Ubuntu Manpage: read - read from a file descriptor

NAME

       read - read from a file descriptor

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

       ssize_t read(int fd, void buf[.count], size_t count);

DESCRIPTION

       read() attempts to read up to count bytes from file descriptor fd into the buffer starting
       at buf.

       On files that support seeking, the read operation commences at the file  offset,  and  the
       file  offset is incremented by the number of bytes read.  If the file offset is at or past
       the end of file, no bytes are read, and read() returns zero.

       If count is zero, read() may detect the errors described below.  In  the  absence  of  any
       errors,  or  if  read() does not check for errors, a read() with a count of 0 returns zero
       and has no other effects.

       According to POSIX.1, if count is greater than SSIZE_MAX, the  result  is  implementation-
       defined; see NOTES for the upper limit on Linux.

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.  See also NOTES.

       On error, -1 is returned, and errno is set to indicate the error.  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. See open(2) for further
details on the O_NONBLOCK flag.

EAGAIN or EWOULDBLOCK
The file descriptor fd refers to a socket and has been marked nonblocking
(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 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 terminal, 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. A further
possible cause of EIO on networked filesystems is when an advisory lock had been
taken out on the file descriptor and this lock has been lost. See the Lost locks
section of fcntl(2) for further details.

EISDIR fd refers to a directory.

Other errors may occur, depending on the object connected to fd.

STANDARDS

       POSIX.1-2008.

HISTORY

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       On   Linux,   read()   (and  similar  system  calls)  will  transfer  at  most  0x7ffff000
       (2,147,479,552) bytes, returning the number of bytes actually transferred.  (This is  true
       on both 32-bit and 64-bit systems.)

       On NFS filesystems, reading small amounts of data will update the timestamp only 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.

BUGS

       According  to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions with Regular File
       Operations"):

           All of the following functions shall be atomic with  respect  to  each  other  in  the
           effects  specified  in  POSIX.1-2008  when  they  operate on regular files or symbolic
           links: ...

       Among the APIs subsequently listed are read() and readv(2).  And among  the  effects  that
       should  be atomic across threads (and processes) are updates of the file offset.  However,
       before Linux 3.14, this was not the case:  if  two  processes  that  share  an  open  file
       description  (see  open(2))  perform a read() (or readv(2)) at the same time, then the I/O
       operations were not atomic with respect to updating the file offset, with the result  that
       the reads in the two processes might (incorrectly) overlap in the blocks of data that they
       obtained.  This problem was fixed in Linux 3.14.