1FFLUSH(3P) POSIX Programmer's Manual FFLUSH(3P)
2
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
13 fflush — flush a stream
14
16 #include <stdio.h>
17 int fflush(FILE *stream);
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21 The functionality described on this reference page is aligned with the
22 ISO C standard. Any conflict between the requirements described here
23 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
24 defers to the ISO C standard.
25 If stream points to an output stream or an update stream in which the
27 most recent operation was not input, fflush() shall cause any unwritten
28 data for that stream to be written to the file, and the last data modi‐
29 fication and last file status change timestamps of the underlying file
30 shall be marked for update.
31 If stream is a null pointer, fflush() shall perform this flushing
33 action on all streams for which the behavior is defined above.
34 For a stream open for reading, if the file is not already at EOF, and
36 the file is one capable of seeking, the file offset of the underlying
37 open file description shall be set to the file position of the stream,
38 and any characters pushed back onto the stream by ungetc() or ungetwc()
39 that have not subsequently been read from the stream shall be discarded
40 (without further changing the file offset).
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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.
46
48 The fflush() function shall fail if:
49 EAGAIN The O_NONBLOCK flag is set for the file descriptor underlying
51 stream and the thread would be delayed in the write operation.
52 EBADF The file descriptor underlying stream is not valid.
54 EFBIG An attempt was made to write a file that exceeds the maximum
56 file size.
57 EFBIG An attempt was made to write a file that exceeds the file size
59 limit of the process.
60 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.
64 EINTR The fflush() function was interrupted by a signal.
66 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.
73 ENOMEM The underlying stream was created by open_memstream() or
75 open_wmemstream() and insufficient memory is available.
76 ENOSPC There was no free space remaining on the device containing the
78 file or in the buffer used by the fmemopen() function.
79 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.
83 The fflush() function may fail if:
85 ENXIO A request was made of a nonexistent device, or the request was
87 outside the capabilities of the device.
88 The following sections are informative.
90
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.
100 char *user;
102 char *oldpasswd;
103 char *newpasswd;
104 ssize_t llen;
105 size_t blen;
106 struct termios term;
107 tcflag_t saveflag;
108 printf("User name: ");
110 fflush(stdout);
111 blen = 0;
112 llen = getline(&user, &blen, stdin);
113 user[llen-1] = 0;
114 tcgetattr(fileno(stdin), &term);
115 saveflag = term.c_lflag;
116 term.c_lflag &= ~ECHO;
117 tcsetattr(fileno(stdin), TCSANOW, &term);
118 printf("Old password: ");
119 fflush(stdout);
120 blen = 0;
121 llen = getline(&oldpasswd, &blen, stdin);
122 oldpasswd[llen-1] = 0;
123 printf("\nNew password: ");
125 fflush(stdout);
126 blen = 0;
127 llen = getline(&newpasswd, &blen, stdin);
128 newpasswd[llen-1] = 0;
129 term.c_lflag = saveflag;
130 tcsetattr(fileno(stdin), TCSANOW, &term);
131 free(user);
132 free(oldpasswd);
133 free(newpasswd);
134
136 None.
137
139 Data buffered by the system may make determining the validity of the
140 position of the current file descriptor impractical. Thus, enforcing
141 the repositioning of the file descriptor after fflush() on streams open
142 for read() is not mandated by POSIX.1‐2008.
143
145 None.
146
148 Section 2.5, Standard I/O Streams, fmemopen(), getrlimit(), open_mem‐
149 stream(), ulimit()
150 The Base Definitions volume of POSIX.1‐2008, <stdio.h>
152
154 Portions of this text are reprinted and reproduced in electronic form
155 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
156 -- Portable Operating System Interface (POSIX), The Open Group Base
157 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
158 cal and Electronics Engineers, Inc and The Open Group. (This is
159 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
164 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 .
169IEEE/The Open Group 2013 FFLUSH(3P)