1fopen(3)                   Library Functions Manual                   fopen(3)


6       fopen, fdopen, freopen - stream open functions


9       Standard C library (libc, -lc)


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


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


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


142       EINVAL The  mode  provided  to  fopen(), fdopen(), or freopen() was in‐
143              valid.
145       The fopen(), fdopen(), and freopen() functions may also  fail  and  set
146       errno for any of the errors specified for the routine malloc(3).
148       The  fopen() function may also fail and set errno for any of the errors
149       specified for the routine open(2).
151       The fdopen() function may also fail and set errno for any of the errors
152       specified for the routine fcntl(2).
154       The  freopen()  function may also fail and set errno for any of the er‐
155       rors specified for the routines open(2), fclose(3), and fflush(3).


158       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
159       tributes(7).
161       ┌────────────────────────────────────────────┬───────────────┬─────────┐
162Interface                                   Attribute     Value   
163       ├────────────────────────────────────────────┼───────────────┼─────────┤
164fopen(), fdopen(), freopen()                │ Thread safety │ MT-Safe │
165       └────────────────────────────────────────────┴───────────────┴─────────┘


168       fopen()
169       freopen()
170              C11, POSIX.1-2008.
172       fdopen()
173              POSIX.1-2008.


176       fopen()
177       freopen()
178              POSIX.1-2001, C89.
180       fdopen()
181              POSIX.1-2001.


184   glibc notes
185       The  GNU  C  library  allows  the  following  extensions for the string
186       specified in mode:
188       c (since glibc 2.3.3)
189              Do not make the open operation, or  subsequent  read  and  write
190              operations, thread cancelation points.  This flag is ignored for
191              fdopen().
193       e (since glibc 2.7)
194              Open the file with the O_CLOEXEC flag.   See  open(2)  for  more
195              information.  This flag is ignored for fdopen().
197       m (since glibc 2.3)
198              Attempt to access the file using mmap(2), rather than I/O system
199              calls  (read(2),  write(2)).   Currently,  use  of  mmap(2)   is
200              attempted only for a file opened for reading.
202       x      Open the file exclusively (like the O_EXCL flag of open(2)).  If
203              the file already  exists,  fopen()  fails,  and  sets  errno  to
204              EEXIST.  This flag is ignored for fdopen().
206       In  addition to the above characters, fopen() and freopen() support the
207       following syntax in mode:
209           ,ccs=string
211       The given string is taken as the name of a coded character set and  the
212       stream  is  marked  as  wide-oriented.  Thereafter, internal conversion
213       functions convert I/O to and from the character  set  string.   If  the
214       ,ccs=string  syntax  is not specified, then the wide-orientation of the
215       stream is determined by the first file operation.  If that operation is
216       a  wide-character  operation,  the  stream is marked wide-oriented, and
217       functions to convert to the coded character set are loaded.


220       When  parsing  for  individual  flag  characters  in  mode  (i.e.,  the
221       characters preceding the "ccs" specification), the glibc implementation
222       of fopen() and freopen() limits the number of  characters  examined  in
223       mode to 7 (or, before glibc 2.14, to 6, which was not enough to include
224       possible specifications such as "rb+cmxe").  The current implementation
225       of fdopen() parses at most 5 characters in mode.


228       open(2),    fclose(3),    fileno(3),    fmemopen(3),    fopencookie(3),
229       open_memstream(3)
233Linux man-pages 6.05              2023-07-20                          fopen(3)