1FREOPEN(3P) POSIX Programmer's Manual FREOPEN(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
12 freopen — open a stream
13
15 #include <stdio.h>
16 FILE *freopen(const char *restrict pathname, const char *restrict mode,
18 FILE *restrict stream);
19
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.1‐2017
24 defers to the ISO C standard.
25 The freopen() function shall first attempt to flush the stream associ‐
27 ated with stream as if by a call to fflush(stream). Failure to flush
28 the stream successfully shall be ignored. If pathname is not a null
29 pointer, freopen() shall close any file descriptor associated with
30 stream. Failure to close the file descriptor successfully shall be
31 ignored. The error and end-of-file indicators for the stream shall be
32 cleared.
33 The freopen() function shall open the file whose pathname is the string
35 pointed to by pathname and associate the stream pointed to by stream
36 with it. The mode argument shall be used just as in fopen().
37 The original stream shall be closed regardless of whether the subse‐
39 quent open succeeds.
40 If pathname is a null pointer, the freopen() function shall attempt to
42 change the mode of the stream to that specified by mode, as if the name
43 of the file currently associated with the stream had been used. In this
44 case, the file descriptor associated with the stream need not be closed
45 if the call to freopen() succeeds. It is implementation-defined which
46 changes of mode are permitted (if any), and under what circumstances.
47 After a successful call to the freopen() function, the orientation of
49 the stream shall be cleared, the encoding rule shall be cleared, and
50 the associated mbstate_t object shall be set to describe an initial
51 conversion state.
52 If pathname is not a null pointer, or if pathname is a null pointer and
54 the specified mode change necessitates the file descriptor associated
55 with the stream to be closed and reopened, the file descriptor associ‐
56 ated with the reopened stream shall be allocated and opened as if by a
57 call to open() with the following flags:
58 ┌─────────────────┬───────────────────────────┐
60 │ freopen() Mode │ open() Flags │
61 ├─────────────────┼───────────────────────────┤
62 │r or rb │ O_RDONLY │
63 │w or wb │ O_WRONLY|O_CREAT|O_TRUNC │
64 │a or ab │ O_WRONLY|O_CREAT|O_APPEND │
65 │r+ or rb+ or r+b │ O_RDWR │
66 │w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC │
67 │a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND │
68 └─────────────────┴───────────────────────────┘
70 Upon successful completion, freopen() shall return the value of stream.
71 Otherwise, a null pointer shall be returned, and errno shall be set to
72 indicate the error.
73
75 The freopen() function shall fail if:
76 EACCES Search permission is denied on a component of the path prefix,
78 or the file exists and the permissions specified by mode are
79 denied, or the file does not exist and write permission is
80 denied for the parent directory of the file to be created.
81 EBADF The file descriptor underlying the stream is not a valid file
83 descriptor when pathname is a null pointer.
84 EINTR A signal was caught during freopen().
86 EISDIR The named file is a directory and mode requires write access.
88 ELOOP A loop exists in symbolic links encountered during resolution of
90 the path argument.
91 EMFILE All file descriptors available to the process are currently
93 open.
94 ENAMETOOLONG
96 The length of a component of a pathname is longer than
97 {NAME_MAX}.
98 ENFILE The maximum allowable number of files is currently open in the
100 system.
101 ENOENT The mode string begins with 'r' and a component of pathname does
103 not name an existing file, or mode begins with 'w' or 'a' and a
104 component of the path prefix of pathname does not name an exist‐
105 ing file, or pathname is an empty string.
106 ENOENT or ENOTDIR
108 The pathname argument contains at least one non-<slash> charac‐
109 ter and ends with one or more trailing <slash> characters. If
110 pathname without the trailing <slash> characters would name an
111 existing file, an [ENOENT] error shall not occur.
112 ENOSPC The directory or file system that would contain the new file
114 cannot be expanded, the file does not exist, and it was to be
115 created.
116 ENOTDIR
118 A component of the path prefix names an existing file that is
119 neither a directory nor a symbolic link to a directory, or the
120 pathname argument contains at least one non-<slash> character
121 and ends with one or more trailing <slash> characters and the
122 last pathname component names an existing file that is neither a
123 directory nor a symbolic link to a directory.
124 ENXIO The named file is a character special or block special file, and
126 the device associated with this special file does not exist.
127 EOVERFLOW
129 The named file is a regular file and the size of the file cannot
130 be represented correctly in an object of type off_t.
131 EROFS The named file resides on a read-only file system and mode
133 requires write access.
134 The freopen() function may fail if:
136 EBADF The mode with which the file descriptor underlying the stream
138 was opened does not support the requested mode when pathname is
139 a null pointer.
140 EINVAL The value of the mode argument is not valid.
142 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
144 resolution of the path argument.
145 ENAMETOOLONG
147 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
148 tion of a symbolic link produced an intermediate result with a
149 length that exceeds {PATH_MAX}.
150 ENOMEM Insufficient storage space is available.
152 ENXIO A request was made of a nonexistent device, or the request was
154 outside the capabilities of the device.
155 ETXTBSY
157 The file is a pure procedure (shared text) file that is being
158 executed and mode requires write access.
159 The following sections are informative.
161
163 Directing Standard Output to a File
164 The following example logs all standard output to the /tmp/logfile
165 file.
166 #include <stdio.h>
169 ...
170 FILE *fp;
171 ...
172 fp = freopen ("/tmp/logfile", "a+", stdout);
173 ...
174
176 The freopen() function is typically used to attach the pre-opened
177 streams associated with stdin, stdout, and stderr to other files.
178 Since implementations are not required to support any stream mode
180 changes when the pathname argument is NULL, portable applications can‐
181 not rely on the use of freopen() to change the stream mode, and use of
182 this feature is discouraged. The feature was originally added to the
183 ISO C standard in order to facilitate changing stdin and stdout to
184 binary mode. Since a 'b' character in the mode has no effect on POSIX
185 systems, this use of the feature is unnecessary in POSIX applications.
186 However, even though the 'b' is ignored, a successful call to fre‐
187 open(NULL, "wb", stdout) does have an effect. In particular, for regu‐
188 lar files it truncates the file and sets the file-position indicator
189 for the stream to the start of the file. It is possible that these
190 side-effects are an unintended consequence of the way the feature is
191 specified in the ISO/IEC 9899:1999 standard, but unless or until the
192 ISO C standard is changed, applications which successfully call fre‐
193 open(NULL, "wb", stdout) will behave in unexpected ways on conforming
194 systems in situations such as:
195 { appl file1; appl file2; } > file3
198 which will result in file3 containing only the output from the second
200 invocation of appl.
201
203 None.
204
206 None.
207
209 Section 2.5, Standard I/O Streams, fclose(), fdopen(), fflush(), fmemo‐
210 pen(), fopen(), mbsinit(), open(), open_memstream()
211 The Base Definitions volume of POSIX.1‐2017, <stdio.h>
213
215 Portions of this text are reprinted and reproduced in electronic form
216 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
217 table Operating System Interface (POSIX), The Open Group Base Specifi‐
218 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
219 Electrical and Electronics Engineers, Inc and The Open Group. In the
220 event of any discrepancy between this version and the original IEEE and
221 The Open Group Standard, the original IEEE and The Open Group Standard
222 is the referee document. The original Standard can be obtained online
223 at http://www.opengroup.org/unix/online.html .
224 Any typographical or formatting errors that appear in this page are
226 most likely to have been introduced during the conversion of the source
227 files to man page format. To report such errors, see https://www.ker‐
228 nel.org/doc/man-pages/reporting_bugs.html .
229IEEE/The Open Group 2017 FREOPEN(3P)