问如何正确使用posix_memalign

EN

#define BUF_SIZE 256
#define SECTOR 512
#define FILE_SIZE 1024 * 1024 * 1024 //1G


int main( int argc, char **argv ){

    int fd, nr;
    char fl_nm[]={"/dev/nvme0n1p1"};

    char* aligned_buf_w = NULL;
    char* aligned_buf_r = NULL;

    void* ad = NULL;
    if (posix_memalign(&ad, SECTOR, BUF_SIZE)) {
        perror("posix_memalign failed"); exit (EXIT_FAILURE);
    }

    aligned_buf_w = (char *)(ad);


    ad = NULL;
    if (posix_memalign(&ad, SECTOR, BUF_SIZE)) {
        perror("posix_memalign failed"); exit (EXIT_FAILURE);
    }
    aligned_buf_r = (char *)(ad);

    memset(aligned_buf_w, '*', BUF_SIZE * sizeof(char));

    printf("BEFORE READ BEGIN\n");
    printf("\t aligned_buf_w::%ld\n",strlen(aligned_buf_w));
    printf("\t aligned_buf_r::%ld\n",strlen(aligned_buf_r));
    printf("BEFORE READ END\n");


    fd = open(fl_nm, O_RDWR | O_DIRECT);
    pwrite(fd, aligned_buf_w, BUF_SIZE, 0);


    //write error checking
    if(nr == -1){
        perror("[error in write 2]\n");
    }


    nr = pread(fd, aligned_buf_r, BUF_SIZE, 0);
    //read error checking
    if(nr == -1){
        perror("[error in read 2]\n");
    }


    printf("AFTER READ BEGIN\n");
    printf("\taligned_buf_r::%ld \n",strlen(aligned_buf_r));
    printf("AFTER READ END\n");


    //error checking for close process
    if(close(fd) == -1){
        perror("[error in close]\n");
    }else{
        printf("[succeeded in close]\n");
    }


    return  0;
}

BEFORE READ BEGIN
         aligned_buf_w::512
         aligned_buf_r::0
BEFORE READ END
AFTER READ BEGIN
        aligned_buf_r::512 
AFTER READ END
[succeeded in close]

BEFORE READ BEGIN
         aligned_buf_w::256
         aligned_buf_r::0
BEFORE READ END
[error in read 2]
: Invalid argument
AFTER READ BEGIN
        aligned_buf_r::0 
AFTER READ END
[succeeded in close]