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



PPRROOLLOOGG
       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.

NNAAMMEE
       fflush -- flush a stream

SSYYNNOOPPSSIISS
       #include <stdio.h>

       int fflush(FILE *_s_t_r_e_a_m);

DDEESSCCRRIIPPTTIIOONN
       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 _s_t_r_e_a_m points to an output stream or an update stream in which the
       most recent operation was not input, _f_f_l_u_s_h() 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 _u_n_g_e_t_c() or _u_n_g_e_t_w_c() that have not subsequently been
       read from the stream shall be discarded (without further changing the
       file offset).

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

RREETTUURRNN VVAALLUUEE
       Upon successful completion, _f_f_l_u_s_h() shall return 0; otherwise, it
       shall set the error indicator for the stream, return EOF, and set _e_r_r_n_o
       to indicate the error.

EERRRROORRSS
       The _f_f_l_u_s_h() function shall fail if:

       EEAAGGAAIINN The O_NONBLOCK flag is set for the file descriptor underlying
              _s_t_r_e_a_m and the thread would be delayed in the write operation.

       EEBBAADDFF  The file descriptor underlying _s_t_r_e_a_m is not valid.

       EEFFBBIIGG  An attempt was made to write a file that exceeds the maximum
              file size.

       EEFFBBIIGG  An attempt was made to write a file that exceeds the file size
              limit of the process.

       EEFFBBIIGG  The file is a regular file and an attempt was made to write at
              or beyond the offset maximum associated with the corresponding
              stream.

       EEIINNTTRR  The _f_f_l_u_s_h() function was interrupted by a signal.

       EEIIOO    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.

       EENNOOMMEEMM The underlying stream was created by _o_p_e_n___m_e_m_s_t_r_e_a_m() or
              _o_p_e_n___w_m_e_m_s_t_r_e_a_m() and insufficient memory is available.

       EENNOOSSPPCC There was no free space remaining on the device containing the
              file or in the buffer used by the _f_m_e_m_o_p_e_n() function.

       EEPPIIPPEE  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 _f_f_l_u_s_h() function may fail if:

       EENNXXIIOO  A request was made of a nonexistent device, or the request was
              outside the capabilities of the device.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
   SSeennddiinngg PPrroommppttss ttoo SSttaannddaarrdd OOuuttppuutt
       The following example uses _p_r_i_n_t_f() calls to print a series of prompts
       for information the user must enter from standard input. The _f_f_l_u_s_h()
       calls force the output to standard output. The _f_f_l_u_s_h() function is
       used because standard output is usually buffered and the prompt may not
       immediately be printed on the output or terminal. The _g_e_t_l_i_n_e()
       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);

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       None.

RRAATTIIOONNAALLEE
       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 _f_f_l_u_s_h() on streams open
       for _r_e_a_d() is not mandated by POSIX.1-2008.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _S_e_c_t_i_o_n _2_._5, _S_t_a_n_d_a_r_d _I_/_O _S_t_r_e_a_m_s, _f_m_e_m_o_p_e_n(), _g_e_t_r_l_i_m_i_t(),
       _o_p_e_n___m_e_m_s_t_r_e_a_m(), _u_l_i_m_i_t()

       The Base Definitions volume of POSIX.1-2017, <<ssttddiioo..hh>>

CCOOPPYYRRIIGGHHTT
       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)
