FFLUSH(3)                  POSIX Programmer's Manual                 FFLUSH(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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.1-2017
       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
       modification and last file status change timestamps of the underlying
       file shall be marked for update.

       For a stream open for reading with an underlying file description, 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).

       If stream is a null pointer, fflush() shall perform this flushing
       action on all streams for which the behavior is defined above.

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("\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__memstream(), ulimit()

       The Base Definitions volume of POSIX.1-2017, <stdio.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  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.opengroup.org/unix/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                  2017                            FFLUSH(3)
