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NAME

       fopen, fdopen, freopen - stream open functions

SYNOPSIS

       #include <stdio.h>

       FILE *fopen(const char *pathname, const char *mode);

       FILE *fdopen(int fd, const char *mode);

       FILE *freopen(const char *pathname, const char *mode, FILE *stream);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fdopen(): _POSIX_C_SOURCE

DESCRIPTION

       The  fopen()  function  opens  the  file whose name is the string pointed to by pathname and associates a
       stream with it.

       The argument mode points to a string beginning with one of the following sequences (possibly followed  by
       additional characters, as described below):

       r      Open text file for reading.  The stream is positioned at the beginning of the file.

       r+     Open for reading and writing.  The stream is positioned at the beginning of the file.

       w      Truncate  file  to  zero  length or create text file for writing.  The stream is positioned at the
              beginning of the file.

       w+     Open for reading and writing.  The file  is  created  if  it  does  not  exist,  otherwise  it  is
              truncated.  The stream is positioned at the beginning of the file.

       a      Open  for  appending  (writing  at  end  of file).  The file is created if it does not exist.  The
              stream is positioned at the end of the file.

       a+     Open for reading and appending (writing at end of file).  The file  is  created  if  it  does  not
              exist.   Output  is  always  appended to the end of the file.  POSIX is silent on what the initial
              read position is when using this mode.  For glibc, the initial file position for reading is at the
              beginning  of the file, but for Android/BSD/MacOS, the initial file position for reading is at the
              end of the file.

       The mode string can also include the letter 'b' either as a last character or as a character between  the
       characters  in any of the two-character strings described above.  This is strictly for compatibility with
       C89 and has no effect; the 'b' is ignored on all  POSIX  conforming  systems,  including  Linux.   (Other
       systems  may  treat text files and binary files differently, and adding the 'b' may be a good idea if you
       do I/O to a binary file and expect that your program may be ported to non-UNIX environments.)

       See NOTES below for details of glibc extensions for mode.

       Any created file will have the mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH (0666),  as
       modified by the process's umask value (see umask(2)).

       Reads  and writes may be intermixed on read/write streams in any order.  Note that ANSI C requires that a
       file positioning function intervene between output and input, unless an input operation  encounters  end-
       of-file.  (If this condition is not met, then a read is allowed to return the result of writes other than
       the most recent.)  Therefore it is good practice (and indeed sometimes necessary under Linux) to  put  an
       fseek(3)  or fgetpos(3) operation between write and read operations on such a stream.  This operation may
       be an apparent no-op (as in fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect).

       Opening a file in append mode (a as the first character of mode) causes all subsequent  write  operations
       to this stream to occur at end-of-file, as if preceded the call:

           fseek(stream, 0, SEEK_END);

       The  file  descriptor  associated with the stream is opened as if by a call to open(2) with the following
       flags:

              ┌─────────────┬───────────────────────────────┐
              │fopen() modeopen() flags                  │
              ├─────────────┼───────────────────────────────┤
              │     r       │ O_RDONLY                      │
              ├─────────────┼───────────────────────────────┤
              │     w       │ O_WRONLY | O_CREAT | O_TRUNC  │
              ├─────────────┼───────────────────────────────┤
              │     a       │ O_WRONLY | O_CREAT | O_APPEND │
              ├─────────────┼───────────────────────────────┤
              │     r+      │ O_RDWR                        │
              ├─────────────┼───────────────────────────────┤
              │     w+      │ O_RDWR | O_CREAT | O_TRUNC    │
              ├─────────────┼───────────────────────────────┤
              │     a+      │ O_RDWR | O_CREAT | O_APPEND   │
              └─────────────┴───────────────────────────────┘
   fdopen()
       The fdopen() function associates a stream with the existing file descriptor, fd.  The mode of the  stream
       (one  of  the  values  "r",  "r+",  "w",  "w+",  "a",  "a+") must be compatible with the mode of the file
       descriptor.  The file position indicator of the new stream is set to that belonging to fd, and the  error
       and end-of-file indicators are cleared.  Modes "w" or "w+" do not cause truncation of the file.  The file
       descriptor is not dup'ed, and will be closed when the stream created by fdopen() is closed.   The  result
       of applying fdopen() to a shared memory object is undefined.

   freopen()
       The  freopen() function opens the file whose name is the string pointed to by pathname and associates the
       stream pointed to by stream with it.  The original stream (if it exists) is closed.  The mode argument is
       used just as in the fopen() function.

       If the pathname argument is a null pointer, freopen() changes the mode of the stream to that specified in
       mode; that is, freopen() reopens the pathname that is associated with the stream.  The specification  for
       this behavior was added in the C99 standard, which says:

              In  this  case,  the  file descriptor associated with the stream need not be closed if the call to
              freopen() succeeds.  It is implementation-defined which changes of mode are  permitted  (if  any),
              and under what circumstances.

       The  primary  use  of the freopen() function is to change the file associated with a standard text stream
       (stderr, stdin, or stdout).

RETURN VALUE

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

ERRORS

       EINVAL The mode provided to fopen(), fdopen(), or freopen() was invalid.

       The fopen(), fdopen() and freopen() functions may also fail and set errno for any of the errors specified
       for the routine malloc(3).

       The fopen() function may also fail and set errno for any of the errors specified for the routine open(2).

       The fdopen() function may also fail and set errno for  any  of  the  errors  specified  for  the  routine
       fcntl(2).

       The  freopen()  function  may  also  fail  and set errno for any of the errors specified for the routines
       open(2), fclose(3), and fflush(3).

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌─────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├─────────────────────────────┼───────────────┼─────────┤
       │fopen(), fdopen(), freopen() │ Thread safety │ MT-Safe │
       └─────────────────────────────┴───────────────┴─────────┘

CONFORMING TO

       fopen(), freopen(): POSIX.1-2001, POSIX.1-2008, C89, C99.

       fdopen(): POSIX.1-2001, POSIX.1-2008.

NOTES

   Glibc notes
       The GNU C library allows the following extensions for the string specified in mode:

       c (since glibc 2.3.3)
              Do not make the open operation, or subsequent  read  and  write  operations,  thread  cancellation
              points.  This flag is ignored for fdopen().

       e (since glibc 2.7)
              Open  the  file  with the O_CLOEXEC flag.  See open(2) for more information.  This flag is ignored
              for fdopen().

       m (since glibc 2.3)
              Attempt to access the file using mmap(2),  rather  than  I/O  system  calls  (read(2),  write(2)).
              Currently, use of mmap(2) is attempted only for a file opened for reading.

       x      Open  the file exclusively (like the O_EXCL flag of open(2)).  If the file already exists, fopen()
              fails, and sets errno to EEXIST.  This flag is ignored for fdopen().

       In addition to the above characters, fopen() and freopen() support the following syntax in mode:

           ,ccs=string

       The given string is taken as the name of a coded character set and the stream is marked as wide-oriented.
       Thereafter,  internal  conversion  functions  convert  I/O  to and from the character set string.  If the
       ,ccs=string syntax is not specified, then the wide-orientation of the stream is determined by  the  first
       file operation.  If that operation is a wide-character operation, the stream is marked wide-oriented, and
       functions to convert to the coded character set are loaded.

BUGS

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

SEE ALSO

       open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3), open_memstream(3)

COLOPHON

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