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 read‐
       ing 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 non‐
              blocking (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  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 per‐
       formance.

       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 3.44 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                              2009-02-23                            READ(2)