Types, Functions, and Control Flow in C

Programming FoundationsLesson 6 of 875 minutesLab included

Learning objectives

By the end of this lesson

• Declare and print common integer types.
• Trace values through a C function and loop.
• Validate argc before using argv.
• Convert text with error checking and return meaningful status codes.

C makes representation harder to ignore

#include <stdio.h>

int sum_to(int limit) {
    int total = 0;
    for (int value = 1; value <= limit; value++) {
        total += value;
    }
    return total;
}

int main(void) {
    printf("%d\n", sum_to(5));
    return 0;
}

Compile with warnings and trace value and total for every iteration. C requires declared types and does not protect every operation from overflow or invalid conversion.

Validate command-line shape first

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

int main(int argc, char **argv) {
    if (argc != 2) {
        fprintf(stderr, "usage: %s number\n", argv[0]);
        return 2;
    }

    char *end = NULL;
    errno = 0;
    long value = strtol(argv[1], &end, 10);
    if (errno != 0 || end == argv[1] || *end != '\0') {
        fprintf(stderr, "invalid integer\n");
        return 3;
    }

    printf("value=%ld\n", value);
    return 0;
}

argc counts arguments including the program name. argv contains pointers to their strings. strtol reports where parsing stopped, allowing the program to reject 12oops instead of silently accepting 12.

Test no argument, 12, 12oops, and a very large number. After each run, print $? to record the exit status.

Knowledge check

Question 1 of 2

Why check argc before argv[1]?

The second argument may not existargc converts text automaticallyargv[1] always contains the filename

Check answer

What this gives us as researchers

You can trace explicit C values and identify validation assumptions around arguments and conversion. Next we add arrays, strings, addresses, and pointers.