将多个结构保存到二进制文件（C）

Question

I'm writing down a function that should save 3 structures (2 of them are arrays of structs) in a binary file. Here's my function:

void saveFile(Struct1 *s1, Struct2 *s2, Struct3 s3) {
    FILE *fp = NULL;
    fp = fopen("save.bin", "w+b");

    if (fp == NULL) {
        printf("Save failed.\n");
    }

    fwrite(s1, sizeof(Struct1), struct3.nElements, fp);
    fwrite(s2, sizeof(Struct2), NELEMENTS, fp);
    fwrite(&s3, sizeof(Struct3), 1, fp);

    printf("Save done.\n");
}

s1 have struct3.nElements , s2 have NELEMENTS (that's a constant) and s3 is just one struct and not an array. When I try to open the save.bin with HexEditor it gives very different results from the ones I was expecting, I'm wondering if I used correctly the fwrite function, especially for array of structs.

Answer 1

There are small issues with you function that might cause problems:

• you define the function as taking s3 by value. Why not pass a pointer to the third struct ? Is the saveFile function properly declared before the calling code? Are you sure the calling code passes the struct by value?

• You forget to close the stream. The handle gets lost, and the contents is not flushed to disk until the program exits.

• You open the file in "w+b" mode: write with read. It is correct to use binary mode, but unnecessary to add the + for read. Just use "wb" .

• If fopen fails, you output a diagnostic message, but you do not return from the function. You will invoke undefined behavior when trying to write to a NULL stream pointer.

Regarding your question, the dump of the file does not correspond to what you expect... give us more information, such as the definitions of the different structures and the hex dump. Here are some ideas:

• Some of the fields in the structures might need a specific aligned and thus be separated from the previous field by padding bytes. The values of those padding bytes is not necessarily 0 : if the structures are in automatic storage or allocated with malloc , their initial state is undefined and can change as a side effect of storing other fields.
• Integers can have different sizes and be stored in little endian or big endian order in the file, depending on the specific architecture your program is compiled for. For this reason, values stored by your program should only be read back with the appropriate, but reasonably similar code, running on the same architecture and OS.
• If your structures contain pointers, you cannot really make sense from the values stored in the output file.