NAME

write, pwrite, writev – write on a file

SYNOPSIS

#include <unistd.h>

ssize_t write(int fildes , const void *buf , size_t nbyte );

DESCRIPTION

write() attempts to write nbyte bytes from the buffer pointed to by buf to the file descriptor specified by fildes . If nbyte is zero and the file is a regular file, write() returns zero and has no other results.

On devices capable of seeking, the actual writing of data starts at 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.

If the O_APPEND flag of the file status flags is set, the file pointer is set to the end of the file prior to each write(). . The system guarantees that no intervening file modification operation will occur between changing the file offset and the write operation.

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()), 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.

If a write() requests that more bytes be written than there is room for(emfor example, if the write would exceed the process file size limit , the system file size limit, or the free space on the device(emonly 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.

When attempting to write to a file descriptor that supports nonblocking writes and 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 thread. 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.

RETURN VALUES

On success, write() returns the number of bytes actually written. Otherwise, it returns -1 and sets errno to indicate the error.

ERRORS

This function is a member of Unison’s IOLIB family of functions. IOLIB is implemented as a message passing and generalized interface layer. Each Unison I/O server is responsible for its own error reporting.

For an exact list of error codes returned by a particular server, refer to that server’s documentation in the Unison Programmer’s Guide for each specific platform.

Servers may implement these errors codes in response to this function.

write() fails and the file pointer remains unchanged if one or more of the following are true:

EAGAIN

Mandatory file/record locking is set, O_NDELAY or O_NONBLOCK is set, and there is a blocking record lock.
Total amount of system memory available when reading using raw I/O is temporarily insufficient.
An attempt is made to write to a stream that can not accept data with the O_NDELAY or O_NONBLOCK flag set.
If a write() to a pipe or FIFOof {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 situation to occur.

EFAULT

buf points to an illegal address.

EFBIG

An attempt is made to write a file that exceeds the thread’s file size limit or the maximum file size

EINVAL

An attempt is made to write to a stream linked below a multiplexor.

ENOLCK

Enforced record locking was enabled and {LOCK_MAX} regions are already locked in the system.
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.

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.

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.

Upon successful completion write() marks for update the st_ctime and st_mtime fields of the file.

SEE ALSO

chmod(), ioctl(), lseek(), open()