write(2) UNIX System V 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, pipe, or ioctl
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 valid 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) UNIX System V 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
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.
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write(2) UNIX System V write(2)
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.
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.
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write(2) UNIX System V write(2)
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.
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.
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write(2) UNIX System V write(2)
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).
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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