1FOPEN(3)                   Linux Programmer's Manual                  FOPEN(3)
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4

NAME

6       fopen, fdopen, freopen - stream open functions
7

SYNOPSIS

9       #include <stdio.h>
10
11       FILE *fopen(const char *pathname, const char *mode);
12
13       FILE *fdopen(int fd, const char *mode);
14
15       FILE *freopen(const char *pathname, const char *mode, FILE *stream);
16
17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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19       fdopen(): _POSIX_C_SOURCE
20

DESCRIPTION

22       The fopen() function opens the file whose name is the string pointed to
23       by pathname and associates a stream with it.
24
25       The argument mode points to a string beginning with one of the  follow‐
26       ing sequences (possibly followed by additional characters, as described
27       below):
28
29       r      Open text file for reading.  The stream  is  positioned  at  the
30              beginning of the file.
31
32       r+     Open  for  reading and writing.  The stream is positioned at the
33              beginning of the file.
34
35       w      Truncate file to zero length or create text  file  for  writing.
36              The stream is positioned at the beginning of the file.
37
38       w+     Open  for  reading  and writing.  The file is created if it does
39              not exist, otherwise it is truncated.  The stream is  positioned
40              at the beginning of the file.
41
42       a      Open  for  appending (writing at end of file).  The file is cre‐
43              ated if it does not exist.  The stream is positioned at the  end
44              of the file.
45
46       a+     Open  for  reading  and appending (writing at end of file).  The
47              file is created if it does not exist.  Output is always appended
48              to  the  end  of  the file.  POSIX is silent on what the initial
49              read position is when using this mode.  For glibc,  the  initial
50              file  position  for reading is at the beginning of the file, but
51              for Android/BSD/MacOS, the initial file position for reading  is
52              at the end of the file.
53
54       The  mode string can also include the letter 'b' either as a last char‐
55       acter or as a character between the characters in any of the  two-char‐
56       acter strings described above.  This is strictly for compatibility with
57       C89 and has no effect; the 'b' is ignored on all POSIX conforming  sys‐
58       tems,  including Linux.  (Other systems may treat text files and binary
59       files differently, and adding the 'b' may be a good idea if you do  I/O
60       to a binary file and expect that your program may be ported to non-UNIX
61       environments.)
62
63       See NOTES below for details of glibc extensions for mode.
64
65       Any created file will have the mode  S_IRUSR  |  S_IWUSR  |  S_IRGRP  |
66       S_IWGRP  | S_IROTH | S_IWOTH (0666), as modified by the process's umask
67       value (see umask(2)).
68
69       Reads and writes may be intermixed on read/write streams in any  order.
70       Note  that  ANSI  C requires that a file positioning function intervene
71       between output and input, unless an input operation encounters  end-of-
72       file.   (If this condition is not met, then a read is allowed to return
73       the result of writes other than the most recent.)  Therefore it is good
74       practice  (and  indeed  sometimes  necessary  under  Linux)  to  put an
75       fseek(3) or fgetpos(3) operation between write and read  operations  on
76       such  a  stream.   This  operation  may  be  an  apparent  no-op (as in
77       fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect).
78
79       Opening a file in append mode (a as the first character of mode) causes
80       all subsequent write operations to this stream to occur at end-of-file,
81       as if preceded the call:
82
83           fseek(stream, 0, SEEK_END);
84
85       The file descriptor associated with the stream is opened  as  if  by  a
86       call to open(2) with the following flags:
87
88              ┌─────────────┬───────────────────────────────┐
89fopen() mode open() flags                  
90              ├─────────────┼───────────────────────────────┤
91r       │ O_RDONLY                      │
92              ├─────────────┼───────────────────────────────┤
93w       │ O_WRONLY | O_CREAT | O_TRUNC  │
94              ├─────────────┼───────────────────────────────┤
95a       │ O_WRONLY | O_CREAT | O_APPEND │
96              ├─────────────┼───────────────────────────────┤
97r+      │ O_RDWR                        │
98              ├─────────────┼───────────────────────────────┤
99w+      │ O_RDWR | O_CREAT | O_TRUNC    │
100              ├─────────────┼───────────────────────────────┤
101a+      │ O_RDWR | O_CREAT | O_APPEND   │
102              └─────────────┴───────────────────────────────┘
103   fdopen()
104       The  fdopen()  function  associates  a  stream  with  the existing file
105       descriptor, fd.  The mode of the stream (one of the values  "r",  "r+",
106       "w",  "w+",  "a",  "a+")  must  be compatible with the mode of the file
107       descriptor.  The file position indicator of the new stream  is  set  to
108       that  belonging  to  fd,  and  the error and end-of-file indicators are
109       cleared.  Modes "w" or "w+" do not cause truncation of the  file.   The
110       file  descriptor is not dup'ed, and will be closed when the stream cre‐
111       ated by fdopen() is closed.  The  result  of  applying  fdopen()  to  a
112       shared memory object is undefined.
113
114   freopen()
115       The  freopen() function opens the file whose name is the string pointed
116       to by pathname and associates the stream pointed to by stream with  it.
117       The  original  stream  (if  it exists) is closed.  The mode argument is
118       used just as in the fopen() function.
119
120       If the pathname argument is a null pointer, freopen() changes the  mode
121       of the stream to that specified in mode; that is, freopen() reopens the
122       pathname that is associated with the  stream.   The  specification  for
123       this behavior was added in the C99 standard, which says:
124
125              In  this  case,  the  file descriptor associated with the stream
126              need not be closed if the call to  freopen()  succeeds.   It  is
127              implementation-defined  which  changes of mode are permitted (if
128              any), and under what circumstances.
129
130       The primary use of the freopen() function is to change the file associ‐
131       ated with a standard text stream (stderr, stdin, or stdout).
132

