fflush(3p) — Linux manual page


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

PROLOG         top

       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         top

       fflush — flush a stream

SYNOPSIS         top

       #include <stdio.h>

       int fflush(FILE *stream);

DESCRIPTION         top

       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 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         top

       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         top

       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

       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         top

   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: ");
           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: ");
           blen = 0;
           llen = getline(&oldpasswd, &blen, stdin);
           oldpasswd[llen-1] = 0;

           printf("\nNew password: ");
           blen = 0;
           llen = getline(&newpasswd, &blen, stdin);
           newpasswd[llen-1] = 0;
           term.c_lflag = saveflag;
           tcsetattr(fileno(stdin), TCSANOW, &term);



RATIONALE         top

       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.



SEE ALSO         top

       Section 2.5, Standard I/O Streams, fmemopen(3p), getrlimit(3p),
       open_memstream(3p), ulimit(3p)

       The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)

COPYRIGHT         top

       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 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 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                          FFLUSH(3P)

Pages that refer to this page: stdio.h(0p)freopen(3p)open_memstream(3p)open_wmemstream(3p)popen(3p)