Performing atomic writes using pwritev2() syscall

atomic_write.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/uio.h>
#include <errno.h>

#define RWF_ATOMIC	(0x00000020)

int main(int argc, char *argv[]) {
    int ret = 0;
    char *random_data;

    if (argc != 4) {
        printf("Usage: %s <file> <off (KB)> <len (KB)>\n", argv[0]);
        return 1;
    }
    char *filename  = argv[1];

    // Convert user-supplied buffer size (in KB) to bytes
    int off_in_KB = atoi(argv[2]);
    size_t off = off_in_KB * 1024;

    int len_in_KB = atoi(argv[3]);
    size_t len = len_in_KB * 1024;

    // Open a file for writing
    int fd = open(filename, O_DIRECT | O_WRONLY | O_CREAT | O_DSYNC, 0666);
    if (fd == -1) {
        ret = errno;
        perror("open");
        return ret;
    }

    // Allocate a buffer to read random data
    posix_memalign(&random_data, 512, len);
    if (random_data == NULL) {
        ret = errno;
        perror("malloc");
        close(fd);
        return ret;
    }


    // Read random data from /dev/urandom
    int urandom_fd = open("/dev/urandom", O_RDONLY);
    if (urandom_fd == -1) {
        ret = errno;
        perror("open /dev/urandom");
        free(random_data);
        close(fd);
        return ret;
    }

    ssize_t bytes_read = read(urandom_fd, random_data, len);
    if (bytes_read != len) {
        ret = errno;
        perror("read /dev/urandom");
        free(random_data);
        close(urandom_fd);
        close(fd);
        return ret;
    }

    // Create an iovec structure for the random data
    struct iovec iov;
    iov.iov_base = random_data;
    iov.iov_len = bytes_read;

    printf("iov.iov_base: %p\n", iov.iov_base);
    printf("iov.iov_len: %d\n", iov.iov_len);

    // Write the random data using pwritev2
    int flags = RWF_ATOMIC;
    ssize_t bytes_written = pwritev2(fd, &iov, 1, off, flags);
    if (bytes_written == -1) {
        ret = errno;
        perror("pwritev2");
        free(random_data);
        close(urandom_fd);
        close(fd);
        return ret;
    }

    printf("Wrote %zd bytes of random data to the file.\n", bytes_written);

    // Close file and /dev/urandom
    free(random_data);
    close(urandom_fd);
    close(fd);

    return 0;
}