fopen

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



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() mode open() 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).

       ┌─────────────────────────────┬───────────────┬─────────┐
       │Interface                    Attribute     Value   │
       ├─────────────────────────────┼───────────────┼─────────┤
       │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
       This page is part of release 5.02 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



GNU                               2019-05-09                          FOPEN(3)