The use of fork and exec together (gcc)

vim test_First.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int is_First(int value)
{
 int i,sum;
 sum=0;
 for(i=1;i<value+1;i++)
  if(value%i==0)
   ++sum;
 if(sum==2)
  return 1;
 else
  return 0;
}
int main(int argc, char *argv[])
{
 int i,j,k,result,sqrt_result;
 int how_many=atoi(argv[1]);
 i=0;
 j=1;
 do
 {
  if(is_First(j))
  {
   ++i;
   k=j;
  }
  ++j;
 }while(i<how_many);
 result=k;
 sqrt_result=sqrt(1.0*result);
 printf("%d,  (sqrt) = %f",result,sqrt(result));
 printf("\n");

 return 0;

}

gcc -o test_First test_First.c -lm

vim text_Opposite.c

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
 int i,j,size_Text;
 char *result;
 char *test;
 size_Text=strlen(argv[1]);
 test=malloc(sizeof(char)*size_Text);
 strcpy(test,argv[1]);
 test[size_Text+1]='\0';
 result=malloc(sizeof(char)*size_Text);

 result[size_Text]='\0';
 for(i=0;i<size_Text+1;i++)
  result[i]=test[i];
 for(i=0;i<size_Text+1;i++)
  result[i]=test[size_Text-(i+1)];
 printf("%s ",test);
 printf("\n"); 
 printf("%s ",result);
 printf("\n");
 free(test);
 free(result);
 printf("\n");


}

gcc -o text_Opposite text_Opposite.c

vim function.h

#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <stdio.h>


int test_FE(char *text, char **arg)
{
 pid_t child_P;
 child_P=fork();
 if(child_P!=0)
  return child_P;
 else
  {
   execvp(text,arg);
   fprintf(stderr,"execvp error\n");
   abort();
  }

}

vim main_prog.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include "function.h"


int main()
{

 char *ARG1[]={
 "./test_First","10","/",NULL};
 char *ARG2[]={
 "./text_Opposite","Example","/",NULL};
 test_FE("./test_First",ARG1);
 test_FE("./text_Opposite",ARG2);
 return 0;

}

gcc -o main_prog main_prog.c
./main_prog