write(2) write(2)
NAME [Toc] [Back]
write, writev, pwrite - write on a file
SYNOPSIS [Toc] [Back]
#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 [Toc] [Back]
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.
For ordinary files, if the O_DSYNC file status flag is set, the write
does not return until both the file data and the file attributes
required to retrieve the data are physically updated. If the O_SYNC
flag is set, the behavior is identical to that of O_DSYNC, with the
addition that all file attributes changed by the write operation,
including access time, modification time and status change time, are
also physically updated before returning to the calling process.
For block special files, if the O_DSYNC or the O_SYNC flag is set, the
write does not return until the data is physically updated. How the
data reaches the physical media is implementation and hardware
dependent.
A write to an ordinary file is prevented if enforcement-mode file and
record locking is set, another process owns a lock on the segment of
the file being written, and the following apply:
+ If O_NDELAY or O_NONBLOCK is set, the write returns -1 and
sets errno to [EAGAIN].
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: August 2003
write(2) write(2)
+ If O_NDELAY and O_NONBLOCK are clear, the write does not
complete until the blocking record lock is removed.
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 operation.
If a write() requests that more bytes be written than there is room
for, for example, the ulimit or the physical end of a medium, 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 will return 20. The next write of a non-zero
number of bytes will give a failure return (except as noted below) and
the implementation will generate a SIGXFSZ signal for the process.
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 written. If the value of nbyte is greater than
{SSIZE_MAX}, the result is implementation dependent.
After a write() to a regular file has successfully returned:
+ Any successful read() 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.
+ Any subsequent successful write() to the same byte position in
the file will overwrite that file data.
Write requests to a pipe or FIFO will be handled the same as a regular
file with the following exceptions:
+ There is no file offset associated with a pipe, hence each
write request will append to the end of the pipe.
+ The system-dependent maximum number of bytes that a pipe or
FIFO can store is PIPSIZ as defined in <sys/inode.h>.
+ The minimum value of PIPSIZ on any HP-UX system is 8192.
When writing a pipe with the O_NDELAY or O_NONBLOCK file status flag
set, the following apply:
+ If nbyte is less than or equal to PIPSIZ and sufficient room
exists in the pipe or FIFO, the write() succeeds and returns
the number of bytes written.
Hewlett-Packard Company - 2 - HP-UX 11i Version 2: August 2003
write(2) write(2)
+ If nbyte is less than or equal to PIPSIZ but insufficient room
exists in the pipe or FIFO, the write() returns having written
nothing. If O_NONBLOCK is set, -1 is returned and errno is
set to [EAGAIN]. If O_NDELAY is set, 0 is returned.
+ If nbyte is greater than PIPSIZ and the pipe or FIFO is full,
the write returns having written nothing. If O_NONBLOCK is
set, -1 is returned and errno is set to [EAGAIN]. If O_NDELAY
is set, 0 is returned.
+ If nbyte is greater than PIPSIZ, and some room exists in the
pipe or FIFO, as much data as fits in the pipe or FIFO is
written, and write() returns the number of bytes actually
written, an amount less than the number of bytes requested.
When writing a pipe and the O_NDELAY and O_NONBLOCK file status flags
are clear, the write() always executes correctly (blocking as
necessary), and returns the number of bytes written.
When attempting to write to a file descriptor (other than a pipe or
FIFO) that supports non-blocking writes and cannot accept the data
immediately, the following apply:
+ If the O_NONBLOCK flag is clear, write() will block until the
data can be accepted.
+ If the O_NONBLOCK flag is set, write() will not block the
process. If some data can be written without blocking the
process, write() will write what it can and return the number
of bytes written. Otherwise, it will return -1 and errno will
be set to [EAGAIN].
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 character special devices, if the stopio() call was used on the
same device after it was opened, write() returns -1, sets errno to
[EBADF], and issues the SIGHUP signal to the process. write() also
clears the potential and granted privilege vectors on the file.
If fildes refers to a STREAM, 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 determined by the topmost
STREAM module. If nbyte falls within the packet size range, nbyte
bytes will be written. If nbyte does not fall within the range and the
minimum packet size value is 0, write() will break the buffer into
maximum packet size segments prior to sending the data downstream (the
last segment may contain less than the maximum packet size). If nbyte
does not fall within the range and the minimum value is non-zero,
Hewlett-Packard Company - 3 - HP-UX 11i Version 2: August 2003
write(2) write(2)
write() will fail with errno set to ERANGE. Writing a zero-length
buffer ( nbyte is 0) to a STREAMS device sends 0 bytes with 0
returned. However, writing a zero-length buffer to a STREAMS-based
pipe or FIFO sends no message and 0 is returned. The process may issue
I_SWROPT ioctl() to enable zero-length messages to be sent across the
pipe or FIFO.
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, the
following apply:
+ If O_NONBLOCK is clear, and the STREAM cannot accept data (the
STREAM write queue is full due to internal flow control
conditions), write() will block until data can be accepted.
+ If O_NONBLOCK is set and the STREAM cannot accept data,
write() will return -1 and set errno to [EAGAIN].
+ If O_NONBLOCK is set and part of the buffer has been written
while a condition in which the STREAM cannot accept additional
data occurs, write() will terminate and return the number of
bytes written.
In addition, write() and writev() 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.
If the write is performed by any user other than the owner or a user
who has appropriate privileges, write() clears the set-user-ID, setgroup-ID,
and sticky bits on all nondirectory files. If the write is
performed by the owner or a user who has appropriate privileges, the
behavior is file-system dependent. In some file systems, the write
clears the set-user-ID, set-group-ID, and sticky bits on a
nondirectory file. In other file systems, the write does not clear
these bits on a nondirectory file.
