[中文版本](/posts/c教程/zh-CN/05-输入输出与文件/047-Command-Line Arguments (argc, argv))

Every C program can receive input directly from the command line, noscanf, nofgets, just arguments passed when you run the executable. This is how professional C tools (likegcc,ls, andgrep) receive filenames, options, and flags.

The Function Signature

Yourmain function can take two parameters:

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

Parameter	Meaning
argc	Argument count (number of command-line arguments)
argv	Argument vector (array of C strings, each argument)

argv[0] is the program name itself, andargv[1] onward are the user-provided arguments.

Example:

./hello world test

Then:

• argc == 3
• argv[0] = "./hello"
• argv[1] = "world"
• argv[2] = "test"

Tiny Code: Print Command-Line Arguments

#include <stdio.h>
int main(int argc, char *argv[]) {
    printf("Argument count: %d\n", argc);
    for (int i = 0; i < argc; i++)
        printf("argv[%d] = %s\n", i, argv[i]);
    return 0;
}

Run it:

./args foo bar 123

Output:

Argument count: 4
argv[0] = ./args
argv[1] = foo
argv[2] = bar
argv[3] = 123

Checking for Missing Arguments

If your program needs arguments, checkargc before accessing them.

#include <stdio.h>
int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
        return 1;
    }
    printf("Opening file: %s\n", argv[1]);
    return 0;
}

Run it:

./fileop

Output:

Usage: ./fileop <filename>

Run again:

./fileop data.txt

Output:

Opening file: data.txt

Converting String Arguments to Numbers

All command-line arguments are strings. To use them as numbers, convert using:

• atoi()– string to int
• atof()– string to float
• strtol()/strtod()– safer and more flexible alternatives

Example:

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
    if (argc != 3) {
        fprintf(stderr, "Usage: %s <a> <b>\n", argv[0]);
        return 1;
    }
    int a = atoi(argv[1]);
    int b = atoi(argv[2]);
    printf("%d + %d = %d\n", a, b, a + b);
    return 0;
}

Run:

./sum 10 25

Output:

10 + 25 = 35

Handling Options (Flags)

You can build simple command-line tools that handle options manually:

#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]) {
    int verbose = 0;
    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-v") == 0)
            verbose = 1;
    }
    if (verbose)
        printf("Verbose mode on\n");
    else
        printf("Run quietly\n");
    return 0;
}

Run:

./tool -v

Output:

Verbose mode on

Tiny Code: Mini File Echo Tool

This program prints the content of a file passed as an argument:

#include <stdio.h>
int main(int argc, char *argv[]) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <file>\n", argv[0]);
        return 1;
    }
    FILE *fp = fopen(argv[1], "r");
    if (!fp) {
        perror("Error");
        return 1;
    }
    char buf[128];
    while (fgets(buf, sizeof(buf), fp))
        printf("%s", buf);
    fclose(fp);
    return 0;
}

Run:

./echo mytext.txt

Why It Matters

argc andargv make your C programs scriptable and composable:

• Automate tasks from the command line
• Integrate with shell scripts or pipelines
• Process multiple input files
• Implement command-line flags and options

Every real-world C utility, fromls togcc, depends on this pattern.

Try It Yourself

Write a program that takes a list of integers and prints their sum.

Add a-r flag to reverse the order of printed arguments.

Build a “greet” tool:

./greet Alice Bob Charlie

→Hello, Alice! Hello, Bob! Hello, Charlie!

Write a “compare” tool that checks if two file names are identical.

Combineargc and file I/O: copy one file to another with

./copy source.txt dest.txt

With command-line arguments, your C programs evolve from static exercises to flexible, real-world tools. Next, you’ll explore reading configuration files, a powerful way to let your programs adapt automatically without recompilation.