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)