Comparison of two C files to verify the requirement for aligned structs for direct IO

direct_io.c

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <stdint.h>

#define BUF_SIZE 4096

struct __attribute__((packed)) Header {
    char tag;           // 1 byte — forces misalignment of subsequent fields
    unsigned long id;   // 8 bytes, but at offset 1 due to packing
    char data[BUF_SIZE - 9];
};

int main() {
    // malloc returns ~16-byte aligned memory, NOT 4096-aligned
    struct Header *buf = malloc(sizeof(struct Header));
    if (!buf) { perror("malloc"); return 1; }

    memset(buf, 0, sizeof(*buf));
    buf->tag = 'X';
    buf->id = 42;
    strcpy(buf->data, "hello direct io");

    printf("buffer address: %p\n", (void *)buf);
    printf("buffer size:    %zu\n", sizeof(*buf));
    printf("aligned to 512? %s\n", ((uintptr_t)buf % 512 == 0) ? "yes" : "no");
    printf("aligned to 4096? %s\n", ((uintptr_t)buf % 4096 == 0) ? "yes" : "no");

    // open with O_DIRECT
    int fd = open("/tmp/direct_io_test.dat", O_CREAT | O_WRONLY | O_DIRECT, 0644);
    if (fd < 0) { perror("open"); free(buf); return 1; }

    ssize_t written = write(fd, buf, BUF_SIZE);
    if (written < 0) {
        printf("write failed: %s (errno=%d)\n", strerror(errno), errno);
    } else {
        printf("write succeeded: %zd bytes\n", written);
    }

    close(fd);
    free(buf);
    return 0;
}

/**
* Second half of the program
*/

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <stdint.h>

#define BUF_SIZE 4096
#define ALIGNMENT 4096

struct __attribute__((packed)) Header {
    char tag;
    unsigned long id;
    char data[BUF_SIZE - 9];
};

int main() {
    struct Header *buf = NULL;
    int ret = posix_memalign((void **)&buf, ALIGNMENT, sizeof(struct Header));
    if (ret != 0) { fprintf(stderr, "posix_memalign failed: %s\n", strerror(ret)); return 1; }

    memset(buf, 0, sizeof(*buf));
    buf->tag = 'X';
    buf->id = 42;
    strcpy(buf->data, "hello direct io");

    printf("buffer address: %p\n", (void *)buf);
    printf("buffer size:    %zu\n", sizeof(*buf));
    printf("aligned to 512? %s\n", ((uintptr_t)buf % 512 == 0) ? "yes" : "no");
    printf("aligned to 4096? %s\n", ((uintptr_t)buf % 4096 == 0) ? "yes" : "no");

    int fd = open("/tmp/direct_io_test_aligned.dat", O_CREAT | O_WRONLY | O_DIRECT, 0644);
    if (fd < 0) { perror("open"); free(buf); return 1; }

    ssize_t written = write(fd, buf, BUF_SIZE);
    if (written < 0) {
        printf("write failed: %s (errno=%d)\n", strerror(errno), errno);
    } else {
        printf("write succeeded: %zd bytes\n", written);
    }

    close(fd);
    free(buf);
    return 0;
}