Functions to do this are in standard library. You don't need to specify some library.
The most simple function way to execute an external program is just: system("/bin/ls");
popen() is another function to execute an external program while capturing the standard output of it into your own program.
The "real" stuff is: fork() in combination with one of the exec()-family of functions.
Please read these man pages for more info:
man system
man popen
man fork
man exec
And here's a freely downloadable book which has an entire chapter on the subject:
http://www.advancedlinuxprogramming.com/
You could also search this forum, as it has been asked and answered many times before...