|FOPEN(3)||Linux Programmer's Manual||FOPEN(3)|
NAME¶fopen, fdopen, freopen - stream open functions
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)):
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):
- Open text file for reading. The stream is positioned at the beginning of the file.
- Open for reading and writing. The stream is positioned at the beginning of the file.
- Truncate file to zero length or create text file for writing. The stream is positioned at the beginning of the file.
- 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.
- 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.
- 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.
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|
|w||O_WRONLY | O_CREAT | O_TRUNC|
|a||O_WRONLY | O_CREAT | O_APPEND|
|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.
- The mode provided to fopen(), fdopen(), or freopen() was invalid.
ATTRIBUTES¶For an explanation of the terms used in this section, see attributes(7).
|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.
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.
- 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().
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 4.16 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/.