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Linux 2.6 - man page for read (linux section 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.

       On files that support seeking, the read operation commences at the  current  file  offset,
       and  the file offset is incremented by the number of bytes read.  If the current file off-
       set is at or past the end of file, no bytes are read, and read() returns zero.

       If count is zero, read() may detect the errors described below.	In  the  absence  of  any
       errors,	or  if	read() does not check for errors, a read() with a count of 0 returns zero
       and has no other effects.

       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() was interrupted by a  sig-
       nal.   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_NON-
	      BLOCK), 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 application 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 speci-
	      fied 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 terminal, and either it is ignor-
	      ing  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 filesystems, reading small amounts of data will update the timestamp only 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.

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 3.55 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					    2013-02-12					  READ(2)


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