fopen

• FILE *fopen(char *filename, char *mode ); tries to open the file named filename. If sucessful, it returns a pointer to the file. If unsucessful, it returns NULL.

• The mode can be any of the following depending on what you want to do):

    Mode        Meaning
    ----        -------
     r          Opens file for reading
     w          Creates a new file (or erases an existing one)
                for writing
     a          Appends (opens for writing at end of file)
     b          Specifies that file contains binary data
     +          Specifies that file is being opened for update
                (both reading and writing)
  

  Mode parameters can be combined: for example, "r+b" and "a+" are both valid file modes.

• Binary files need to be handled specially on some systems because the usual end of file character (ctrl-d) may be used to store data. Also, on some systems text files have their carriage return handled differently.

• The filename can either be something like "foobar" in which case it is assumed to be a file the in the current directory, "/usr/local/tmp/foobar", or "foo/bar/README".

• When you are done using the file, call the fclose(FILE *) function. While it is true that exit() automatically closes all open files, it is still good programming practice to close all open files explicitly. In particular, most operating systems have a limit on the number of files a program can have open at the same time.