/* sum.c */
#include <stdio.h>
#include <stdlib.h>
void printhelp(char progname[]);
 
int main(int argc, char *argv[])
{
    int i;
    int sum = 0;
 
    /* Simple sanity check */
    if (argc == 1)
    {
        printhelp(argv[0]);
        return 1;
    }
 
    for (i=1; i<argc; i++)
    {
        sum = sum + atoi(argv[i]);
    }
    printf("Total sum: %i\n", sum);
    return 0;
}
 
void printhelp(char progname[])
{
    printf("%s integer ...\n", progname);
    printf("This program takes any number of "
        "integer values and sums them up\n");
}

$ make sum
cc     sum.c   -o sum

$ ./sum
./sum integer ...
This program takes any number of integer values and sums them up

$ echo $?
1

$ ./sum 45 55 12
Total sum: 112

$ echo $?
0

* atoi() converts a string into an integer.
* atol() converts a string into a long integer.
* atoll() converts a string into a long long integer.
* atof() converts a string into a floating-point number (of type double)