Changes to fopen function

The fopen function is usually used for opening a file and returning a file descriptor to that file. The concept of a file is very general in Unix, and by using the term file, we don't necessarily mean a file located on the filesystem. The fopen function has the following signatures:

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);

Code Box 12-2: Various signatures of the family of fopen functions

As you can see, all of the preceding signatures accept a mode input. This input parameter is a string that determines how the file should be opened. The following description in Shell Box 12-2 is obtained from the FreeBSD manual for the fopen function and explains how mode should be used:

$ man 3 fopen
...
The argument mode points to a string beginning with one of the following letters:
     "r"     Open for reading.  The...