1FFLUSH(3P)                 POSIX Programmer's Manual                FFLUSH(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
10

NAME

12       fflush — flush a stream
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SYNOPSIS

15       #include <stdio.h>
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17       int fflush(FILE *stream);
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DESCRIPTION

20       The functionality described on this reference page is aligned with  the
21       ISO C  standard.  Any  conflict between the requirements described here
22       and the ISO C standard is unintentional. This  volume  of  POSIX.1‐2017
23       defers to the ISO C standard.
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25       If  stream  points to an output stream or an update stream in which the
26       most recent operation was not input, fflush() shall cause any unwritten
27       data for that stream to be written to the file, and the last data modi‐
28       fication and last file status change timestamps of the underlying  file
29       shall be marked for update.
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31       For  a  stream open for reading with an underlying file description, if
32       the file is not already at EOF, and the file is one capable of seeking,
33       the file offset of the underlying open file description shall be set to
34       the file position of the stream, and any characters  pushed  back  onto
35       the  stream  by  ungetc()  or ungetwc() that have not subsequently been
36       read from the stream shall be discarded (without further  changing  the
37       file offset).
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39       If  stream  is  a  null  pointer,  fflush() shall perform this flushing
40       action on all streams for which the behavior is defined above.
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RETURN VALUE

43       Upon successful completion, fflush()  shall  return  0;  otherwise,  it
44       shall set the error indicator for the stream, return EOF, and set errno
45       to indicate the error.
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ERRORS

48       The fflush() function shall fail if:
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50       EAGAIN The O_NONBLOCK flag is set for the  file  descriptor  underlying
51              stream and the thread would be delayed in the write operation.
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53       EBADF  The file descriptor underlying stream is not valid.
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55       EFBIG  An  attempt  was  made  to write a file that exceeds the maximum
56              file size.
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58       EFBIG  An attempt was made to write a file that exceeds the  file  size
59              limit of the process.
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61       EFBIG  The  file  is a regular file and an attempt was made to write at
62              or beyond the offset maximum associated with  the  corresponding
63              stream.
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65       EINTR  The fflush() function was interrupted by a signal.
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67       EIO    The process is a member of a background process group attempting
68              to write to its controlling terminal, TOSTOP is set, the calling
69              thread  is  not  blocking  SIGTTOU,  the process is not ignoring
70              SIGTTOU, and the process group of the process is orphaned.  This
71              error  may  also be returned under implementation-defined condi‐
72              tions.
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74       ENOMEM The  underlying  stream  was  created  by  open_memstream()   or
75              open_wmemstream() and insufficient memory is available.
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77       ENOSPC There  was  no free space remaining on the device containing the
78              file or in the buffer used by the fmemopen() function.
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80       EPIPE  An attempt is made to write to a pipe or FIFO that is  not  open
81              for  reading by any process. A SIGPIPE signal shall also be sent
82              to the thread.
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84       The fflush() function may fail if:
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86       ENXIO  A request was made of a nonexistent device, or the  request  was
87              outside the capabilities of the device.
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89       The following sections are informative.
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EXAMPLES

92   Sending Prompts to Standard Output
93       The  following example uses printf() calls to print a series of prompts
94       for information the user must enter from standard input.  The  fflush()
95       calls  force  the  output  to standard output. The fflush() function is
96       used because standard output is usually buffered and the prompt may not
97       immediately  be  printed on the output or terminal. The getline() func‐
98       tion calls read strings from standard input and place  the  results  in
99       variables, for use later in the program.
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102           char *user;
103           char *oldpasswd;
104           char *newpasswd;
105           ssize_t llen;
106           size_t blen;
107           struct termios term;
108           tcflag_t saveflag;
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110           printf("User name: ");
111           fflush(stdout);
112           blen = 0;
113           llen = getline(&user, &blen, stdin);
114           user[llen-1] = 0;
115           tcgetattr(fileno(stdin), &term);
116           saveflag = term.c_lflag;
117           term.c_lflag &= ~ECHO;
118           tcsetattr(fileno(stdin), TCSANOW, &term);
119           printf("Old password: ");
120           fflush(stdout);
121           blen = 0;
122           llen = getline(&oldpasswd, &blen, stdin);
123           oldpasswd[llen-1] = 0;
124
125           printf("\nNew password: ");
126           fflush(stdout);
127           blen = 0;
128           llen = getline(&newpasswd, &blen, stdin);
129           newpasswd[llen-1] = 0;
130           term.c_lflag = saveflag;
131           tcsetattr(fileno(stdin), TCSANOW, &term);
132           free(user);
133           free(oldpasswd);
134           free(newpasswd);
135

APPLICATION USAGE

137       None.
138

RATIONALE

140       Data  buffered  by  the system may make determining the validity of the
141       position of the current file descriptor  impractical.  Thus,  enforcing
142       the repositioning of the file descriptor after fflush() on streams open
143       for read() is not mandated by POSIX.1‐2008.
144

FUTURE DIRECTIONS

146       None.
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SEE ALSO

149       Section 2.5, Standard I/O Streams, fmemopen(),  getrlimit(),  open_mem‐
150       stream(), ulimit()
151
152       The Base Definitions volume of POSIX.1‐2017, <stdio.h>
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155       Portions  of  this text are reprinted and reproduced in electronic form
156       from IEEE Std 1003.1-2017, Standard for Information Technology --  Por‐
157       table  Operating System Interface (POSIX), The Open Group Base Specifi‐
158       cations Issue 7, 2018 Edition, Copyright (C) 2018 by the  Institute  of
159       Electrical  and  Electronics Engineers, Inc and The Open Group.  In the
160       event of any discrepancy between this version and the original IEEE and
161       The  Open Group Standard, the original IEEE and The Open Group Standard
162       is the referee document. The original Standard can be  obtained  online
163       at http://www.opengroup.org/unix/online.html .
164
165       Any  typographical  or  formatting  errors that appear in this page are
166       most likely to have been introduced during the conversion of the source
167       files  to  man page format. To report such errors, see https://www.ker
168       nel.org/doc/man-pages/reporting_bugs.html .
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172IEEE/The Open Group                  2017                           FFLUSH(3P)
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