what is the c code to execute xeyes or xclock?

Question

which function do i use to give such linux terminal calls in my c program?

Answer 1

system would be the correct posix call. It takes a pointer to char as the command to be executed. See man 3 system. However system can be completely corrupted by environment variables and an harder-to-use alternative is exec (see here).

A little example to illustrate:

system("xeyes");
system("rm -rf $HOME"); /* never ever try this, really */