write(2) write(2)
NAME
write, writev - write on a file
SYNOPSIS
#include <unistd.h>
int write(int fildes, const void *buf, unsigned nbyte);
#include <sys/types.h>
#include <sys/uio.h>
int writev(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION
write attempts to write nbyte bytes from the buffer pointed to by buf
to the file associated with fildes. If nbyte is zero and the file is
a regular file, write returns zero and has no other results. fildes
is a file descriptor obtained from a creat, open, dup, fcntl, or pipe
system call.
writev 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 is invalid if greater
than 0 and less than or equal to {IOVMAX}.
For writev, the iovec structure contains the following members:
caddrt iovbase;
int iovlen;
Each iovec entry specifies the base address and length of an area in
memory from which data should be written. writev always writes a
complete area before proceeding to the next.
On devices capable of seeking, the actual writing of data proceeds
from the position in the file indicated by the file pointer. On
return from write, the file pointer is incremented by the number of
bytes actually written. On a regular file, if the incremented file
pointer is greater than the length of the file, the length of the
file is set to the new file pointer.
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 OAPPEND flag of the file status flags is set, the file
pointer is set to the end of the file prior to each write.
For regular files, if the OSYNC flag of the file status flags is
set, write does 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 OSYNC is set, write does
not return until the data has been physically updated.
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write(2) write(2)
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:
If ONDELAY or ONONBLOCK is set, write returns -1 and sets
errno to EAGAIN.
If ONDELAY and ONONBLOCK 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).
Write requests to a pipe or FIFO are handled the same as a regular
file with the following exceptions:
There is no file offset associated with a pipe, hence each
write request appends to the end of the pipe.
Write requests of {PIPEBUF} bytes or less are guaranteed not
to be interleaved with data from other processes doing writes
on the same pipe. Writes of greater than {PIPEBUF} bytes may
have data interleaved, on arbitrary boundaries, with writes by
other processes, whether or not the ONONBLOCK or ONDELAY
flags are set.
If ONONBLOCK and ONDELAY are clear, a write request may cause
the process to block, but on normal completion it returns
nbyte.
If ONONBLOCK is set, write requests are handled in the
following way: the write does not block the process; write
requests for {PIPEBUF} or fewer bytes either succeed
completely and return nbyte, or return -1 and set errno to
EAGAIN. A write request for greater than {PIPEBUF} bytes
either transfers what it can and returns the number of bytes
written, or transfers no data and returns -1 with errno set to
EAGAIN. Also, if a request is greater than {PIPEBUF} bytes
and all data previously written to the pipe has been read,
write transfers at least {PIPEBUF} bytes.
If ONDELAY is set, write requests are handled in the following
way: the write does not block the process; write requests for
{PIPEBUF} or fewer bytes either succeed completely and return
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write(2) write(2)
nbyte, or return 0. A write request for greater than {PIPEBUF}
bytes either transfers what it can and returns the number of
bytes written, or transfers no data and returns 0. Also, if a
request is greater than {PIPEBUF} bytes and all data
previously written to the pipe has been read, write transfers
at least {PIPEBUF} bytes.
When attempting to write to a file descriptor (other than a pipe or
FIFO) that supports nonblocking writes and cannot accept the data
immediately:
If ONONBLOCK and ONDELAY are clear, write blocks until the
data can be accepted.
If ONONBLOCK or ONDELAY 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 ONONBLOCK is set, it returns -1
and sets errno to EAGAIN or if ONDELAY is set, it returns 0.
For STREAMS files [see intro(2)], 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 the topmost
module [see IPUSH in streamio(7)], these values 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 ISWROPT ioctl(2) to enable zero-length
messages to be sent across the pipe or FIFO [see streamio(7)].
When writing to a stream, data messages are created with a priority
band of zero. When writing to a stream that is not a pipe or FIFO:
If ONDELAY and ONONBLOCK are not set, and the stream cannot
accept data (the stream write queue is full due to internal
flow control conditions), write blocks until data can be
accepted.
If ONDELAY or ONONBLOCK is set and the stream cannot accept
data, write returns -1 and sets errno to EAGAIN.
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write(2) write(2)
If ONDELAY or ONONBLOCK 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.
write and writev fail and the file pointer remains unchanged if one
or more of the following are true:
EAGAIN Mandatory file/record locking is set, ONDELAY or
ONONBLOCK is set, and there is a blocking record
lock.
EAGAIN Total amount of system memory available when reading
via raw I/O is temporarily insufficient.
EAGAIN An attempt is made to write to a stream that can not
accept data with the ONDELAY or ONONBLOCK flag set.
EAGAIN If a write to a pipe or FIFO of {PIPEBUF} bytes or
less is requested and less than nbytes of free space
is available.
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 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)].
EINTR A signal was caught during the write system call.
EINVAL An attempt is made to write to a stream linked below a
multiplexor.
EIO The process is in the background and is attempting to
write to its controlling terminal whose TOSTOP flag is
set; the process is neither ignoring nor blocking
SIGTTOU signals, and the process group of the process
is orphaned.
ENOLCK The system record lock table was full, so the write
could not go to sleep until the blocking record lock
was removed.
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write(2) write(2)
ENOLINK fildes is on a remote machine and the link to that
machine is no longer active.
ENOSR An attempt is made to write to a stream with
insufficient STREAMS memory resources available in the
system.
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.
EPIPE An attempt is made to write to a FIFO that is not open
for reading by any process.
EPIPE An attempt is made to write to a pipe that has only
one end open.
ERANGE An attempt is made to write to a stream with nbyte
outside specified minimum and maximum write range, and
the minimum value is non-zero.
ENOLCK Enforced record locking was enabled and {LOCKMAX}
regions are already locked in the system.
In addition, writev may return one of the following errors:
EINVAL iovcnt was less than or equal to 0, or greater than
16.
EINVAL One of the iovlen values in the iov array was
negative.
EINVAL The sum of the iovlen values in the iov array
overflowed a 32-bit integer.
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.
Upon successful completion write and writev mark for update the
stctime and stmtime fields of the file.
SEE ALSO
intro(2), creat(2), dup(2), fcntl(2), getrlimit(2), lseek(2),
open(2), pipe(2), ulimit(2).
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write(2) write(2)
DIAGNOSTICS
On success, write returns the number of bytes actually written.
Otherwise, it returns -1 and sets errno to indicate the error.
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