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 hap­
       pen 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.

       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  sub­
       stantially  increase server load and decrease performance.


SEE ALSO

       close(2), fcntl(2), ioctl(2), lseek(2), readdir(2),  read­
       link(2), select(2), write(2), fread(3)