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READ(2)                             Linux Programmer's Manual                             READ(2)

       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 off-
       set 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  sig-
       nal.   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.

              The  file  descriptor fd refers to a socket and has been marked nonblocking (O_NON-
              BLOCK), 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 speci-
              fied 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 ignor-
              ing  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 NFS file systems, 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.

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

       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and    information    about    reporting    bugs,    can    be    found     at

Linux                                       2013-02-12                                    READ(2)

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