• 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; 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 3.19 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/.
  

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