|FOPEN(3)||Linux Programmer's Manual||FOPEN(3)|
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);
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. 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|
|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|
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:
The primary use of the freopen() function is to change the file associated with a standard text stream (stderr, stdin, or stdout).
- 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).
|fopen (), fdopen (), freopen ()||Thread safety||MT-Safe|
fdopen(): POSIX.1-2001, POSIX.1-2008.
- 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().
In addition to the above characters, fopen() and freopen() support the following syntax in mode:
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.