atomic_replace.c

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

struct hdr { uint32_t magic; uint32_t version; uint64_t len; };

static int atomic_replace(const char *path, const void *buf, size_t len, uint32_t version)
{
    char tmp[4096];
    snprintf(tmp, sizeof tmp, "%s.tmp.%ld", path, (long)getpid());
    int fd = open(tmp, O_WRONLY | O_CREAT | O_TRUNC | O_EXCL, 0644);
    if (fd < 0) return -1;
    struct hdr h;
    h.magic = 0xA0A0F11E;
    h.version = version;
    h.len = len;
    ssize_t n = write(fd, &h, sizeof h);
    if (n != (ssize_t)sizeof h) goto fail;
    n = write(fd, buf, len);
    if (n != (ssize_t)len) goto fail;
    if (fsync(fd) < 0) goto fail;
    if (close(fd) < 0) goto fail;
    if (rename(tmp, path) < 0) goto fail;
    char dir[4096];
    strncpy(dir, path, sizeof dir);
    dir[sizeof dir - 1] = 0;
    char *slash = strrchr(dir, '/');
    if (slash) {
        *slash = 0;
        int dfd = open(dir, O_RDONLY | O_DIRECTORY);
        if (dfd >= 0) {
            fsync(dfd);
            close(dfd);
        }
    }
    return 0;
fail:
    {
        int e = errno;
        close(fd);
        unlink(tmp);
        errno = e;
        return -1;
    }
}

int main(int argc, char **argv)
{
    if (argc < 3) return 1;
    const char *path = argv[1];
    const char *msg = argv[2];
    if (atomic_replace(path, msg, strlen(msg), 1) < 0) {
        perror("atomic_replace");
        return 1;
    }
    return 0;
}