journal_kv.c

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>

typedef struct {
    uint32_t klen;
    uint32_t vlen;
} rec_hdr_t;

typedef struct {
    char *key;
    char *val;
} kv_t;

typedef struct {
    kv_t *items;
    size_t len;
    size_t cap;
} kv_map_t;

static void map_init(kv_map_t *m)
{
    m->items = NULL;
    m->len = 0;
    m->cap = 0;
}

static void map_set(kv_map_t *m, const char *k, const char *v)
{
    for (size_t i = 0; i < m->len; i++) {
        if (strcmp(m->items[i].key, k) == 0) {
            free(m->items[i].val);
            m->items[i].val = strdup(v);
            return;
        }
    }
    if (m->len == m->cap) {
        size_t ncap = m->cap ? m->cap * 2 : 8;
        kv_t *ni = realloc(m->items, ncap * sizeof(kv_t));
        if (!ni) return;
        m->items = ni;
        m->cap = ncap;
    }
    m->items[m->len].key = strdup(k);
    m->items[m->len].val = strdup(v);
    m->len++;
}

static void map_free(kv_map_t *m)
{
    for (size_t i = 0; i < m->len; i++) {
        free(m->items[i].key);
        free(m->items[i].val);
    }
    free(m->items);
}

static int journal_append(int fd, const char *k, const char *v)
{
    rec_hdr_t h;
    h.klen = (uint32_t)strlen(k);
    h.vlen = (uint32_t)strlen(v);
    if (write(fd, &h, sizeof h) != (ssize_t)sizeof h) return -1;
    if (write(fd, k, h.klen) != (ssize_t)h.klen) return -1;
    if (write(fd, v, h.vlen) != (ssize_t)h.vlen) return -1;
    if (fsync(fd) < 0) return -1;
    return 0;
}

static int journal_replay(int fd, kv_map_t *m)
{
    lseek(fd, 0, SEEK_SET);
    rec_hdr_t h;
    for (;;) {
        ssize_t n = read(fd, &h, sizeof h);
        if (n == 0) break;
        if (n != (ssize_t)sizeof h) return -1;
        char *k = malloc(h.klen + 1);
        char *v = malloc(h.vlen + 1);
        if (!k || !v) {
            free(k);
            free(v);
            return -1;
        }
        if (read(fd, k, h.klen) != (ssize_t)h.klen) {
            free(k);
            free(v);
            return -1;
        }
        if (read(fd, v, h.vlen) != (ssize_t)h.vlen) {
            free(k);
            free(v);
            return -1;
        }
        k[h.klen] = 0;
        v[h.vlen] = 0;
        map_set(m, k, v);
        free(k);
        free(v);
    }
    return 0;
}

int main(void)
{
    const char *path = "journal_kv.log";
    int fd = open(path, O_WRONLY | O_CREAT | O_APPEND, 0644);
    if (fd < 0) {
        perror("open");
        return 1;
    }
    journal_append(fd, "user", "alice");
    journal_append(fd, "mode", "debug");
    journal_append(fd, "user", "bob");
    close(fd);
    fd = open(path, O_RDONLY);
    if (fd < 0) {
        perror("open");
        return 1;
    }
    kv_map_t m;
    map_init(&m);
    if (journal_replay(fd, &m) < 0) {
        perror("journal_replay");
        close(fd);
        return 1;
    }
    close(fd);
    for (size_t i = 0; i < m.len; i++) {
        printf("%s=%s\n", m.items[i].key, m.items[i].val);
    }
    map_free(&m);
    return 0;
}