write(2) write(2)
NAME
write, writev - write on a file
SYNOPSIS
#include <unistd.h>
ssize_t write(int fildes, const void *buf, size_t nbyte);
#include <sys/types.h>
#include <sys/uio.h>
ssize_t 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 0
and the file is a regular file, write returns 0 and has no
other results. If the value of nbyte is greater than
{SSIZE_MAX} the result is undefined. 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 only if greater than 0 and less than or equal
to {MAXIOVCNT}.
For writev, the iovec structure contains the following
members:
void * iov_base;
size_t iov_len;
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 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.
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write(2) write(2)
If the O_APPEND flag of the file status flags is set, the file
pointer is set to the end of the file before each write.
For regular files, if the O_SYNC 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 O_SYNC is
set, write does not return until the data has been physically
updated.
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 O_NDELAY or O_NONBLOCK is set, write returns -1 and
sets errno to EAGAIN.
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).
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 {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 the O_NONBLOCK or O_NDELAY flags are set.
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write(2) write(2)
If O_NONBLOCK and O_NDELAY are clear, a write request
may cause the process to block, but on normal completion
it returns nbyte.
If O_NONBLOCK is set, write requests are handled in the
following way: the write does not block the process;
write requests for {PIPE_BUF} or fewer bytes either
succeed completely and return nbyte, or return -1 and
set errno to EAGAIN. A write request for greater than
{PIPE_BUF} 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 {PIPE_BUF} bytes and all data
previously written to the pipe has been read, write
transfers at least {PIPE_BUF} bytes.
If O_NDELAY is set, write requests are handled in the
following way: the write does not block the process;
write requests for {PIPE_BUF} or fewer bytes either
succeed completely and return nbyte, or return 0. A
write request for greater than {PIPE_BUF} 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 {PIPE_BUF} bytes and all data
previously 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 or FIFO) that supports nonblocking writes and cannot
accept the data immediately:
If O_NONBLOCK and O_NDELAY are clear, write blocks until
the data can be accepted.
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.
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 I_PUSH in streamio(7)], these
Copyright 1994 Novell, Inc. Page 3
write(2) write(2)
values cannot 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 0, 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 0) to a STREAMS device sends a
zero-length message with 0 returned. However, writing a
zero-length buffer to a pipe or FIFO sends no message and 0 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(7)].
When writing to a stream, data messages are created with a
priority band of 0. When writing to a stream that is not a
pipe or FIFO:
If O_NDELAY and O_NONBLOCK are not set, and the stream
cannot accept data (the stream write queue is full
because of internal flow control conditions), write
blocks until data can be accepted.
If O_NDELAY and O_NONBLOCK are not set, and the and the
stream cannot accept data, write returns -1 and sets
errno to EAGAIN.
If O_NDELAY and O_NONBLOCK are not set, and the 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.
Return Values
On success, write and writev return the number of bytes
actually written and mark for update the st_ctime and st_mtime
fields of the file. On failure, write and writev return -1
and set errno to identify the error.
Errors
In the following conditions, write and writev fail and set
errno to:
Copyright 1994 Novell, Inc. Page 4
write(2) write(2)
EAGAIN Mandatory file/record locking is set, O_NDELAY
or O_NONBLOCK 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
cannot accept data with the O_NDELAY or
O_NONBLOCK flag set.
EAGAIN If 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 fildes is not a valid file descriptor open for
writing.
EDEADLK The write was going to go to sleep and cause a
deadlock 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.
EIO fildes points to a device special file that is
in the closing state.
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write(2) write(2)
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 The device associated with the file descriptor
is a block-special or character-special file
and the file-pointer value is out of range.
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
{LOCK_MAX} regions are already locked in the
system.
In addition, in the following conditions writev fails and sets
errno to:
EINVAL iovcnt was less than or equal to 0, or greater
than 16.
EINVAL An iov_len value in the iov array was negative.
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write(2) write(2)
EINVAL The sum of the iov_len 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.
After carrier loss, M_HANGUP is set, and a subsequent write
will return -1 with errno set to EIO. To write after
disconnecting and reconnecting the line, set the CLOCAL flag
to tell the driver to ignore the state of the line and the
driver will not send M_HANGUP to the stream head. If CLOCAL
is not set, and hangup occurs, the application is responsible
for re-establishing the connection.
REFERENCES
creat(2), dup(2), fcntl(2), getrlimit(2), intro(2), lseek(2),
open(2), pipe(2), pwrite(2), read(2), types(5), ulimit(2)
NOTICES
Considerations for Threads Programming
Open file descriptors are a process resource and available to
any sibling thread; if used concurrently, actions by one
thread can interfere with those of a sibling.
While one thread is blocked, siblings might still be
executing.
Copyright 1994 Novell, Inc. Page 7