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.

   If count is greater than SSIZE_MAX, the result is unspecified.

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.

   EISDIR fd refers to a directory.

   Other errors may occur, depending on the object connected to fd.  POSIX
   allows a read() that is interrupted after reading some data  to  return
   -1  (with  errno set to EINTR) or to return the number of bytes already
   read.

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 4.09 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/.

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