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System Calls write(2)
NAME
write, pwrite, writev - write on a file
SYNOPSIS
#include <unistd.h>
ssize_t write(int fildes, const void *buf, size_t nbyte);
ssize_t pwrite(int fildes, const void *buf, size_t nbyte,
off_t offset);
#include <sys/uio.h>
ssize_t 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 O_SYNC 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.
If the O_APPEND 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.
SunOS 5.10 Last change: 1 Nov 2003 1
System Calls write(2)
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 O_NDELAY or O_NONBLOCK is set, write() returns -1
and sets errno to EAGAIN.
o 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 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.
After a write() to a regular file has successfully returned:
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 modified.
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
SunOS 5.10 Last change: 1 Nov 2003 2
System Calls write(2)
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 inter-
leaved, on arbitrary boundaries, with writes by other
processes, whether or not the O_NONBLOCK or O_NDELAY
flags are set.
o If O_NONBLOCK and O_NDELAY are clear, a write request
may cause the process to block, but on normal comple-
tion it returns nbyte.
o If O_NONBLOCK and O_NDELAY are set, write() does not
block the process. If a write() request for PIPE_BUF or
fewer bytes succeeds completely write() returns nbyte.
Otherwise, if O_NONBLOCK is set, it returns -1 and sets
errno to EAGAIN or if O_NDELAY is set, it returns 0. A
write() request for greater than {PIPE_BUF} bytes
transfers what it can and returns the number of bytes
written or it transfers no data and, if O_NONBLOCK is
set, returns -1 with errno set to EAGAIN or if O_NDELAY
is set, it returns 0. Finally, 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, a FIFO, a socket, or a STREAM) that supports nonblock-
ing writes and cannot accept the data immediately:
o If O_NONBLOCK and O_NDELAY are clear, write() blocks
until the data can be accepted.
o 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.
Upon successful completion, where nbyte is greater than 0,
write() will mark for update the st_ctime and st_mtime
fields of the file, and if the file is a regular file, the
S_ISUID and S_ISGID 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
SunOS 5.10 Last change: 1 Nov 2003 3
System Calls write(2)
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 I_SWROPT 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 O_NDELAY and O_NONBLOCK 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 O_NDELAY or O_NONBLOCK is set and the STREAM cannot
accept data, write() returns -1 and sets errno to
EAGAIN.
o If O_NDELAY or O_NONBLOCK 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.
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 performs the same action as write(),
except that it writes into a given position without changing
the file pointer. 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 {IOV_MAX}. See intro(2) for a
SunOS 5.10 Last change: 1 Nov 2003 4
System Calls write(2)
definition of {IOV_MAX}.
The iovec structure contains the following members:
caddr_t iov_base;
int iov_len;
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 iov_len
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 iov_len values is greater than SSIZE_MAX,
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
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.
ERRORS
The write(), pwrite(), and writev() functions will fail if:
EAGAIN Mandatory file/record locking is set,
O_NDELAY or O_NONBLOCK 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 O_NDELAY or O_NONBLOCK flag
set; or 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 The fildes argument is not a valid file
descriptor open for writing.
SunOS 5.10 Last change: 1 Nov 2003 5
System Calls write(2)
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
system 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 esta-
blished in the file description associated
with fildes.
EINTR A signal was caught during the write opera-
tion and no data was transferred.
EIO The process is in the background and is
attempting to write to its controlling ter-
minal whose TOSTOP flag is set, or the pro-
cess is neither ignoring nor blocking
SIGTTOU signals and the process group of
the process is orphaned.
ENOLCK Enforced record locking was enabled and
{LOCK_MAX} 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.
SunOS 5.10 Last change: 1 Nov 2003 6
System Calls write(2)
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 writ-
ten 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 descriptor created by
socket(3SOCKET), using type SOCK_STREAM 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 associ-
ated 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 ssize_t.
The pwrite() function fails and the file pointer remains
unchanged if:
ESPIPE The fildes argument is associated with a
pipe or FIFO.
SunOS 5.10 Last change: 1 Nov 2003 7
System Calls write(2)
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 writ-
ten 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 {IOV_MAX}; one of the
iov_len values in the iov array was nega-
tive; or the sum of the iov_len values in
the iov array overflowed an ssize_t.
USAGE
The pwrite() function has a transitional interface for 64-
bit file offsets. See lf64(5).
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Standard |
|_____________________________|_____________________________|
| MT-Level | write() is Async-Signal-Safe|
|_____________________________|_____________________________|
SEE ALSO
SunOS 5.10 Last change: 1 Nov 2003 8
System Calls write(2)
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)
SunOS 5.10 Last change: 1 Nov 2003 9
Man(1) output converted with
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This page was generated on Wed Sep 12 11:27:06 GMT 2007
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