fd_cache.c

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

typedef struct {
    char *path;
    int fd;
    int age;
} fd_entry_t;

typedef struct {
    fd_entry_t *items;
    int cap;
    int tick;
} fd_cache_t;

static void fd_cache_init(fd_cache_t *c, int cap)
{
    c->items = calloc(cap, sizeof(fd_entry_t));
    c->cap = cap;
    c->tick = 0;
}

static void fd_cache_close_entry(fd_entry_t *e)
{
    if (e->fd >= 0) {
        close(e->fd);
        e->fd = -1;
    }
    free(e->path);
    e->path = NULL;
    e->age = 0;
}

static void fd_cache_destroy(fd_cache_t *c)
{
    for (int i = 0; i < c->cap; i++) {
        fd_cache_close_entry(&c->items[i]);
    }
    free(c->items);
}

static int fd_cache_get(fd_cache_t *c, const char *path)
{
    c->tick++;
    for (int i = 0; i < c->cap; i++) {
        if (c->items[i].path && strcmp(c->items[i].path, path) == 0) {
            c->items[i].age = c->tick;
            return c->items[i].fd;
        }
    }
    int victim = 0;
    int oldest = c->tick + 1;
    for (int i = 0; i < c->cap; i++) {
        if (c->items[i].path == NULL) {
            victim = i;
            oldest = -1;
            break;
        }
        if (c->items[i].age < oldest) {
            oldest = c->items[i].age;
            victim = i;
        }
    }
    fd_cache_close_entry(&c->items[victim]);
    int fd = open(path, O_RDONLY);
    if (fd < 0) return -1;
    c->items[victim].path = strdup(path);
    c->items[victim].fd = fd;
    c->items[victim].age = c->tick;
    return fd;
}

static int cached_read_line(fd_cache_t *c, const char *path, char *buf, size_t sz)
{
    int fd = fd_cache_get(c, path);
    if (fd < 0) return -1;
    lseek(fd, 0, SEEK_SET);
    ssize_t n = read(fd, buf, sz - 1);
    if (n < 0) return -1;
    buf[n] = 0;
    return 0;
}

int main(int argc, char **argv)
{
    if (argc < 2) {
        fprintf(stderr, "usage: %s file1 [file2 ...]\n", argv[0]);
        return 1;
    }
    fd_cache_t cache;
    fd_cache_init(&cache, 4);
    char buf[256];
    for (int round = 0; round < 3; round++) {
        for (int i = 1; i < argc; i++) {
            if (cached_read_line(&cache, argv[i], buf, sizeof buf) == 0) {
                printf("[%d] %s: %.*s", round, argv[i], (int)strcspn(buf, "\n") + 1, buf);
            } else {
                perror("read");
            }
        }
    }
    fd_cache_destroy(&cache);
    return 0;
}