WRITE

NAME

SYNOPSIS

ssize_t write(int fd, const void *buf, size_t count);  

DESCRIPTION

The number of bytes written may be less than count if, for example, there is insufficient space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered (see setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. (See also pipe(7).)

For a seekable file (i.e., one to which lseek(2) may be applied, for example, a regular file) writing takes place at the current file offset, and the file offset is incremented by the number of bytes actually written. If the file was open(2)ed with O_APPEND, the file offset is first set to the end of the file before writing. The adjustment of the file offset and the write operation are performed as an atomic step.

POSIX requires that a read(2) which can be proved to occur after a write() has returned returns the new data. Note that not all file systems are POSIX conforming.  

RETURN VALUE

If count is zero and fd refers to a regular file, then write() may return a failure status if one of the errors below is detected. If no errors are detected, 0 will be returned without causing any other effect. If count is zero and fd refers to a file other than a regular file, the results are not specified.  

ERRORS

EAGAIN

The file descriptor fd refers to a file other than a socket and has been marked non-blocking (O_NONBLOCK), and the write would block.

EAGAIN or EWOULDBLOCK

The file descriptor fd refers to a socket and has been marked non-blocking (O_NONBLOCK), and the write would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.

EBADF

fd is not a valid file descriptor or is not open for writing.

EFAULT

buf is outside your accessible address space.

EFBIG

An attempt was made to write a file that exceeds the implementation-defined maximum file size or the process's file size limit, or to write at a position past the maximum allowed offset.

EINTR

The call was interrupted by a signal before any data was written; see signal(7).

EINVAL

fd is attached to an object which is unsuitable for writing; or the file was opened with the O_DIRECT flag, and either the address specified in buf, the value specified in count, or the current file offset is not suitably aligned.

EIO

A low-level I/O error occurred while modifying the inode.

ENOSPC

The device containing the file referred to by fd has no room for the data.

EPIPE

fd is connected to a pipe or socket whose reading end is closed. When this happens the writing process will also receive a SIGPIPE signal. (Thus, the write return value is seen only if the program catches, blocks or ignores this signal.)

Other errors may occur, depending on the object connected to fd.  

CONFORMING TO

Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.  

NOTES

If a write() is interrupted by a signal handler before any bytes are written, then the call fails with the error EINTR; if it is interrupted after at least one byte has been written, the call succeeds, and returns the number of bytes written.  

SEE ALSO

COLOPHON