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READ   (2) manpage
READ
2
1997-07-12
Linux 2.0.32
Linux Programmer's Manual
  • 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
      EINTR
      The call was interrupted by a signal before any data was read.
      EAGAIN
      Non-blocking I/O has been selected using
      O_NONBLOCK and no data was immediately available for reading.
      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.
      EBADF
      fd
       is not a valid file descriptor or is not open for reading.
      EINVAL
      fd
       is attached to an object which is unsuitable for reading.
      EFAULT
      buf
       is outside your accessible address space.
      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, SVID, AT&T, POSIX, X/OPEN, BSD 4.3
  • 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 atime updates to the server and client side reads satisfied from the
      client's cache will not cause 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 unneccesary. So, O_NONBLOCK may not be available on files
      and/or disks.
  • SEE ALSO
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