fopen(3) — Linux manual page

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 *restrict pathname, const char *restrict mode);
       FILE *fdopen(int fd, const char *mode);
       FILE *freopen(const char *restrict pathname, const char *restrict mode,
                     FILE *restrict 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 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         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 5.13 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                            2021-03-22                       FOPEN(3)

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