read

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



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 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.

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.

SEE ALSO
       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
       This page is part of release 5.13 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)