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)

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

       EINTR  The call was interrupted by a signal before any data was read.

       EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.

       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.

       EBADF  fd is not a valid file descriptor or is not open for reading.

       EINVAL fd is attached to an object which is unsuitable for reading.

       EFAULT buf is outside your accessible address space.

       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, SVID, AT&T, POSIX, X/OPEN, BSD 4.3

RESTRICTIONS

       On NFS file systems, reading small amounts of data will only update the time stamp 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 atime updates to the server and client side reads satis-
       fied from the client's cache will not cause 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  filesystems  and disks were considered to be fast enough that the implementation of O_NONBLOCK was deemed unneccesary. So, O_NONBLOCK
       may not be available on files and/or disks.

SEE ALSO

       close(2), fcntl(2), ioctl(2), lseek(2), readdir(2), readlink(2), select(2), write(2), fread(3), readv(3)

Linux 2.0.32							    1997-07-12								   READ(2)