C Fundamentals

After this lesson, you will be able to: Use C's data types, operators, control flow, functions, and main signature.

Basic syntax. C is small; the danger is in details.

Prerequisites:C Setup

Types + operators + control flow

Read each line.

tsx

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    // Primitive types, sizes are platform-dependent, EXCEPT stdint.h
    int n = 42;                 // typically 32-bit
    long ln = 100000L;          // at least 32-bit
    long long lln = 999LL;       // at least 64-bit
    float f = 3.14f;
    double d = 3.14159;
    char c = 'A';                // 1 byte; really a small int
    bool b = true;                // from stdbool.h

    // Use stdint.h for portable widths
    // int32_t, int64_t, uint8_t, uint64_t, exact sizes

    // Operators
    int sum = n + 8;
    int rem = n % 5;
    int isEven = (n % 2 == 0);

    // Control flow
    if (n > 0) printf("pos\n");
    else if (n == 0) printf("zero\n");
    else printf("neg\n");

    for (int i = 0; i < 5; i++) printf("%d\n", i);

    int j = 0;
    while (j < 5) { printf("%d\n", j); j++; }

    switch (n) {
        case 0: printf("zero\n"); break;
        case 42: printf("answer\n"); break;
        default: printf("other\n");
    }

    return 0;
}

Functions + main signatures

Declare before use; main can take args.

tsx

#include <stdio.h>

// Forward declaration
int add(int a, int b);

int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "usage: %s <name>\n", argv[0]);
        return 1;
    }
    printf("Hello, %s\n", argv[1]);
    printf("2 + 3 = %d\n", add(2, 3));
    return 0;
}

int add(int a, int b) {
    return a + b;
}

printf format specifiers

%d / %i, int. %u, unsigned. %ld, long. %lld, long long. %f, float/double. %.2f, 2 decimals. %c, char. %s, string. %p, pointer. %x, hex. %%, literal percent. Mismatched format + type is undefined behavior. Always match: `printf("%d\n", 3.14)` is wrong.

Common mistakes only experienced C devs avoid

Using `int` for sizes (use `size_t`). Mismatched printf format and type. Integer overflow without thinking. INT_MAX + 1 is undefined behavior in C. Forgetting `void main()` is non-standard. Use `int main(void)` or `int main(int argc, char *argv[])`.

Quick Check

What's wrong with `printf("%d\n", 3.14);`?

Pick the bug.

NothingFormat specifier %d expects int; passing double = undefined behaviorIt's slowCompile error

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