1FOPEN(3)                   Linux Programmer's Manual                  FOPEN(3)
2
3
4

NAME

6       fopen, fdopen, freopen - stream open functions
7

SYNOPSIS

9       #include <stdio.h>
10
11       FILE *fopen(const char *restrict pathname, const char *restrict mode);
12       FILE *fdopen(int fd, const char *mode);
13       FILE *freopen(const char *restrict pathname, const char *restrict mode,
14                     FILE *restrict stream);
15
16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
17
18       fdopen():
19           _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 be‐
30              ginning 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 | S_IW‐
66       GRP  |  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 de‐
105       scriptor, 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 de‐
107       scriptor.  The file position indicator of the new stream is set to that
108       belonging  to fd, and the error and end-of-file indicators are cleared.
109       Modes "w" or "w+" do not cause truncation of the file.   The  file  de‐
110       scriptor  is  not dup'ed, and will be closed when the stream created by
111       fdopen() is closed.  The result of applying fdopen() to a shared memory
112       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 im‐
127              plementation-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 in‐
140              valid.
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 er‐
152       rors 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  at‐
156       tributes(7).
157
158       ┌────────────────────────────────────────────┬───────────────┬─────────┐
159Interface                                   Attribute     Value   
160       ├────────────────────────────────────────────┼───────────────┼─────────┤
161fopen(), fdopen(), freopen()                │ Thread safety │ MT-Safe │
162       └────────────────────────────────────────────┴───────────────┴─────────┘
163

CONFORMING TO

165       fopen(), freopen(): POSIX.1-2001, POSIX.1-2008, C89, C99.
166
167       fdopen(): POSIX.1-2001, POSIX.1-2008.
168

NOTES

170   Glibc notes
171       The GNU C library allows the following extensions for the string speci‐
172       fied in mode:
173
174       c (since glibc 2.3.3)
175              Do not make the open operation, or subsequent read and write op‐
176              erations,  thread cancellation points.  This flag is ignored for
177              fdopen().
178
179       e (since glibc 2.7)
180              Open the file with the O_CLOEXEC flag.  See open(2) for more in‐
181              formation.  This flag is ignored for fdopen().
182
183       m (since glibc 2.3)
184              Attempt to access the file using mmap(2), rather than I/O system
185              calls (read(2), write(2)).  Currently, use  of  mmap(2)  is  at‐
186              tempted only for a file opened for reading.
187
188       x      Open the file exclusively (like the O_EXCL flag of open(2)).  If
189              the file already exists, fopen() fails, and sets errno  to  EEX‐
190              IST.  This flag is ignored for fdopen().
191
192       In  addition to the above characters, fopen() and freopen() support the
193       following syntax in mode:
194
195           ,ccs=string
196
197       The given string is taken as the name of a coded character set and  the
198       stream  is  marked  as  wide-oriented.  Thereafter, internal conversion
199       functions convert I/O to and from the character  set  string.   If  the
200       ,ccs=string  syntax  is not specified, then the wide-orientation of the
201       stream is determined by the first file operation.  If that operation is
202       a  wide-character  operation,  the  stream is marked wide-oriented, and
203       functions to convert to the coded character set are loaded.
204

BUGS

206       When parsing for individual flag characters in mode (i.e., the  charac‐
207       ters  preceding  the  "ccs" specification), the glibc implementation of
208       fopen() and freopen() limits the number of characters examined in  mode
209       to  7 (or, in glibc versions before 2.14, to 6, which was not enough to
210       include possible specifications such as "rb+cmxe").  The current imple‐
211       mentation of fdopen() parses at most 5 characters in mode.
212

SEE ALSO

214       open(2),  fclose(3),  fileno(3), fmemopen(3), fopencookie(3), open_mem‐
215       stream(3)
216

COLOPHON

218       This page is part of release 5.13 of the Linux  man-pages  project.   A
219       description  of  the project, information about reporting bugs, and the
220       latest    version    of    this    page,    can     be     found     at
221       https://www.kernel.org/doc/man-pages/.
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225GNU                               2021-03-22                          FOPEN(3)
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