System Calls write(2)
NAME
write, pwrite, writev - write on a file
SYNOPSIS
#include
ssizet write(int fildes, const void *buf, sizet nbyte);
ssizet pwrite(int fildes, const void *buf, sizet nbyte,
offt offset);
#include
ssizet writev(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION
The write() function attempts to write nbyte bytes from the
buffer pointed to by buf to the file associated with the
open file descriptor, fildes.
If nbyte is 0, write() will return 0 and have no other
results if the file is a regular file; otherwise, the
results are unspecified.
On a regular file or other file capable of seeking, the
actual writing of data proceeds from the position in the
file indicated by the file offset associated with fildes.
Before successful return from write(), the file offset is
incremented by the number of bytes actually written. On a
regular file, if this incremented file offset is greater
than the length of the file, the length of the file will be
set to this file offset.
If the OSYNC bit has been set, write I/O operations on the
file descriptor complete as defined by synchronized I/O file
integrity completion.
If fildes refers to a socket, write() is equivalent to
send(3SOCKET) with no flags set.
On a file not capable of seeking, writing always takes place
starting at the current position. The value of a file
offset associated with such a device is undefined.
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System Calls write(2)
If the OAPEND flag of the file status flags is set, the
file offset will be set to the end of the file prior to each
write and no intervening file modification operation will
occur between changing the file offset and the write opera-
tion.
For regular files, no data transfer will occur past the
offset maximum established in the open file description with
fildes.
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:
o If ONDELAY or ONONBLOCK is set, write() returns
-1 and sets errno to EAGAIN.
o 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 pro-
cess 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).
If write() is interrupted by a signal before it writes any
data, it will return -1 with errno set to EINTR.
If write() is interrupted by a signal after it successfully
writes some data, it will return the number of bytes writ-
ten.
If write() exceeds the process file size limit, the applica-
tion generates a SIGXFSZ signal, whose default behavior is
to dump core.
After a write() to a regular file has successfully returned:
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System Calls write(2)
o Any successful read(2) from each byte position in
the file that was modified by that write will
return the data specified by the write() for that
position until such byte positions are again modi-
fied.
o Any subsequent successful write() to the same byte
position in the file will overwrite that file data.
Write requests to a pipe or FIFO are handled the same as a
regular file with the following exceptions:
o There is no file offset associated with a pipe,
hence each write request appends to the end of the
pipe.
o 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.
o If ONONBLOCK and ONDELAY are clear, a write
request may cause the process to block, but on nor-
mal completion it returns nbyte.
o If ONONBLOCK and ONDELAY are set, write() does
not block the process. If a write() request for
PIPEBUF or fewer bytes succeeds completely
write() returns nbyte. Otherwise, if ONONBLOCK is
set, it returns -1 and sets errno to EAGAIN or if
ONDELAY is set, it returns 0. A write() request
for greater than {PIPEBUF} bytes transfers what
it can and returns the number of bytes written or
it transfers no data and, if ONONBLOCK is set,
returns -1 with errno set to EAGAIN or if ONDELAY
is set, it returns 0. Finally, 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, a FIFO, a socket, or a STREAM) that supports nonblock-
ing writes and cannot accept the data immediately:
o If ONONBLOCK and ONDELAY are clear, write()
blocks until the data can be accepted.
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System Calls write(2)
o 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.
Upon successful completion, where nbyte is greater than 0,
write() will mark for update the stctime and stmtime
fields of the file, and if the file is a regular file, the
SISUID and SISGID bits of the file mode may be cleared.
For STREAMS files (see Intro(2) and streamio(7I)), 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, and 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(7I)).
When writing to a STREAM, data messages are created with a
priority band of zero. When writing to a socket or to a
STREAM that is not a pipe or a FIFO:
o 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.
o If ONDELAY or ONONBLOCK is set and the STREAM
cannot accept data, write() returns -1 and sets
errno to EAGAIN.
o 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
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System Calls write(2)
data, write() terminates and returns the number of
bytes written.
The write() and writev() functions will fail if the STREAM
head had processed an asynchronous error before the call.
In this case, the value of errno does not reflect the result
of write() or writev() but reflects the prior error.
pwrite()
The pwrite() function is equivalent to write(), except that
it writes into a given position and does not change the file
offset (regardless of whether OAPEND is set). The first
three arguments to pwrite() are the same as write(), with
the addition of a fourth argument offset for the desired
position inside the file.
writev()
The writev() function performs the same action as write(),
but gathers the output data from the iovcnt buffers speci-
fied by the members of the iov array: iov[0], iov[1], ...,
iov[iovcnt-1]. The iovcnt buffer is valid if greater than 0
and less than or equal to {IOVMAX}. See Intro(2) for a
definition of {IOVMAX}.
