read(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

READ(2)                 Linux Programmer's Manual                READ(2)

NAME         top

       read - read from a file descriptor

SYNOPSIS         top

       #include <unistd.h>

       ssize_t read(int fd, void *buf, size_t count);

DESCRIPTION         top

       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         top

       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         top

       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         top

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES         top

       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         top

       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.

SEE ALSO         top

       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         top

       This page is part of release 5.12 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                          2021-03-22                        READ(2)

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