WRITE(2) WRITE(2)
NAME
write - write on a file
SYNOPSIS
int write (fildes, buf, nbyte)
int fildes;
char *buf;
unsigned nbyte;
DESCRIPTION
fildes is a file descriptor obtained from a creat(2),
open(2), dup(2), fcntl(2), or pipe(2) system call.
write attempts to write nbyte bytes from the buffer pointed
to by buf to the file associated with the fildes.
On devices capable of seeking, the actual writing of data
proceeds from the position in the file indicated by the file
pointer. Upon return from write, the file pointer is
incremented by the number of bytes actually written.
On devices incapable of seeking, writing always takes place
starting at the current position. The value of a file
pointer associated with such a device is undefined.
If the O_APPEND flag of the file status flags is set, the
file pointer will be set to the end of the file prior to
each write.
For regular files, if the O_SYNC flag of the file status
flags is set, the write will not return until both the file
data and file status have been physically updated. This
function is for special applications that require extra
reliability at the cost of performance. For block special
files, if O_SYNC is set, the write will not return until the
data has been physically updated.
A write to a regular file will be 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
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WRITE(2) WRITE(2)
file to be written. If O_NDELAY is not set, the write will
sleep until the blocking record lock is removed.
For STREAMS [see intro(2)] files, 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 contained in the topmost stream module. Unless the user
pushes [see I_PUSH in streamio(7)] the topmost module, these
values can not be set or tested from user level. If nbyte
falls within the packet size range, nbyte bytes will be
written. If nbyte does not fall within the range and the
minimum packet size value is zero, write will break the
buffer into maximum packet size segments prior to sending
the data downstream (the last segment may contain less than
the maximum packet size). If nbyte does not fall within the
range and the minimum value is non-zero, write will fail
with errno set to ERANGE. Writing a zero-length buffer
(nbyte is zero) sends zero bytes with zero returned.
For STREAMS files, if O_NDELAY is not set and the stream can
not accept data (the stream write queue is full due to
internal flow control conditions), write will block until
data can be accepted. O_NDELAY will prevent a process from
blocking due to flow control conditions. If O_NDELAY is set
and the stream can not accept data, write will fail. If
O_NDELAY is set and part of the buffer has been written when
a condition in which the stream can not accept additional
data occurs, write will terminate and return the number of
bytes written.
write will fail and the file pointer will remain unchanged
if one or more of the following are true:
[EAGAIN] Mandatory file/record locking was set,
O_NDELAY was set, and there was a blocking
record lock.
[EAGAIN] Total amount of system memory available when
reading via raw IO is temporarily
insufficient.
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WRITE(2) WRITE(2)
[EAGAIN] Attempt to write to a stream that can not
accept data with the O_NDELAY flag set.
[EBADF] fildes 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.
[EFAULT] buf points outside the process's allocated
address space.
[EFBIG] An attempt was made to write a file that
exceeds the process's file size limit or the
maximum file size [see ulimit(2)].
[EINTR] A signal was caught during the write system
call.
[EINVAL] Attempt to write to a stream linked below a
multiplexor.
[ENOLCK] The system record lock table was full, so the
write could not go to sleep until the
blocking record lock was removed.
[ENOLINK] fildes 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.
[ENXIO] A hangup occurred on the stream being written
to.
[EPIPE and SIGPIPE signal]
An attempt is made to write to a pipe that is
not open for reading by any process.
[ERANGE] Attempt to write to a stream with nbyte
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WRITE(2) WRITE(2)
outside specified minimum and maximum write
range, and the minimum value is non-zero.
If a write requests that more bytes be written than there is
room for (e.g., the ulimit [see ulimit(2)] or the physical
end of a medium), 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 will return 20. The next write of a non-zero
number of bytes will give a failure return (except as noted
below).
If the file being written is a pipe (or FIFO) and the
O_NDELAY flag of the file flag word is set, then write to a
full pipe (or FIFO) will return a count of 0. Otherwise
(O_NDELAY clear), writes to a full pipe (or FIFO) will block
until space becomes available.
A write to a STREAMS file can 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.
SEE ALSO
creat(2), dup(2), fcntl(2), intro(2), lseek(2), open(2),
pipe(2), ulimit(2).
DIAGNOSTICS
Upon successful completion the number of bytes actually
written is returned. Otherwise, -1 is returned and errno is
set to indicate the error.
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