read

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



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  pro-
              cess  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  satis-
       fied 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
       close(2),  fcntl(2),  ioctl(2),  lseek(2),   readdir(2),   readlink(2),
       select(2), write(2), fread(3), readv(3)



Linux 2.0.32                      1997-07-12                           READ(2)