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

       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.

       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.

       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 pro-
	      cess 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.

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  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 filesystems 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)



Linux 2.0.32			  1997-07-12			       READ(2)
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