The UNIX manual has an entry for all available functions. Function documentation is stored in section 3 of the manual, and there are many other useful system calls in section 2. If you already know the name of the function you want, you can read the page by typing (to find about strcat).
man 3 strcat
If you don't know the name of the function, a full list is included in the introductory page for section 3 of the manual. To read this, type
man 3 intro
There are approximately 700 functions described here. This number tends to increase with each upgrade of the system.
On any manual page, the SYNOPSIS section will include information on the use of the function. For example
#include <time.h> char *ctime(time_t *clock)
This means that you must have
in your file before you call ctime. And that function ctime takes a pointer to type time_t as an argument, and returns a string (char *). time_t will probably be defined in the same manual page.
The DESCRIPTION section will then give a short description of what the function does. For example
ctime() converts a long integer, pointed to by clock, to a 26-character string of the form produced by asctime().
Further related reading is suggested in the SEE ALSO section.
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