RETURN VALUE

134       Upon  successful  completion  fopen(),  fdopen() and freopen() return a
135       FILE pointer.  Otherwise, NULL is returned and errno is set to indicate
136       the error.
137

ERRORS

139       EINVAL The  mode  provided  to  fopen(),  fdopen(),  or  freopen()  was
140              invalid.
141
142       The fopen(), fdopen() and freopen() functions may  also  fail  and  set
143       errno for any of the errors specified for the routine malloc(3).
144
145       The  fopen() function may also fail and set errno for any of the errors
146       specified for the routine open(2).
147
148       The fdopen() function may also fail and set errno for any of the errors
149       specified for the routine fcntl(2).
150
151       The  freopen()  function  may  also  fail  and set errno for any of the
152       errors specified for the routines open(2), fclose(3), and fflush(3).
153

ATTRIBUTES

155       For  an  explanation  of  the  terms  used   in   this   section,   see
156       attributes(7).
157
158       ┌─────────────────────────────┬───────────────┬─────────┐
159Interface                    Attribute     Value   
160       ├─────────────────────────────┼───────────────┼─────────┤
161fopen(), fdopen(), freopen() │ Thread safety │ MT-Safe │
162       └─────────────────────────────┴───────────────┴─────────┘

CONFORMING TO

164       fopen(), freopen(): POSIX.1-2001, POSIX.1-2008, C89, C99.
165
166       fdopen(): POSIX.1-2001, POSIX.1-2008.
167

NOTES

169   Glibc notes
170       The GNU C library allows the following extensions for the string speci‐
171       fied in mode:
172
173       c (since glibc 2.3.3)
174              Do not make the open operation, or  subsequent  read  and  write
175              operations,  thread  cancellation  points.  This flag is ignored
176              for fdopen().
177
178       e (since glibc 2.7)
179              Open the file with the O_CLOEXEC flag.   See  open(2)  for  more
180              information.  This flag is ignored for fdopen().
181
182       m (since glibc 2.3)
183              Attempt to access the file using mmap(2), rather than I/O system
184              calls  (read(2),  write(2)).   Currently,  use  of  mmap(2)   is
185              attempted only for a file opened for reading.
186
187       x      Open the file exclusively (like the O_EXCL flag of open(2)).  If
188              the file already exists, fopen() fails, and sets errno  to  EEX‐
189              IST.  This flag is ignored for fdopen().
190
191       In  addition to the above characters, fopen() and freopen() support the
192       following syntax in mode:
193
194           ,ccs=string
195
196       The given string is taken as the name of a coded character set and  the
197       stream  is  marked  as  wide-oriented.  Thereafter, internal conversion
198       functions convert I/O to and from the character  set  string.   If  the
199       ,ccs=string  syntax  is not specified, then the wide-orientation of the
200       stream is determined by the first file operation.  If that operation is
201       a  wide-character  operation,  the  stream is marked wide-oriented, and
202       functions to convert to the coded character set are loaded.
203

BUGS

205       When parsing for individual flag characters in mode (i.e., the  charac‐
206       ters  preceding  the  "ccs" specification), the glibc implementation of
207       fopen() and freopen() limits the number of characters examined in  mode
208       to  7 (or, in glibc versions before 2.14, to 6, which was not enough to
209       include possible specifications such as "rb+cmxe").  The current imple‐
210       mentation of fdopen() parses at most 5 characters in mode.
211

SEE ALSO

213       open(2),  fclose(3),  fileno(3), fmemopen(3), fopencookie(3), open_mem‐
214       stream(3)
215

COLOPHON

217       This page is part of release 5.04 of the Linux  man-pages  project.   A
218       description  of  the project, information about reporting bugs, and the
219       latest    version    of    this    page,    can     be     found     at
220       https://www.kernel.org/doc/man-pages/.
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224GNU                               2019-05-09                          FOPEN(3)
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