the file to be written:
o If O_NDELAY or O_NONBLOCK is set, write() returns −1 and
sets errno to EAGAIN.
o If O_NDELAY and O_NONBLOCK are clear, write() sleeps until
all blocking locks are removed or the write() is terminated
by a signal.
If a write() requests that more bytes be written than there is room
for—for example, if the write would exceed the process file size limit
(see getrlimit(2) and ulimit(2)), the system file size limit, or the
free space on the device—only as many bytes as there is room for will
be written. For example, suppose there is space for 20 bytes more in a
file before reaching a limit. A write() of 512-bytes returns 20. The
next write() of a non-zero number of bytes gives a failure return
(except as noted for pipes and FIFO below).
If write() is interrupted by a signal before it writes any data, it
will return −1 with errno set to EINTR.
If write() is interrupted by a signal after it successfully writes some
data, it will return the number of bytes written.
If write() exceeds the process file size limit, the application gener‐
ates a SIGXFSZ signal, whose default behavior is to dump core.
After a write() to a regular file has successfully returned:
o Any successful read(2) from each byte position in the file
that was modified by that write will return the data speci‐
fied by the write() for that position until such byte posi‐
tions are again modified.
o Any subsequent successful write() to the same byte position
in the file will overwrite that file data.
Write requests to a pipe or FIFO are handled the same as a regular file
with the following exceptions:
o There is no file offset associated with a pipe, hence each
write request appends to the end of the pipe.
o Write requests of {PIPE_BUF} bytes or less are guaranteed
not to be interleaved with data from other processes doing
writes on the same pipe. Writes of greater than {PIPE_BUF}
bytes may have data interleaved, on arbitrary boundaries,
with writes by other processes, whether or not the O_NON‐
BLOCK or O_NDELAY flags are set.
o If O_NONBLOCK and O_NDELAY are clear, a write request may
cause the process to block, but on normal completion it
returns nbyte.
o If O_NONBLOCK and O_NDELAY are set, write() does not block
the process. If a write() request for PIPE_BUF or fewer
bytes succeeds completely write() returns nbyte. Otherwise,
if O_NONBLOCK is set, it returns −1 and sets errno to EAGAIN
or if O_NDELAY is set, it returns 0. A write() request for
greater than {PIPE_BUF} bytes transfers what it can and
returns the number of bytes written or it transfers no data
and, if O_NONBLOCK is set, returns −1 with errno set to
EAGAIN or if O_NDELAY is set, it returns 0. Finally, if a
request is greater than PIPE_BUF bytes and all data previ‐
ously written to the pipe has been read, write() transfers
at least PIPE_BUF bytes.
When attempting to write to a file descriptor (other than a pipe, a
FIFO, a socket, or a stream) that supports nonblocking writes and can‐
not accept the data immediately:
o If O_NONBLOCK and O_NDELAY are clear, write() blocks until
the data can be accepted.
o If O_NONBLOCK or O_NDELAY is set, write() does not block the
process. If some data can be written without blocking the
process, write() writes what it can and returns the number
of bytes written. Otherwise, if O_NONBLOCK is set, it
returns −1 and sets errno to EAGAIN or if O_NDELAY is set,
it returns 0.
Upon successful completion, where nbyte is greater than 0, write() will
mark for update the st_ctime and st_mtime fields of the file, and if
the file is a regular file, the S_ISUID and S_ISGID bits of the file
mode may be cleared.
For streams files (see Intro(2) and streamio(7I)), the operation of
write() is determined by the values of the minimum and maximum nbyte
range ("packet size") accepted by the stream. These values are con‐
tained in the topmost stream module, and can not be set or tested from
user level. If nbyte falls within the packet size range, nbyte bytes
are written. If nbyte does not fall within the range and the minimum
packet size value is zero, write() breaks the buffer into maximum
packet size segments prior to sending the data downstream (the last
segment may be smaller than the maximum packet size). If nbyte does
not fall within the range and the minimum value is non-zero, write()
fails and sets errno to ERANGE. Writing a zero-length buffer (nbyte is
zero) to a streams device sends a zero length message with zero
returned. However, writing a zero-length buffer to a pipe or FIFO sends
no message and zero is returned. The user program may issue the
I_SWROPT ioctl(2) to enable zero-length messages to be sent across the
pipe or FIFO (see streamio(7I)).
When writing to a stream, data messages are created with a priority
band of zero. When writing to a socket or to a stream that is not a
pipe or a FIFO:
o If O_NDELAY and O_NONBLOCK are not set, and the stream can‐
not accept data (the stream write queue is full due to
internal flow control conditions), write() blocks until data
can be accepted.
o If O_NDELAY or O_NONBLOCK is set and the stream cannot
accept data, write() returns -1 and sets errno to EAGAIN.
o If O_NDELAY or O_NONBLOCK is set and part of the buffer has
already been written when a condition occurs in which the
stream cannot accept additional data, write() terminates and
returns the number of bytes written.
