Manpages - fopen.3

fopen, fdopen, freopen - stream open functions

  #include <stdio.h>

  FILE *fopen(const char *restrict pathname",constchar*restrict"mode);
  FILE *fdopen(int fd, const char *mode);
  FILE *freopen(const char *restrict pathname",constchar*restrict"mode,
   FILE *restrict stream);

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

*fdopen*():

      _POSIX_C_SOURCE

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

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

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).

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

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

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

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.

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.

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

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