1FREOPEN(3P)                POSIX Programmer's Manual               FREOPEN(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.
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11

NAME

13       freopen — open a stream
14

SYNOPSIS

16       #include <stdio.h>
17
18       FILE *freopen(const char *restrict pathname, const char *restrict mode,
19           FILE *restrict stream);
20

DESCRIPTION

22       The functionality described on this reference page is aligned with  the
23       ISO C  standard.  Any  conflict between the requirements described here
24       and the ISO C standard is unintentional. This  volume  of  POSIX.1‐2008
25       defers to the ISO C standard.
26
27       The  freopen() function shall first attempt to flush the stream associ‐
28       ated with stream as if by a call to fflush(stream).  Failure  to  flush
29       the  stream  successfully  shall  be ignored. If pathname is not a null
30       pointer, freopen() shall close  any  file  descriptor  associated  with
31       stream.   Failure  to  close  the file descriptor successfully shall be
32       ignored.  The error and end-of-file indicators for the stream shall  be
33       cleared.
34
35       The freopen() function shall open the file whose pathname is the string
36       pointed to by pathname and associate the stream pointed  to  by  stream
37       with it. The mode argument shall be used just as in fopen().
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39       The  original  stream  shall be closed regardless of whether the subse‐
40       quent open succeeds.
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42       If pathname is a null pointer, the freopen() function shall attempt  to
43       change the mode of the stream to that specified by mode, as if the name
44       of the file currently associated with the stream had been used. In this
45       case, the file descriptor associated with the stream need not be closed
46       if the call to freopen() succeeds. It is  implementation-defined  which
47       changes of mode are permitted (if any), and under what circumstances.
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49       After  a  successful call to the freopen() function, the orientation of
50       the stream shall be cleared, the encoding rule shall  be  cleared,  and
51       the  associated  mbstate_t  object  shall be set to describe an initial
52       conversion state.
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54       If pathname is not a null pointer, or if pathname is a null pointer and
55       the  specified  mode change necessitates the file descriptor associated
56       with the stream to be closed and reopened, the file descriptor  associ‐
57       ated  with the reopened stream shall be allocated and opened as if by a
58       call to open() with the following flags:
59
60                   ┌─────────────────┬───────────────────────────┐
61freopen() Mode  open() Flags        
62                   ├─────────────────┼───────────────────────────┤
63r or rb          │ O_RDONLY                  │
64w or wb          │ O_WRONLY|O_CREAT|O_TRUNC  │
65a or ab          │ O_WRONLY|O_CREAT|O_APPEND │
66r+ or rb+ or r+b │ O_RDWR                    │
67w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC    │
68a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND   │
69                   └─────────────────┴───────────────────────────┘

RETURN VALUE

71       Upon successful completion, freopen() shall return the value of stream.
72       Otherwise,  a null pointer shall be returned, and errno shall be set to
73       indicate the error.
74

ERRORS

76       The freopen() function shall fail if:
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78       EACCES Search permission is denied on a component of the  path  prefix,
79              or  the  file  exists  and the permissions specified by mode are
80              denied, or the file does  not  exist  and  write  permission  is
81              denied for the parent directory of the file to be created.
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83       EBADF  The  file  descriptor  underlying the stream is not a valid file
84              descriptor when pathname is a null pointer.
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86       EINTR  A signal was caught during freopen().
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88       EISDIR The named file is a directory and mode requires write access.
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90       ELOOP  A loop exists in symbolic links encountered during resolution of
91              the path argument.
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93       EMFILE All  file  descriptors  available  to  the process are currently
94              open.
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96       ENAMETOOLONG
97              The  length  of  a  component  of  a  pathname  is  longer  than
98              {NAME_MAX}.
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100       ENFILE The  maximum  allowable number of files is currently open in the
101              system.
102
103       ENOENT The mode string begins with 'r' and a component of pathname does
104              not  name an existing file, or mode begins with 'w' or 'a' and a
105              component of the path prefix of pathname does not name an exist‐
106              ing file, or pathname is an empty string.
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108       ENOENT or ENOTDIR
109              The  pathname argument contains at least one non-<slash> charac‐
110              ter and ends with one or more trailing  <slash>  characters.  If
111              pathname  names  an  existing  file, an [ENOENT] error shall not
112              occur.
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114       ENOSPC The directory or file system that would  contain  the  new  file
115              cannot  be  expanded,  the file does not exist, and it was to be
116              created.
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118       ENOTDIR
119              A component of the path prefix names an existing  file  that  is
120              neither  a  directory nor a symbolic link to a directory, or the
121              pathname argument contains at least  one  non-<slash>  character
122              and  ends  with  one or more trailing <slash> characters and the
123              last pathname component names an existing file that is neither a
124              directory nor a symbolic link to a directory.
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126       ENXIO  The named file is a character special or block special file, and
127              the device associated with this special file does not exist.
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129       EOVERFLOW
130              The named file is a regular file and the size of the file cannot
131              be represented correctly in an object of type off_t.
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133       EROFS  The  named  file  resides  on  a  read-only file system and mode
134              requires write access.
135
136       The freopen() function may fail if:
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138       EBADF  The mode with which the file descriptor  underlying  the  stream
139              was  opened does not support the requested mode when pathname is
140              a null pointer.
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142       EINVAL The value of the mode argument is not valid.
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144       ELOOP  More than {SYMLOOP_MAX} symbolic links were  encountered  during
145              resolution of the path argument.
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147       ENAMETOOLONG
148              The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
149              tion of a symbolic link produced an intermediate result  with  a
150              length that exceeds {PATH_MAX}.
151
152       ENOMEM Insufficient storage space is available.
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154       ENXIO  A  request  was made of a nonexistent device, or the request was
155              outside the capabilities of the device.
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157       ETXTBSY
158              The file is a pure procedure (shared text) file  that  is  being
159              executed and mode requires write access.
160
161       The following sections are informative.
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EXAMPLES

164   Directing Standard Output to a File
165       The  following  example  logs  all  standard output to the /tmp/logfile
166       file.
167
168           #include <stdio.h>
169           ...
170           FILE *fp;
171           ...
172           fp = freopen ("/tmp/logfile", "a+", stdout);
173           ...
174

APPLICATION USAGE

176       The freopen() function is  typically  used  to  attach  the  pre-opened
177       streams associated with stdin, stdout, and stderr to other files.
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179       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
196           { appl file1; appl file2; } > file3
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198       which will result in file3 containing only the output from  the  second
199       invocation of appl.
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RATIONALE

202       None.
203

FUTURE DIRECTIONS

205       None.
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SEE ALSO

208       Section 2.5, Standard I/O Streams, fclose(), fdopen(), fflush(), fmemo‐
209       pen(), fopen(), mbsinit(), open(), open_memstream()
210
211       The Base Definitions volume of POSIX.1‐2008, <stdio.h>
212
214       Portions of this text are reprinted and reproduced in  electronic  form
215       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
216       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
217       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
218       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
219       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/online.html .
224
225       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 .
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232IEEE/The Open Group                  2013                          FREOPEN(3P)
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