Command line arguments allow you to pass input to a C program when you run it from the terminal. Instead of hardcoding values, the user can provide them at runtime:
./myprogram hello 42 3.14C receives these arguments through two special parameters of main():
int argc - argument count: total number of arguments (including the program name itself).char *argv[] - argument vector: array of strings, where argv[0] is the program name.| Index | Value | Description |
|---|---|---|
argv[0] | "./myprogram" | Program name (always present) |
argv[1] | "hello" | First user argument |
argv[2] | "42" | Second user argument (string!) |
argv[argc] | NULL | Always NULL - marks end of array |
#include <stdio.h>
int main(int argc, char *argv[]) {
printf("Program name: %s\n", argv[0]);
printf("Argument count: %d\n", argc);
printf("\nAll arguments:\n");
for (int i = 0; i < argc; i++) {
printf(" argv[%d] = \"%s\"\n", i, argv[i]);
}
return 0;
}
/*
Run: ./args hello world 123
Output:
Program name: ./args
Argument count: 4
All arguments:
argv[0] = "./args"
argv[1] = "hello"
argv[2] = "world"
argv[3] = "123"
*/Since all arguments are strings, use the standard library functions to convert them:
| Function | Header | Converts to | Example |
|---|---|---|---|
atoi(str) | stdlib.h | int | atoi("42") -> 42 |
atof(str) | stdlib.h | double | atof("3.14") -> 3.14 |
atol(str) | stdlib.h | long | atol("100000") -> 100000 |
strtol(str, &end, base) | stdlib.h | long | Safer, detects errors |
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[]) {
// Usage: ./calc 10 + 5
if (argc != 4) {
printf("Usage: %s <num1> <op> <num2>\n", argv[0]);
printf("Example: ./calc 10 + 5\n");
return 1;
}
double a = atof(argv[1]);
char op = argv[2][0];
double b = atof(argv[3]);
double result;
switch (op) {
case '+': result = a + b; break;
case '-': result = a - b; break;
case '*': result = a * b; break;
case '/':
if (b == 0) { printf("Error: division by zero\n"); return 1; }
result = a / b;
break;
default:
printf("Unknown operator: %c\n", op);
return 1;
}
printf("%.2f %c %.2f = %.2f\n", a, op, b, result);
return 0;
}
/*
Run: ./calc 10 + 5 -> 10.00 + 5.00 = 15.00
Run: ./calc 7 / 2 -> 7.00 / 2.00 = 3.50
Run: ./calc 3 * 4 -> 3.00 * 4.00 = 12.00
*/Always validate argc before accessing argv elements. Accessing argv[1] when argc == 1 is undefined behaviour.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
int main(int argc, char *argv[]) {
if (argc < 2) {
fprintf(stderr, "Error: no arguments provided\n");
fprintf(stderr, "Usage: %s <number>\n", argv[0]);
return EXIT_FAILURE;
}
// Use strtol for safe integer conversion (detects errors)
char *endptr;
errno = 0;
long num = strtol(argv[1], &endptr, 10);
if (errno != 0 || *endptr != '\0') {
fprintf(stderr, "Error: '%s' is not a valid integer\n", argv[1]);
return EXIT_FAILURE;
}
printf("You entered: %ld\n", num);
printf("Squared: %ld\n", num * num);
return EXIT_SUCCESS;
}
/*
Run: ./validate 7 -> You entered: 7 / Squared: 49
Run: ./validate abc -> Error: 'abc' is not a valid integer
Run: ./validate -> Error: no arguments provided
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