How to read command-line arguments in C ? (argc, argv tutorial)

The main function in C receives command line parameters through argc and argv: argc is the number of parameters (at least 1, including the program name), argv is an array of pointers pointing to each parameter string, argv[argc] is a null pointer, and argc must be checked before accessing to prevent out-of-bounds.

In C , command-line arguments are passed to the main function via two parameters: argc (argument count) and argv (argument vector). argc is an integer telling you how many arguments were supplied, including the program name. argv is an array of C-style strings ( char* ) pointing to each argument.

Understanding argc and argv

argc is always at least 1 — because argv[0] holds the program's name or path (as invoked). Each subsequent element ( argv[1] , argv[2] , etc.) corresponds to the arguments typed after the program name. The array ends with a null pointer ( argv[argc] == nullptr ), so it's safe to loop from 0 to argc - 1 .

• ./myapp hello world → argc = 3 , argv[0] = "./myapp" , argv[1] = "hello" , argv[2] = "world"
• ./myapp -v --input=file.txt → argc = 4 ; no automatic parsing — you handle splitting and meaning

Basic access and safety checks

Always check argc before accessing argv[i] to avoid undefined behavior. For example, if your program expects at least one user-provided argument, verify argc > 1 .

• Use if (argc \n"; return 1; }
• Convert argv[i] to std::string for easier manipulation: std::string arg(argv[1]);
• Remember: argv elements are null-terminated C strings — safe to pass to C library functions like strcmp or atoi , but prefer C alternatives ( std::stoi , std::string::compare ) when possible

Simple argument parsing pattern

For basic flag-based usage (eg, -h , --help , -o output.txt ), iterate through argv starting at index 1 and inspect each string:

• Check argv[i][0] == '-' to detect flags
• Handle short options like -v ( argv[i][1] == 'v' ) and long options like --verbose using std::string(argv[i]).compare("--verbose") == 0
• If a flag expects a value (eg, -o file.txt ), ensure i 1 before reading <code>argv[i 1]

Modern alternatives (C 17 and beyond)

While argc/argv remains standard, consider wrapping them for clarity and safety:

• Create a std::vector<:string></:string> from argv : std::vector<:string> args(argv, argv argc);</:string>
• Use libraries like Poco , argparse , or argh for robust, declarative parsing
• For simple cases, structured loops with range-based for over a vector are more readable than raw pointer arithmetic

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