Part III — Advanced: data structures, concurrency, networking

You can allocate memory and organize a program across files; this Part is where you build the structures that actually hold data — linked lists, trees, hash tables, and the Linux-kernel's intrusive-container trick — then add threads, atomics, and raw sockets on top. It ends with a thread pool and a multi-client chat server that exercises concurrency and networking together, not in isolation.

17. Bit manipulation and bit-fields

#include <stdio.h>
#include <stdint.h>

#define FLAG_READ    (1u << 0)
#define FLAG_WRITE   (1u << 1)
#define FLAG_EXECUTE (1u << 2)

int main(void) {
    uint32_t perms = FLAG_READ | FLAG_WRITE;   // set two flags

    if (perms & FLAG_WRITE) printf("writable\n");   // test a flag
    perms &= ~FLAG_WRITE;                            // clear a flag
    perms ^= FLAG_EXECUTE;                           // toggle a flag

    printf("%u\n", perms);

    // Real-world example: this is exactly how chmod-style permission bits,
    // network protocol flag fields, and hardware register access all work.
    return 0;
}

Bit-fields let a struct pack sub-byte fields, common in wire-format and hardware-register structs (layout is implementation-defined in order, which is why real protocol code prefers explicit shifting/masking over bit-fields for anything that crosses machine boundaries):

typedef struct {
    unsigned int is_admin : 1;
    unsigned int level    : 4;   // 0-15
    unsigned int reserved : 3;
} PermissionByte;   // fits in 1 byte total