c 2/5 25 XP

Command-Line Arguments

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Quick Reference

argc and argv:

// ./calculator 3 + 4
// argc = 4
// argv[0] = "./calculator"
// argv[1] = "3"
// argv[2] = "+"
// argv[3] = "4"
// argv[4] = NULL

Validation pattern:

int main(int argc, char *argv[])
{
    if (argc != 3)
    {
        fprintf(stderr, "Usage: %s input.txt output.txt\n", argv[0]);
        return 1;
    }
    // argv[1] and argv[2] are safe to access
}

String-to-number conversion:

Function	Use	Error handling
`atoi(s)`	String → int	Returns 0 on failure (silent)
`atof(s)`	String → double	Returns 0.0 on failure (silent)
`strtol(s, &end, 10)`	String → long	Reports errors properly

Java vs C:

Java	C
`args[0]` = first user arg	`argv[0]` = program name
`args.length` = user arg count	`argc` = total count (including program name)
`Integer.parseInt(args[0])`	`atoi(argv[1])`

Common Pitfalls

Off-by-one from Java — Java’s args[0] is the first user arg. C’s argv[0] is the program name. User args start at argv[1].
Forgetting argc check — Accessing argv[3] when argc is 2 reads past the array.
Arguments are strings — argv[1] is always char *. Use atoi/atof to convert.
Shell glob expansion — * expands to filenames. Quote or escape special characters.
atoi silent failure — atoi("hello") returns 0 without error. Validate input if it matters.

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