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12.3 Opening Streams

Opening a file with the fopen function creates a new stream and establishes a connection between the stream and a file. This may involve creating a new file.

Everything described in this section is declared in the header file stdio.h.

— Function: FILE * fopen (const char *filename, const char *opentype)

Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd lock | See POSIX Safety Concepts.

The fopen function opens a stream for I/O to the file filename, and returns a pointer to the stream.

The opentype argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters:

r
Open an existing file for reading only.
w
Open the file for writing only. If the file already exists, it is truncated to zero length. Otherwise a new file is created.
a
Open a file for append access; that is, writing at the end of file only. If the file already exists, its initial contents are unchanged and output to the stream is appended to the end of the file. Otherwise, a new, empty file is created.
r+
Open an existing file for both reading and writing. The initial contents of the file are unchanged and the initial file position is at the beginning of the file.
w+
Open a file for both reading and writing. If the file already exists, it is truncated to zero length. Otherwise, a new file is created.
a+
Open or create file for both reading and appending. If the file exists, its initial contents are unchanged. Otherwise, a new file is created. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

As you can see, ‘+’ requests a stream that can do both input and output. When using such a stream, you must call fflush (see Stream Buffering) or a file positioning function such as fseek (see File Positioning) when switching from reading to writing or vice versa. Otherwise, internal buffers might not be emptied properly.

Additional characters may appear after these to specify flags for the call. Always put the mode (‘r’, ‘w+’, etc.) first; that is the only part you are guaranteed will be understood by all systems.

The GNU C Library defines additional characters for use in opentype:

c
The file is opened with cancellation in the I/O functions disabled.
e
The underlying file descriptor will be closed if you use any of the exec... functions (see Executing a File). (This is equivalent to having set FD_CLOEXEC on that descriptor. See Descriptor Flags.)
m
The file is opened and accessed using mmap. This is only supported with files opened for reading.
x
Insist on creating a new file—if a file filename already exists, fopen fails rather than opening it. If you use ‘x’ you are guaranteed that you will not clobber an existing file. This is equivalent to the O_EXCL option to the open function (see Opening and Closing Files).

The ‘x’ modifier is part of ISO C11.

The character ‘b’ in opentype has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including GNU systems). If both ‘+’ and ‘b’ are specified, they can appear in either order. See Binary Streams.

If the opentype string contains the sequence ,ccs=STRING then STRING is taken as the name of a coded character set and fopen will mark the stream as wide-oriented with appropriate conversion functions in place to convert from and to the character set STRING. Any other stream is opened initially unoriented and the orientation is decided with the first file operation. If the first operation is a wide character operation, the stream is not only marked as wide-oriented, also the conversion functions to convert to the coded character set used for the current locale are loaded. This will not change anymore from this point on even if the locale selected for the LC_CTYPE category is changed.

Any other characters in opentype are simply ignored. They may be meaningful in other systems.

If the open fails, fopen returns a null pointer.

When the sources are compiling with _FILE_OFFSET_BITS == 64 on a 32 bit machine this function is in fact fopen64 since the LFS interface replaces transparently the old interface.

You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. See Stream/Descriptor Precautions. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. See File Locks.

— Function: FILE * fopen64 (const char *filename, const char *opentype)

Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd lock | See POSIX Safety Concepts.

This function is similar to fopen but the stream it returns a pointer for is opened using open64. Therefore this stream can be used even on files larger than 2^31 bytes on 32 bit machines.

Please note that the return type is still FILE *. There is no special FILE type for the LFS interface.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name fopen and so transparently replaces the old interface.

— Macro: int FOPEN_MAX

The value of this macro is an integer constant expression that represents the minimum number of streams that the implementation guarantees can be open simultaneously. You might be able to open more than this many streams, but that is not guaranteed. The value of this constant is at least eight, which includes the three standard streams stdin, stdout, and stderr. In POSIX.1 systems this value is determined by the OPEN_MAX parameter; see General Limits. In BSD and GNU, it is controlled by the RLIMIT_NOFILE resource limit; see Limits on Resources.

— Function: FILE * freopen (const char *filename, const char *opentype, FILE *stream)

Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe corrupt fd | See POSIX Safety Concepts.

This function is like a combination of fclose and fopen. It first closes the stream referred to by stream, ignoring any errors that are detected in the process. (Because errors are ignored, you should not use freopen on an output stream if you have actually done any output using the stream.) Then the file named by filename is opened with mode opentype as for fopen, and associated with the same stream object stream.

If the operation fails, a null pointer is returned; otherwise, freopen returns stream.

freopen has traditionally been used to connect a standard stream such as stdin with a file of your own choice. This is useful in programs in which use of a standard stream for certain purposes is hard-coded. In the GNU C Library, you can simply close the standard streams and open new ones with fopen. But other systems lack this ability, so using freopen is more portable.

When the sources are compiling with _FILE_OFFSET_BITS == 64 on a 32 bit machine this function is in fact freopen64 since the LFS interface replaces transparently the old interface.

— Function: FILE * freopen64 (const char *filename, const char *opentype, FILE *stream)

Preliminary: | MT-Safe | AS-Unsafe corrupt | AC-Unsafe corrupt fd | See POSIX Safety Concepts.

This function is similar to freopen. The only difference is that on 32 bit machine the stream returned is able to read beyond the 2^31 bytes limits imposed by the normal interface. It should be noted that the stream pointed to by stream need not be opened using fopen64 or freopen64 since its mode is not important for this function.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name freopen and so transparently replaces the old interface.

In some situations it is useful to know whether a given stream is available for reading or writing. This information is normally not available and would have to be remembered separately. Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C Library.

— Function: int __freadable (FILE *stream)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The __freadable function determines whether the stream stream was opened to allow reading. In this case the return value is nonzero. For write-only streams the function returns zero.

This function is declared in stdio_ext.h.

— Function: int __fwritable (FILE *stream)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The __fwritable function determines whether the stream stream was opened to allow writing. In this case the return value is nonzero. For read-only streams the function returns zero.

This function is declared in stdio_ext.h.

For slightly different kind of problems there are two more functions. They provide even finer-grained information.

— Function: int __freading (FILE *stream)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The __freading function determines whether the stream stream was last read from or whether it is opened read-only. In this case the return value is nonzero, otherwise it is zero. Determining whether a stream opened for reading and writing was last used for writing allows to draw conclusions about the content about the buffer, among other things.

This function is declared in stdio_ext.h.

— Function: int __fwriting (FILE *stream)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The __fwriting function determines whether the stream stream was last written to or whether it is opened write-only. In this case the return value is nonzero, otherwise it is zero.

This function is declared in stdio_ext.h.