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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 Non-blocking  I/O has been selected using O_NONBLOCK and no data
              was immediately available for reading.

       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 performance.

       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 3.15 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/.