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write(2) [redhat man page]

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)

Check Out this Related Man Page

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

NAME
read - read from a file descriptor SYNOPSIS
#include <unistd.h> ssize_t read(int fd, void *buf, size_t count); DESCRIPTION
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf. If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspecified. RETURN VALUE
On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually avail- able right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal. On error, -1 is returned, and errno is set appropriately. In this case it is left unspecified whether the file position (if any) changes. ERRORS
EAGAIN The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and the read would block. EAGAIN or EWOULDBLOCK The file descriptor fd refers to a socket and has been marked nonblocking (O_NONBLOCK), and the read 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 appli- cation should check for both possibilities. EBADF fd is not a valid file descriptor or is not open for reading. EFAULT buf is outside your accessible address space. EINTR The call was interrupted by a signal before any data was read; see signal(7). EINVAL fd is attached to an object which is unsuitable for reading; 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. EINVAL fd was created via a call to timerfd_create(2) and the wrong size buffer was given to read(); see timerfd_create(2) for further information. EIO I/O error. This will happen for example when the process is in a background process group, tries to read from its controlling tty, and either it is ignoring or blocking SIGTTIN or its process group is orphaned. It may also occur when there is a low-level I/O error while reading from a disk or tape. EISDIR fd refers to a directory. Other errors may occur, depending on the object connected to fd. POSIX allows a read() that is interrupted after reading some data to return -1 (with errno set to EINTR) or to return the number of bytes already read. CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. NOTES
On NFS file systems, reading small amounts of data will only update the timestamp the first time, subsequent calls may not do so. This is caused by client side attribute caching, because most if not all NFS clients leave st_atime (last file access time) updates to the server and client side reads satisfied from the client's cache will not cause st_atime updates on the server as there are no server side reads. UNIX semantics can be obtained by disabling client side attribute caching, but in most situations this will substantially increase server load and decrease performance. Many file systems and disks were considered to be fast enough that the implementation of O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK may not be available on files and/or disks. SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2), readlink(2), readv(2), select(2), write(2), fread(3) COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2009-02-23 READ(2)

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