FILE * fopen ( const char * filename, const char * mode );
Opens the file whose name is specified in the parameter filename and
associates it with a stream that can be identified in future operations
by the FILE object whose pointer is returned. The operations that are
allowed on the stream and how these are performed are defined by
the mode parameter.
The running environment supports at least
FOPEN_MAX files open
FOPEN_MAX is a macro constant defined in
C string containing the name of the file to be opened. This paramenter
must follow the file name specifications of the running environment and
can include a path if the system supports it.
C string containing a file access modes. It can be:
|"r"||Open a file for reading. The file must exist.|
|"w"||Create an empty file for writing. If a file with the same name already exists its content is erased and the file is treated as a new empty file.|
|"a"||Append to a file. Writing operations append data at the end of the file. The file is created if it does not exist.|
|"r+"||Open a file for update both reading and writing. The file must exist.|
|"w+"||Create an empty file for both reading and writing. If a file with the same name already exists its content is erased and the file is treated as a new empty file.|
|"a+"||Open a file for reading and appending. All writing operations are performed at the end of the file, protecting the previous content to be overwritten. You can reposition (fseek, rewind) the internal pointer to anywhere in the file for reading, but writing operations will move it back to the end of file. The file is created if it does not exist.|
With the mode specifiers above the file is open as a text file. In
order to open a file as a binary file, a
"b" character has to be
included in the mode string. This additional
"b" character can
either be appended at the end of the string (thus making the following
compound modes: “rb”, “wb”, “ab”, “r+b”, “w+b”, “a+b”) or be inserted
between the letter and the
"+" sign for the mixed modes (“rb+”, “wb+”,
Additional characters may follow the sequence, although they should have
no effect. For example,
"t" is sometimes appended to make explicit the
file is a text file.
In the case of text files, depending on the environment where the
application runs, some special character conversion may occur in
input/output operations to adapt them to a system-specific text file
format. In many environments, such as most UNIX-based systems, it makes
no difference to open a file as a text file or a binary file; Both are
treated exactly the same way, but differentiation is recommended for a
For the modes where both read and writing (or appending) are allowed
(those which include a
"+" sign), the stream should be flushed
(fflush) or repositioned
(fseek, fsetpos, rewind)
between either a reading operation followed by a writing operation or a
writing operation followed by a reading operation.
If the file has been successfully opened the function will return a
pointer to a FILE object that is used
to identify the stream on all further operations involving it.
Otherwise, a null pointer is returned.