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.