read(2)



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  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 fur-
              ther 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.

       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  actu-
       ally transferred.  (This is true on both 32-bit and 64-bit systems.)

       On NFS filesystems, reading small amounts of data will update the time-
       stamp 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 4.13 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                             2017-09-15                           READ(2)

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