Ubuntu Manpage: read - read from a file descriptor

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.

       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  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
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.

CONFORMING TO

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       The  types  size_t  and  ssize_t are, respectively, unsigned and signed integer data types
       specified by POSIX.1.

       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,
       on  Linux  before version 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 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.

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.