NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       fopen, fdopen, freopen - stream open functions

SYNOPSIS         top

       #include <stdio.h>

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

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

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

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

       fdopen(): _POSIX_C_SOURCE

DESCRIPTION         top

       The fopen() function opens the file whose name is the string pointed
       to by path 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.  The initial file
              position for reading is at the beginning of the file, but
              output is always appended to 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 files will have 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 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.

       The freopen() function opens the file whose name is the string
       pointed to by path 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.  The primary use of
       the freopen() function is to change the file associated with a
       standard text stream (stderr, stdin, or stdout).

RETURN VALUE         top

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

ERRORS         top

       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         top

       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         top

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

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

NOTES         top

   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         top

       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         top

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

COLOPHON         top

       This page is part of release 4.07 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                              2016-03-15                         FOPEN(3)