For directories, write() does not clear the set-user-ID, set-group-ID,
and sticky bits.
The writev() function is equivalent to 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]. iovcnt is valid if greater
than 0 and less than or equal to {IOV_MAX}, as defined in <limits.h>.
Each iovec entry specifies the base address and length of an area in
memory from which data should be written. The writev() function will
always write a complete area before proceeding to the next. The iovec
structure is defined in /usr/include/sys/uio.h.
Hewlett-Packard Company - 4 - HP-UX 11i Version 2: August 2003
write(2) write(2)
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.
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.
RETURN VALUE [Toc] [Back]
Upon successful completion, write() and pwrite() will return the
number of bytes actually written to the file associated with fildes.
This number will never be greater than nbyte. Otherwise, -1 is
returned and errno is set to indicate the error.
Upon successful completion, writev() returns the number of bytes
actually written. Otherwise, it returns a value of -1, the file
pointer remains unchanged, and errno is set to indicate an error.
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.
ERRORS [Toc] [Back]
Under the following conditions, write(), pwrite() and writev() fail
and set errno to:
[EAGAIN] The O_NONBLOCK flag was set for the file
descriptor and the process was delayed in the
write() operation.
[EAGAIN] Enforcement-mode file and record locking was set,
O_NDELAY was set, and there was a blocking record
lock.
[EBADF] The fildes argument was not a valid file
descriptor open for writing.
[EDEADLK] A resource deadlock would occur as a result of
this operation (see lockf(2) and fcntl(2)).
[EDQUOT] User's disk quota block limit has been reached for
this file system.
[EFBIG] An attempt was made to write a file that exceeds
the implementation-dependent maximum file size or
the process' file size limit.
Hewlett-Packard Company - 5 - HP-UX 11i Version 2: August 2003
write(2) write(2)
[EFBIG] The file is a regular file and nbyte is greater
than zero and the starting position is greater
than or equal to the offset maximum established in
the open file description associated with fildes.
[EINTR] The write operation was terminated due to the
receipt of a signal, and no data was transferred.
[EINVAL] The STREAM or multiplexer referenced by fildes is
linked (directly or indirectly) downstream from a
multiplexer.
[EIO] A physical I/O error has occurred.
[EIO] The process is a member of a background process
group attempting to write to its controlling
terminal. TOSTOP is set, the process is neither
ignoring nor blocking SIGTTOU, and the process
group of the process is orphaned. This error may
also be returned under implementation-dependent
conditions.
[ENOLCK] The system record lock table is full, preventing
the write from sleeping until the blocking record
lock is removed.
[ENOSPC] Not enough space on the file system. The process
does not possess the limit effective privilege to
override this restriction.
[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.
[EPIPE] An attempt is made to write to a pipe or FIFO that
is not open for reading by any process, or that
only has one end open. A SIGPIPE signal will also
be sent to the process.
[ERANGE] The transfer request size was outside the range
supported by the STREAMS file associated with
fildes.
Under the following conditions, writev() fails and sets errno to:
[EFAULT] iov_base or iov points outside of the allocated
address space. The reliable detection of this
error is implementation dependent.
Hewlett-Packard Company - 6 - HP-UX 11i Version 2: August 2003
write(2) write(2)
[EINVAL] One of the iov_len values in the iov array is
negative.
[EINVAL] The sum of the iov_len values in the iov array
would overflow an ssize_t.
Under the following conditions, the writev() function may fail and set
errno to:
[EINVAL] The iovcnt argument was less than or equal to 0,
or greater than {IOV_MAX}.
Under the following conditions, the pwrite() function fails, the file
pointer remains unchanged and errno is set to:
[EINVAL] The offset argument is invalid, and the value is
negative.
[ESPIPE] The fildes argument is associated with a pipe or
FIFO.
Under the following conditions, write() or writev() fails, the file
offset is updated to reflect the amount of data transferred and errno
is set to:
[EFAULT] buf points outside the process's allocated address
space. The reliable detection of this error is
implementation dependent.
EXAMPLES [Toc] [Back]
Assuming a process opened a file for writing, the following call to
write() attempts to write mybufsize bytes to the file from the buffer
to which mybuf points.
#include <string.h>
int fildes;
size_t mybufsize;
ssize_t nbytes;
char *mybuf = "aeiou and sometimes y";
mybufsize = (size_t)strlen (mybuf);
nbytes = write (fildes, (void *)mybuf, mybufsize);
WARNINGS [Toc] [Back]
Check signal(5) for the appropriateness of signal references on
systems that support sigvector(). See the sigvector(2) manpage.
sigvector() can affect the behavior of the write(), writev() and
pwrite() functions described here.
Hewlett-Packard Company - 7 - HP-UX 11i Version 2: August 2003
write(2) write(2)
Character special devices, and raw disks in particular, apply
constraints on how write() can be used. See specific Section 7 manual
entries for details on particular devices.
AUTHOR [Toc] [Back]
write() was developed by HP, AT&T, and the University of California,
Berkeley.
SEE ALSO [Toc] [Back]
mkfs(1M), chmod(2), creat(2), dup(2), fcntl(2), getrlimit(2),
lockf(2), lseek(2), open(2), pipe(2), sigvector(2), ulimit(2),
ustat(2), signal(5), <limits.h>, <stropts.h>, <sys/uio.h>, <unistd.h>.
STANDARDS CONFORMANCE [Toc] [Back]
write(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1,
POSIX.4
Hewlett-Packard Company - 8 - HP-UX 11i Version 2: August 2003 [ Back ] |
