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RedHat 9 (Linux i386) - man page for write (redhat section 2)

WRITE(2)						     Linux Programmer's Manual							  WRITE(2)

NAME
write - write to a file descriptor
SYNOPSIS
#include <unistd.h> ssize_t write(int fd, const void *buf, size_t count);
DESCRIPTION
write writes up to count bytes to the file referenced by the file descriptor fd from the buffer starting at buf. POSIX requires that a read() which can be proved to occur after a write() has returned returns the new data. Note that not all file systems are POSIX conform- ing.
RETURN VALUE
On success, the number of bytes written are returned (zero indicates nothing was written). On error, -1 is returned, and errno is set appropriately. If count is zero and the file descriptor refers to a regular file, 0 will be returned without causing any other effect. For a special file, the results are not portable.
ERRORS
EBADF fd is not a valid file descriptor or is not open for writing. EINVAL fd is attached to an object which is unsuitable 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' file size limit, or to write at a position past than the maximum allowed offset. 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.) EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and the write would block. EINTR The call was interrupted by a signal before any data was written. ENOSPC The device containing the file referred to by fd has no room for the data. EIO A low-level I/O error occurred while modifying the inode. Other errors may occur, depending on the object connected to fd.
CONFORMING TO
SVr4, SVID, POSIX, X/OPEN, 4.3BSD. SVr4 documents additional error conditions EDEADLK, ENOLCK, ENOLNK, ENOSR, ENXIO, or ERANGE. Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.
NOTES
A successful return from write does not make any guarantee that data has been committed to disk. In fact, on some buggy implementations, it does not even guarantee that space has successfully been reserved for the data. The only way to be sure is to call fsync(2) after you are done writing all your data.
SEE ALSO
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), read(2), select(2), fwrite(3), writev(3) Linux 2.0.32 2001-12-13 WRITE(2)