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.

       On  files  that  support  seeking,  the read operation commences at the current file offset, and the file
       offset is incremented by the number of bytes read.  If the current file offset is at or past the  end  of
       file, no bytes are read, and read() returns zero.

       If  count  is  zero,  read()  may detect the errors described below.  In the absence of any errors, or if
       read() does not check for errors, a read() with a count of 0 returns zero and has no other effects.

       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 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 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 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  terminal,  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  filesystems,  reading  small  amounts  of  data  will  update the timestamp only 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.

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 3.54 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                                              2013-02-12                                            READ(2)