Ubuntu Manpage: read - read from a file descriptor

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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.

       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  5.10  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/.

Linux                                              2018-02-02                                            READ(2)