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FFLUSH(3P)                 POSIX Programmer's Manual                FFLUSH(3P)
PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.
NAME
       fflush -- flush a stream
SYNOPSIS
       #include <stdio.h>
       int fflush(FILE *stream);
DESCRIPTION
       The functionality described on this reference page is aligned with  the
       ISO C  standard.  Any  conflict between the requirements described here
       and the ISO C standard is unintentional. This  volume  of  POSIX.1-2008
       defers to the ISO C standard.
       If  stream  points to an output stream or an update stream in which the
       most recent operation was not input, fflush() shall cause any unwritten
       data for that stream to be written to the file, and the last data modi-
       fication and last file status change timestamps of the underlying  file
       shall be marked for update.
       If  stream  is  a  null  pointer,  fflush() shall perform this flushing
       action on all streams for which the behavior is defined above.
       For a stream open for reading, if the file is not already at  EOF,  and
       the  file  is one capable of seeking, the file offset of the underlying
       open file description shall be set to the file position of the  stream,
       and any characters pushed back onto the stream by ungetc() or ungetwc()
       that have not subsequently been read from the stream shall be discarded
       (without further changing the file offset).
RETURN VALUE
       Upon  successful  completion,  fflush()  shall  return 0; otherwise, it
       shall set the error indicator for the stream, return EOF, and set errno
       to indicate the error.
ERRORS
       The fflush() function shall fail if:
       EAGAIN The  O_NONBLOCK  flag  is set for the file descriptor underlying
              stream and the thread would be delayed in the write operation.
       EBADF  The file descriptor underlying stream is not valid.
       EFBIG  An attempt was made to write a file  that  exceeds  the  maximum
              file size.
       EFBIG  An  attempt  was made to write a file that exceeds the file size
              limit of the process.
       EFBIG  The file is a regular file and an attempt was made to  write  at
              or  beyond  the offset maximum associated with the corresponding
              stream.
       EINTR  The fflush() function was interrupted by a signal.
       EIO    The process is a member of a background process group attempting
              to write to its controlling terminal, TOSTOP is set, the calling
              thread is not blocking SIGTTOU,  the  process  is  not  ignoring
              SIGTTOU, and the process group of the process is orphaned.  This
              error may also be returned under  implementation-defined  condi-
              tions.
       ENOMEM The   underlying  stream  was  created  by  open_memstream()  or
              open_wmemstream() and insufficient memory is available.
       ENOSPC There was no free space remaining on the device  containing  the
              file or in the buffer used by the fmemopen() function.
       EPIPE  An  attempt  is made to write to a pipe or FIFO that is not open
              for reading by any process. A SIGPIPE signal shall also be  sent
              to the thread.
       The fflush() function may fail if:
       ENXIO  A  request  was made of a nonexistent device, or the request was
              outside the capabilities of the device.
       The following sections are informative.
EXAMPLES
   Sending Prompts to Standard Output
       The following example uses printf() calls to print a series of  prompts
       for  information  the user must enter from standard input. The fflush()
       calls force the output to standard output.  The  fflush()  function  is
       used because standard output is usually buffered and the prompt may not
       immediately be printed on the output or terminal. The  getline()  func-
       tion  calls  read  strings from standard input and place the results in
       variables, for use later in the program.
           char *user;
           char *oldpasswd;
           char *newpasswd;
           ssize_t llen;
           size_t blen;
           struct termios term;
           tcflag_t saveflag;
           printf("User name: ");
           fflush(stdout);
           blen = 0;
           llen = getline(&user, &blen, stdin);
           user[llen-1] = 0;
           tcgetattr(fileno(stdin), &term);
           saveflag = term.c_lflag;
           term.c_lflag &= ~ECHO;
           tcsetattr(fileno(stdin), TCSANOW, &term);
           printf("Old password: ");
           fflush(stdout);
           blen = 0;
           llen = getline(&oldpasswd, &blen, stdin);
           oldpasswd[llen-1] = 0;
           printf("\nNew password: ");
           fflush(stdout);
           blen = 0;
           llen = getline(&newpasswd, &blen, stdin);
           newpasswd[llen-1] = 0;
           term.c_lflag = saveflag;
           tcsetattr(fileno(stdin), TCSANOW, &term);
           free(user);
           free(oldpasswd);
           free(newpasswd);
APPLICATION USAGE
       None.
RATIONALE
       Data buffered by the system may make determining the  validity  of  the
       position  of  the  current file descriptor impractical. Thus, enforcing
       the repositioning of the file descriptor after fflush() on streams open
       for read() is not mandated by POSIX.1-2008.
FUTURE DIRECTIONS
       None.
SEE ALSO
       Section  2.5,  Standard I/O Streams, fmemopen(), getrlimit(), open_mem-
       stream(), ulimit()
       The Base Definitions volume of POSIX.1-2008, <stdio.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
       The  Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be  obtained  online
       at http://www.unix.org/online.html .
       Any  typographical  or  formatting  errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files  to  man page format. To report such errors, see https://www.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                           FFLUSH(3P)