Command-Line Arguments in C

C programs can accept arguments from the command line using argc and argv parameters of the main function.

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Syntax

int main(int argc, char *argv[]) {
    /* argc = argument count */
    /* argv = argument vector (array of strings) */
}

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Example 1: Print All Arguments

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Number of arguments: %d\n", argc);
    for (int i = 0; i < argc; i++) {
        printf("argv[%d] = %s\n", i, argv[i]);
    }
    return 0;
}

Running:

./program hello world 123

Output:

Number of arguments: 4
argv[0] = ./program
argv[1] = hello
argv[2] = world
argv[3] = 123

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Example 2: Add Two Numbers from Command Line

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

int main(int argc, char *argv[]) {
    if (argc != 3) {
        printf("Usage: %s num1 num2\n", argv[0]);
        return 1;
    }
    
    int a = atoi(argv[1]);
    int b = atoi(argv[2]);
    printf("Sum = %d\n", a + b);
    
    return 0;
}

Running:

./add 5 7
Sum = 12

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Example 3: File Processing

#include <stdio.h>

int main(int argc, char *argv[]) {
    if (argc < 2) {
        printf("Usage: %s filename\n", argv[0]);
        return 1;
    }
    
    FILE *fp = fopen(argv[1], "r");
    if (!fp) {
        printf("Cannot open: %s\n", argv[1]);
        return 1;
    }
    
    char line[200];
    while (fgets(line, sizeof(line), fp)) {
        printf("%s", line);
    }
    fclose(fp);
    return 0;
}

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Notes

• All command-line arguments are received as strings (char *)
• Use atoi(), atof(), atol() from stdlib.h to convert to numbers
• Always validate argc before accessing argv to avoid out-of-bounds access