write(2) SYSTEM CALLS 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 incre-
mented by the number of bytes actually written. On a regu-
lar 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) SYSTEM CALLS 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 exam-
ple, 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 inter-
leaved, 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 comple-
tion 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;
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write(2) SYSTEM CALLS write(2)
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. 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 max-
imum 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
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write(2) SYSTEM CALLS write(2)
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.
EAGAIN Total amount of system memory available when
reading via raw I/O is temporarily insuffi-
cient.
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 termi-
nal whose TOSTOP flag is set; the process is
neither ignoring nor blocking SIGTTOU sig-
nals, and the process group of the process is
orphaned.
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write(2) SYSTEM CALLS write(2)
ENOLCK The system record lock table was full, so the
write could not go to sleep until the block-
ing 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 avail-
able 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 mes-
sage has been received at the stream head.
In this case, errno is set to the value
included in the error message. Upon success-
ful completion write and writev mark for
update the stctime and stmtime fields of
5
write(2) SYSTEM CALLS write(2)
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 writ-
ten. Otherwise, it returns -1 and sets errno to indicate
the error.
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