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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
       EAGAIN The file descriptor fd refers to a file other than a  socket  and  has  been
              marked non-blocking (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  sig-
              nal(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 performance.

       Many file systems and disks were considered to be fast enough that the  implementa-
       tion  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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-02-23                           READ(2)

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