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. The wri-
tev() function always writes all data from an area before
proceeding to the next.
If fildes refers to a regular file and all of the iovlen
members in the array pointed to by iov are 0, writev() will
return 0 and have no other effect. For other file types,
the behavior is unspecified.
If the sum of the iovlen values is greater than SIZEMAX,
the operation fails and no data is transferred.
RETURN VALUES
Upon successful completion, write() returns the number of
bytes actually written to the file associated with fildes.
This number is never greater than nbyte. Otherwise, -1 is
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System Calls write(2)
returned, the file-pointer remains unchanged, and errno is
set to indicate the error.
Upon successful completion, writev() returns the number of
bytes actually written. Otherwise, it returns -1, the
file-pointer remains unchanged, and errno is set to indicate
an error.
ERORS
The write(), pwrite(), and writev() functions will fail if:
EAGAIN Mandatory file/record locking is set, ONDELAY or
ONONBLOCK is set, and there is a blocking record
lock; an attempt is made to write to a STREAM
that can not accept data with the ONDELAY or
ONONBLOCK flag set; or a write to a pipe or FIFO
of PIPEBUF bytes or less is requested and less
than nbytes of free space is available.
EBADF The fildes argument is not a valid file descrip-
tor open for writing.
EDEADLK The write was going to go to sleep and cause a
deadlock situation to occur.
EDQUOT The user's quota of disk blocks on the file sys-
tem containing the file has been exhausted.
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)).
EFBIG The file is a regular file, nbyte is greater than
0, and the starting position is greater than or
equal to the offset maximum established in the
file description associated with fildes.
EINTR A signal was caught during the write operation
and no data was transferred.
EIO The process is in the background and is attempt-
ing to write to its controlling terminal whose
TOSTOP flag is set, or the process is neither
ignoring nor blocking SIGTOU signals and the
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System Calls write(2)
process group of the process is orphaned.
ENOLCK Enforced record locking was enabled and
{LOCKMAX} regions are already locked in the
system, or the system record lock table was full
and the write could not go to sleep until the
blocking record lock was removed.
ENOLINK The fildes argument 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.
ENOSR An attempt is made to write to a STREAMS with
insufficient STREAMS memory resources available
in the system.
ENXIO A hangup occurred on the STREAM being written to.
EPIPE An attempt is made to write to a pipe or a FIFO
that is not open for reading by any process, or
that has only one end open (or to a file descrip-
tor created by socket(3SOCKET), using type
SOCKSTREAM that is no longer connected to a peer
endpoint). A SIGPIPE signal will also be sent to
the thread. The process dies unless special pro-
visions were taken to catch or ignore the signal.
ERANGE The transfer request size was outside the range
supported by the STREAMS file associated with
fildes.
The write() and pwrite() functions will fail if:
EFAULT The buf argument points to an illegal address.
EINVAL The nbyte argument overflowed an ssizet.
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System Calls write(2)
The pwrite() function fails and the file pointer remains
unchanged if:
ESPIPE The fildes argument is associated with a pipe or
FIFO.
The write() and writev() functions may fail if:
EINVAL The STREAM or multiplexer referenced by fildes is
linked (directly or indirectly) downstream from a
multiplexer.
ENXIO A request was made of a non-existent device, or
the request was outside the capabilities of the
device.
ENXIO A hangup occurred on the STREAM being written to.
A write to a STREAMS file may 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.
The writev() function may fail if:
EINVAL The iovcnt argument was less than or equal to 0 or
greater than {IOVMAX}; one of the iovlen values
in the iov array was negative; or the sum of the
iovlen values in the iov array overflowed an
ssizet.
USAGE
The pwrite() function has a transitional interface for 64-
bit file offsets. See lf64(5).
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
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System Calls write(2)
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Committed
MT-Level write() is Async-Signal-Safe
Standard See standards(5).
SEE ALSO
Intro(2), chmod(2), creat(2), dup(2), fcntl(2),
getrlimit(2), ioctl(2), lseek(2), open(2), pipe(2),
ulimit(2), send(3SOCKET), socket(3SOCKET), attributes(5),
lf64(5), standards(5), streamio(7I)
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