在 C 上使用 fwrite() 时输出错误

Question

I recently started doing a small project in C to learn how to properly handle working with output and input for a file.

The situation is as follows:
When I use a char array all works fine, but when I pass a struct as a parameter to fwrite() the output goes crazy.

FILE *fp;
Acazzo a;
fp = fopen("Inserimento.dat","w");
gets(a.nome);
fflush(stdin);
gets(a.cognome);
fflush(stdin);
gets(a.note);
fflush(stdin);
fwrite(&a,sizeof(a),1,fp);
fclose(fp);
return 0;

That's a really short project for learning, so I skipped some stuff like printf for name surname etc etc.
This is the output I get when I open the file

GIulio ÿÿÿÿSavoca E' un ottimo student

Answer 1

First there's nothing "crazy" about that output. Don't expect any "magic" happening, writing a struct to a file just does write exactly how the struct is stored in RAM, byte by byte .

Second, that's almost never what you want, because the layout in memory for a struct will differ from machine to machine, sometimes even from compiler to compiler. For writing structs to a file and reading them back reliably, you have to do some sort of (de-)serialization using only textual representations of all individual fields.

---

Very basic example of (de-)serializing a struct :

This example is really minimal and stores a struct in a file line-by-line, assuming the struct and all pointer fields are allocated dynamically. I didn't test this code, it might have a bug, just to show a simple idea.

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

typedef struct data
{
    int    id;
    char   *name;
    char   *value;
} data;

static char *copyInput(char *buf)
{
    char *copy;
    size_t endpos = strlen(buf) - 1;
    while (buf[endpos] == '\r' || buf[endpos] == '\n')
    {
        buf[endpos] = 0;
        --endpos;
    }

    copy = malloc(strlen(buf) + 1);
    strcpy(copy, buf);
    return copy;
}

data *data_deserialize(FILE *fp)
{
    char buf[256];
    data *d = calloc(1, sizeof(*d));
    if (!d) return 0;

    if (!fgets(buf, 256, fp)) goto err;
    d->id = atoi(buf);
    if (!fgets(buf, 256, fp)) goto err;
    d->name = copyInput(buf);
    if (!fgets(buf, 256, fp)) goto err;
    d->value = copyInput(buf);

    return d;

err:
    free(d->name);
    free(d->value);
    free(d);
    return 0;
}

int data_serialize(FILE *fp, const data *d)
{
    int rc;

    rc = fprintf(fp, "%d\n", d->id);
    if (rc < 0) return rc;

    rc = fprintf(fp, "%s\n", d->name);
    if (rc < 0) return rc;

    rc = fprintf(fp, "%s\n", d->value);

    return rc;
}