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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_NON-
              BLOCK), 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 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-02-23                           READ(2)