The write() and writev() functions will fail if the stream head had
processed an asynchronous error before the call. In this case, the
value of errno does not reflect the result of write() or writev() but
reflects the prior error.
pwrite()
The pwrite() function is equivalent to write(), except that it writes
into a given position and does not change the file offset (regardless
of whether O_APPEND is set). The first three arguments to pwrite() are
the same as write(), with the addition of a fourth argument offset for
the desired position inside the file.
writev()
The writev() function performs the same action as write(), but gathers
the output data from the iovcnt buffers specified by the members of the
iov array: iov[0], iov[1], ..., iov[iovcnt−1]. The iovcnt buffer is
valid if greater than 0 and less than or equal to {IOV_MAX}. See
Intro(2) for a definition of {IOV_MAX}.
The iovec structure contains the following members:
caddr_t iov_base;
int iov_len;
Each iovec entry specifies the base address and length of an area in
memory from which data should be written. The writev() function always
writes all data from an area before proceeding to the next.
If fildes refers to a regular file and all of the iov_len members in
the array pointed to by iov are 0, writev() will return 0 and have no
other effect. For other file types, the behavior is unspecified.
If the sum of the iov_len values is greater than SSIZE_MAX, the opera‐
tion fails and no data is transferred.
RETURN VALUES
Upon successful completion, write() returns the number of bytes actu‐
ally written to the file associated with fildes. This number is never
greater than nbyte. Otherwise, −1 is returned, the file-pointer remains
unchanged, and errno is set to indicate the error.
Upon successful completion, writev() returns the number of bytes actu‐
ally written. Otherwise, it returns −1, the file-pointer remains
unchanged, and errno is set to indicate an error.
ERRORS
The write(), pwrite(), and writev() functions will fail if:
EAGAIN Mandatory file/record locking is set, O_NDELAY or O_NONBLOCK
is set, and there is a blocking record lock; an attempt is
made to write to a stream that can not accept data with the
O_NDELAY or O_NONBLOCK flag set; or a write to a pipe or
FIFO of PIPE_BUF bytes or less is requested and less than
nbytes of free space is available.
EBADF The fildes argument is not a valid file descriptor open for
writing.
EDEADLK The write was going to go to sleep and cause a deadlock
situation to occur.
EDQUOT The user's quota of disk blocks on the file system contain‐
ing the file has been exhausted.
EFBIG An attempt is made to write a file that exceeds the
process's file size limit or the maximum file size (see
getrlimit(2) and ulimit(2)).
EFBIG The file is a regular file, nbyte is greater than 0, and the
starting position is greater than or equal to the offset
maximum established in the file description associated with
fildes.
EINTR A signal was caught during the write operation and no data
was transferred.
EIO The process is in the background and is attempting to write
to its controlling terminal whose TOSTOP flag is set, or the
process is neither ignoring nor blocking SIGTTOU signals
and the process group of the process is orphaned.
ENOLCK Enforced record locking was enabled and {LOCK_MAX} regions
are already locked in the system, or the system record lock
table was full and the write could not go to sleep until
the blocking record lock was removed.
ENOLINK The fildes argument is on a remote machine and the link to
that machine is no longer active.
ENOSPC During a write to an ordinary file, there is no free space
left on the device.
ENOSR An attempt is made to write to a streams with insufficient
streams memory resources available in the system.
ENXIO A hangup occurred on the stream being written to.
EPIPE An attempt is made to write to a pipe or a FIFO that is not
open for reading by any process, or that has only one end
open (or to a file descriptor created by socket(3SOCKET),
using type SOCK_STREAM that is no longer connected to a peer
endpoint). A SIGPIPE signal will also be sent to the thread.
The process dies unless special provisions were taken to
catch or ignore the signal.
ERANGE The transfer request size was outside the range supported by
the streams file associated with fildes.
The write() and pwrite() functions will fail if:
EFAULT The buf argument points to an illegal address.
EINVAL The nbyte argument overflowed an ssize_t.
The pwrite() function fails and the file pointer remains unchanged if:
ESPIPE The fildes argument is associated with a pipe or FIFO.
The write() and writev() functions may fail if:
EINVAL The stream or multiplexer referenced by fildes is linked
(directly or indirectly) downstream from a multiplexer.
ENXIO A request was made of a non-existent device, or the request
was outside the capabilities of the device.
ENXIO A hangup occurred on the stream being written to.
A write to a streams file may fail if an error message has been
received at the stream head. In this case, errno is set to the value
included in the error message.
The writev() function may fail if:
EINVAL The iovcnt argument was less than or equal to 0 or greater
than {IOV_MAX}; one of the iov_len values in the iov array
was negative; or the sum of the iov_len values in the iov
array overflowed an ssize_t.
USAGE
The pwrite() function has a transitional interface for 64-bit file off‐
sets. See lf64(5).
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Committed │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │write() is Async-Signal-Safe │
├─────────────────────────────┼─────────────────────────────┤
│Standard │See standards(5). │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOIntro(2), chmod(2), creat(2), dup(2), fcntl(2), getrlimit(2), ioctl(2),
lseek(2), open(2), pipe(2), ulimit(2), send(3SOCKET), socket(3SOCKET),
attributes(5), lf64(5), standards(5), streamio(7I)SunOS 5.11 29 Jan 2008 write(2)
