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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.

       If count is zero, read() returns zero and has no other results.  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.  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.

       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 current 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 tty, 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

       On  NFS  file  systems,  reading  small amounts of data will only update the timestamp 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.

       Many  file  systems and disks were considered to be fast enough that the implementation of
       O_NONBLOCK was deemed unnecessary.  So, O_NONBLOCK may not be available  on  files  and/or
       disks.

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 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.