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.1‐2017 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         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 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         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.1‐2017, stdio.h(0p)

COPYRIGHT         top

       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(3P)

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