Argument validation

Learn to validate command-line arguments.

Checking argument count

A program usually expects at least one argument at run time. By checking argc you catch missing input early and show the user a meaningful message.

Example: if (argc < 2) { printf("Error: missing argument\n"); return 1; }. Here argc < 2 is true when only the program name is present (no user argument). Print the error and exit with return 1 so the rest of the code never runs. This is the most common validation pattern in CLI tools and guides users toward correct usage.

Usage message

On error, instead of just saying "missing", you print a usage message showing how to run the program. This lets the user fix the mistake.

Example: printf("Usage: %s <file>\n", argv[0]);. Here argv[0] gives the program name so the message adapts to how the program was called. Keep the message short and clear: state the expected arguments' order and type. Well-designed usage messages are the backbone of tool documentation and steer users to the right command instead of trial and error.

Exercise Write argument validation

Declare a simulated count with int argc = 1;. Check with if (argc < 2); if true, print printf("Hata: argüman eksik\n"); and return 1;. Otherwise print printf("Tamam\n"); and end with return 0;.

Show example solution

#include <stdio.h>

int main(void) {
    int argc = 1;
    if (argc < 2) {
        printf("Hata: argüman eksik\n");
        return 1;
    }
    printf("Tamam\n");
    return 0;
}

Test yourself

When is argc < 2 true?
- When the user gives no argument
- When the user gives two arguments
- When the program fails to compile
- When argc equals zero
Show answer

When the user gives no argument — argc < 2 is true when only the program name is present (no user argument).
Match the validation parts

argc < 2

Missing-argument check
argv[0]

Program name
return 1

Error exit
printf

Message print

Complete the error message

printf("Hata: eksik %d
", ____);

Show answer

printf("Hata: eksik %d
", argc);

Next lesson: Exit codes

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