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READ(2)			   Linux Programmer's Manual		       READ(2)

       read - read from a file descriptor

       #include <unistd.h>

       ssize_t read(int fd, void *buf, size_t count);

       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.

       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.

       EAGAIN The  file descriptor fd refers to a file other than a socket and
	      has been marked non-blocking (O_NONBLOCK), and  the  read	 would

	      The  file	 descriptor  fd refers to a socket and has been marked
	      non-blocking   (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 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 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  fur‐
	      ther 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

       SVr4, 4.3BSD, POSIX.1-2001.

       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.

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

       This  page  is  part of release 3.22 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2009-02-23			       READ(2)

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