I've created a c program and compiled it with gcc, in unix.
The file name is abc.c and it is run by typing the command ./abc

I have another program which creates a child process, and I need this abc program to run on that child process. I've tried execvp(), but it doesn't work.

How can I run such a file on a child process?

I'm a bit confused. Which program is wanting to run which? You want "another program" to run "abc", right? What is this "other program"?

#define PNAME  "/fullpath/to/abc"
#define ARGS NULL
...
int status;.
if (  (child = fork()) == 0) {
     execl(PNAME,basename(PNAME),ARGS);
} else {
     waitpid(child,&status,0);
     /*test for exit status of child process here with the WIF macros if important*/
}