Syntax Basics

A function has a return type, a name, and a parameter list. The compiler reads a file top to bottom, so calling a function before its definition requires a prototype (a declaration) earlier in the file.

#include <stdio.h>

/* prototypes: tell the compiler these functions exist */
int max_of(int a, int b);
void greet(const char *name);

int main(void) {
    greet("C");
    printf("%d\n", max_of(3, 9));
    return 0;
}

int max_of(int a, int b) {
    return a > b ? a : b;
}

void greet(const char *name) {
    printf("Hello, %s!\n", name);
}

Arguments are always passed by value: the function gets a copy, and changing a parameter does not affect the caller. To let a function modify the caller's variable, you pass a pointer, which chapter 6 covers in depth. For a function that takes no arguments, write int f(void); that is what states "no parameters" precisely.

Control flow

C offers if/else, switch, while, do-while, and for, plus break to leave a loop and continue to jump to the next iteration.

for (int i = 1; i <= 15; i++) {
    if (i % 15 == 0) {
        printf("FizzBuzz\n");
    } else if (i % 3 == 0) {
        printf("Fizz\n");
    } else if (i % 5 == 0) {
        printf("Buzz\n");
    } else {
        printf("%d\n", i);
    }
}

int day = 6;
switch (day) {
case 6:
case 7:
    printf("weekend\n");
    break;          /* without break, execution falls into the next case */
default:
    printf("weekday\n");
    break;
}

int n = 0;
do {
    n++;
} while (n < 3);    /* the body always runs at least once */

switch works only on integer values. Forgetting break makes execution fall through to the following case, so end every case with break unless fall-through is deliberate.

printf and scanf

printf substitutes its arguments into the conversion specifiers of a format string. A specifier that does not match the argument type is undefined behavior, so match them carefully.

scanf stores what it reads, so it needs the address of each variable (&). It returns the number of items successfully converted, and you should always check it. Note that scanf reads a double with %lf, unlike printf.

#include <stdio.h>

int main(void) {
    int age;
    double height;

    printf("Enter age and height: ");
    if (scanf("%d %lf", &age, &height) != 2) {
        fprintf(stderr, "invalid input\n");
        return 1;
    }
    printf("age %d, height %.1f\n", age, height);
    return 0;
}

For reading a line of text, prefer fgets(buf, sizeof buf, stdin), which respects the buffer size. A bare %s has no length limit and can overflow the buffer.

Summary

• Programs start at int main(void) or int main(int argc, char *argv[]).
• Declare variables with a type and initialize them immediately; integer division discards the fraction.
• Put prototypes before calls to functions defined later. Arguments are passed by value.
• End switch cases with break.
• Match printf specifiers to argument types exactly and always check what scanf returns.