View Source: read(2) - Waikato Linux Users Group

Edit PageHistory Diff Info LikePages

!!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 available 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(2) 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

!!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 satisfied 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.

!!SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), readdir(2), readlink(2), select(2), write(2), fread(3)

62 pages link to read(2):

This page is a man page (or other imported legacy content). We are unable to automatically determine the license status of this page.

Last edited on Sunday, March 16, 2003 8:28:04 pm by PerryLorier

Edit PageHistory Diff Info LikePages