c语言write与read函数读取写入结构体到文件问题

Question

将结构体赋值后写入文件，再用read函数读取到结构体指针中，并没有获取到打印信息，请教这是怎么回事！！

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>

struct flow
{
    unsigned int num;
    char name[32];
    unsigned char metric;
};

int main(int argc, const char *argv[])
{
    int fd = open("./open.txt",O_RDWR | O_CREAT | O_TRUNC,0666);
    if(fd == -1)
    {
        perror("open failed");
        return -1;
    }

    struct flow t;
    memset(&t, 0, sizeof(struct flow));
    t.num = 5;
    strcpy(t.name, "test");
    t.metric = 127;
    write(fd, &t, sizeof(struct flow));

       struct flow *p = NULL;
    p = (struct flow*)malloc(sizeof(struct flow)); 

    printf("ret1: %d\n", ret1);

    //读取
    int ret1 = lseek (fd, 0, 0);
    if (read(fd, p, sizeof(struct flow) == -1))
    {
        printf("read error\n");
        exit(1);
    }

    //没有打印信息，貌似没有读取成功
    printf("==> %d\n", p->num);
    printf("==> %s\n", p->name);
    printf("==> %d\n", p->metric);
    close(fd);
    return 0;
}

Answer 1

#include <stdio.h>
#include <stdlib.h>

struct flow
{
    unsigned int num;
    char name[32];
    unsigned char metric;
};

int main(int argc, const char *argv[])
{
    FILE *file = fopen("open.bin", "w+b");
    if (!file)
    {
        printf("failed to open file\n");
        return -1;
    }

    struct flow t = {5, "test", 127};
    fwrite(&t, sizeof(struct flow), 1, file);

    struct flow *p = (struct flow *)malloc(sizeof(struct flow));
    rewind(file);
    if (fread(p, sizeof(struct flow), 1, file) != 1)
    {
        printf("read error\n");
        return 1;
    }
    fclose(file);

    printf("==> %d\n", p->num);
    printf("==> %s\n", p->name);
    printf("==> %d\n", p->metric);

    return 0;
}