Command-Line Arguments in C: argc, argv, and Parsing User Input at Startup

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Lesson 33Mixed lesson

Command-Line Arguments in C: argc, argv, and Parsing User Input at Startup

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Lesson format13 sections1 video5 code blocks1 practice itemUpdated Aug 27, 2026

main is not required to take zero parameters. The standard defines two valid signatures: int main(void) and int main(int argc, char *argv[]). The second form gives you access to whatever the user typed after the program name. argc is the count. argv is an array of C strings. argv[0] is the program name itself. argv[argc] is guaranteed to be NULL. These are your program's first contact with the outside world.

Command Line Arguments | C Programming Tutorial

#Iterating Over Arguments#link

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Program name: %s\n", argv[0]);
    printf("Argument count: %d\n", argc);

    for (int i = 1; i < argc; i++) {
        printf("argv[%d] = \"%s\"\n", i, argv[i]);
    }

    return 0;
}

Run args_demo with sample arguments

root@vulnarex:~#gcc -Wall args_demo.c -o args_demo && ./args_demo hello world "two words" 42

Quoted strings in the shell become a single argv entry. "two words" is one argument, not two. The shell handles the parsing before your program ever sees it. Your program receives the already-split array. Every argument is a string. If you need an integer, you convert it with atoi or strtol. There is no type information. It is all text.

#Validating and Converting Arguments#link

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    if (argc != 3) {
        fprintf(stderr, "Usage: %s <filename> <count>\n", argv[0]);
        return 1;
    }

    const char *filename = argv[1];
    char *endptr;
    long count = strtol(argv[2], &endptr, 10);

    if (*endptr != '\0' || count <= 0) {
        fprintf(stderr, "Error: count must be a positive integer\n");
        return 1;
    }

    printf("File: %s, Count: %ld\n", filename, count);
    return 0;
}

Run args_validate with valid input

root@vulnarex:~#gcc -Wall args_validate.c -o args_validate && ./args_validate data.txt 5

Run args_validate with invalid input

root@vulnarex:~#./args_validate data.txt abc

strtol is preferred over atoi because it reports conversion errors via the endptr. atoi returns 0 on failure, which is indistinguishable from a legitimate input of "0". strtol lets you check whether the entire string was consumed. The usage message on stderr follows Unix convention: show the user what they should have typed, and exit with a nonzero status.

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quiz BLOCK (★ 20 XP)

What is argv[0] conventionally set to?

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Command-line arguments are the simplest interface between a user and your program. Every Unix tool, every build script, every cron job passes arguments. Getting the parsing right, with proper validation and usage messages, is the difference between a tool people trust and one they curse. Next: remove, for deleting files from within C.

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Iterating Over Arguments Validating and Converting Arguments

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August 27, 2026

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Laboratory sanity code

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#include <stdio.h>

int main(int argc, char *argv[]) {

printf("Program name: %s\n", argv[0]);

printf("Argument count: %d\n", argc);

for (int i = 1; i < argc; i++) {

printf("argv[%d] = \"%s\"\n", i, argv[i]);

}

return 0;

}

#include <stdio.h>

#include <stdlib.h>

int main(int argc, char *argv[]) {

if (argc != 3) {

fprintf(stderr, "Usage: %s <filename> <count>\n", argv[0]);

return 1;

}

const char *filename = argv[1];

char *endptr;

long count = strtol(argv[2], &endptr, 10);

if (*endptr != '\0' || count <= 0) {

fprintf(stderr, "Error: count must be a positive integer\n");

return 1;

}

printf("File: %s, Count: %ld\n", filename, count);

return 0;

}