fopen opens the file named by filename and associates a stream with it. fopen returns a pointer to be used to identify the stream in subsequent operations.
The mode string used in calls to fopen is one of the following values:
Value Description
r Open for reading only.
w Create for writing. If a file by that name already exists, it will be overwritten.
a Append; open for writing at end-of-file or create for writing if the file does not exist.
r+ Open an existing file for update (reading and writing).
w+ Create a new file for update (reading and writing). If a file by that name already exists, it will be overwritten.
a+ Open for append; open (or create if the file does not exist) for update at the end of the file.
To specify that a given file is being opened or created in text mode append a t to the mode string (rt w+t and so on). Similarly to specify binary mode append a b to the mode string (wb a+b and so on). fopen also allows the t or b to be inserted between the letter and the + character in the mode string; for example rt+ is equivalent to r+t.
If a t or b is not given in the mode string the mode is governed by the global variable _fmode. If _fmode is set to O_BINARY files are opened in binary mode. If _fmode is set to O_TEXT they are opened in text mode. These O_... constants are defined in fcntl.h.
When a file is opened for update, both input and output can be done on the resulting stream; however,
output cannot be directly followed by input without an intervening fseek or rewind
input cannot be directly followed by output without an intervening fseek, rewind, or an input that encounters end-offile
Return Value
On successful completion fopen returns a pointer to the newly opened stream. In the event of error it returns NULL.