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       read - read from a file descriptor


       #include <unistd.h>

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


       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  current  file  offset,
       and  the  file  offset  is  incremented  by the number of bytes read.  If the current 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.


       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.


       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.

              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 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.


       SVr4, 4.3BSD, POSIX.1-2001.


       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.


       According  to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions with Regular File

           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.


       close(2),  fcntl(2),  ioctl(2),  lseek(2),  open(2),  pread(2),  readdir(2),  readlink(2),
       readv(2), select(2), write(2), fread(3)


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