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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)


       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


       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

       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 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.


       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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