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 correspond-
       ing 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 unwrit-
       ten data for that stream to be written to the file, and the last data modification 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 conditions.

       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() function 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("
New 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_memstream(), 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 Technol-
       ogy  --	Portable  Operating  System  Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of
       Electrical 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 Stan-
       dard 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE
/The Open Group						       2013								FFLUSH(3P)