如何在文件中写入一个包含 Malloc 数组的结构

Question

I try to print in a file the content of a structure that has a dynamic array inside and I think I'm not getting it

the struct looks like

struct save {
    char s_ext[5];
    char *s_bits;
    long s_frec[256];
    long int s_sim;
};

here I save the imformation in the struct

struct save *res = malloc(sizeof(struct save));

I try to malloc the array s_bits inside a struct

    res->s_bits = malloc(sizeof(char) * sim);

    if (res->s_bits == NULL) {
        printf("error\n");
    }

    strcpy(res->s_bits, textDeco);
    strcpy(res->s_ext, extension);
    res->s_sim = sim;

    for (i = 0; i < 15; ++i) {
        printf("%ld -> %d:%d, ", i, res->s_bits[i], textDeco[i]);
    }
    printf("\n");
    for (i = 0; i < 256; ++i) {
        res->s_frec[i] = frecCopy[i];
    }

open the file

FILE *save_struct = fopen("codi.dat", "w");

When I try to write the struct on a binary file using fwrite

if (fwrite(res, sizeof(struct save), 1, save_struct) != 0) {
        printf("file created!\n");
    } else {
        printf("error\n");
    }

it doesn't write a the elements of s_bits, which I don't want.

how do i get the elements with fread?

Answer 1

You allocate your struct like this:

struct save *res = malloc(sizeof(struct *res));
if(!res) // handle errror
res->s_bits =  malloc(sim);
if(!res->s_bits) // handle error

When you use fwrite() to store the struct to a file, it will save pointer value res->s_bits but not the array it points to. The way to handle that is write out each field individually. When this gets annoying find a library to help you serialize your data (Cap'n Proto, protobuf, JSON etc). You should also consider SQLite.

As you only have one field like this you could make s_bits a flexible array member:

struct save {
    char s_ext[5];
    long s_frec[256];
    long int s_sim;
    char s_bits[];
};

and now you allocate it like this:

struct save *res = malloc(sizeof(struct *res) + sim);

and you would write it in a similar fashion:

fwrite(res, sizeof(*res) + res->s_sim, 1, save_struct)