在文件中保存带有指针成员的结构

Question

I'm trying to save a struct into a.dat file and read it back in later.

struct myStruct{
  char **one;
  mytype **two;
  mytype2 *three;
}

With an assigning function:

struct MyStruct get_struct() = {
char **pi = ...;
mytype **pa = ...;
mytype2 **po = ...;
MyStruct n = {pi, pa, po};
return n;
}

I originally tried to save this struct into a.dat file by doing this:

struct MyStruct s = get_struct();
myoutfile = fopen("file.dat", "w");
if (myoutfile == NULL) {
    fprintf(stderr, "\nError opend file\n");
    exit(1);
}
fwrite(&s, sizeof(struct MyStruct), 1, myoutfile);
fclose(myoutfile);

and read it back in with:

fread(&t, sizeof(struct MyStruct), 1, myinfile)

Now I learned, that this does not work (segmentation error), because I only save the location where the pointer points to, not the actual thing.

Now my question is, how can I do it properly? I have found some solutions for C++ but I need to stay in C.

EDIT: Later on, I want to call a function which looks like this:

void work_with_struct(MyStruct s){
   char ** xone = s.one;
   mytype **xtwo = s.two;
   mytype2 *xthree = s.three;
}

This post is related to this post, but as I could specify my mistake now, asking in a new post makes more sense to me.

Answer 1

As always in programming, you break up the task to smaller chunks, and break up smaller chunks to yet smaller chunks, until every chunk is easy.

int saveMyStruct (struct myStruct* myStruct, FILE* file) {
   // what do I do here?!?!
   // well it has three members
   // so treat each one in sequence
   int result;
   result = saveStringArray(myStruct->one, file);
   if (result >= 0)
     result = saveMyTypeArray (myStruct->two, file);
   if (result >= 0)
     result = saveMyType (myStruct->three, file);
   return result;
}

Note how the status is checked all the time. If you work with files, you need to check the status all the time.

What next? You need to write three functions mentioned above.

 saveStringArray(char** stringArray, FILE* file)
 {
     // first save the length of the array, then save each individual string
     int length = getStringArrayLength(stringArray);
     int result = fwrite(&length, sizeof(length), 1, file);
     if (result != 1)
        return -1;
     for (i = 0; i < length; ++i)
     {
        result = saveString(stringArray[i], file);
        if (result < 0)
           return -1;
     }
     return i;            
 }

And so on and so forth. I presume your array of pointers is NULL-terminated; if not, you need to have some other way to know its length.

Note how array length is always saved before array elements. This is because you will need to read your array later, and you will need to know where to stop. It will also be easy to allocate your array when you read it.

Answer 2

I'm trying to save a struct into a.dat file and read it back in later.

struct myStruct{
  char **one;
  mytype **two;
  mytype2 *three;
}

With an assigning function:

struct MyStruct get_struct() = {
char **pi = ...;
mytype **pa = ...;
mytype2 **po = ...;
MyStruct n = {pi, pa, po};
return n;
}

I originally tried to save this struct into a.dat file by doing this:

struct MyStruct s = get_struct();
myoutfile = fopen("file.dat", "w");
if (myoutfile == NULL) {
    fprintf(stderr, "\nError opend file\n");
    exit(1);
}
fwrite(&s, sizeof(struct MyStruct), 1, myoutfile);
fclose(myoutfile);

and read it back in with:

fread(&t, sizeof(struct MyStruct), 1, myinfile)

Now I learned, that this does not work (segmentation error), because I only save the location where the pointer points to, not the actual thing.

Now my question is, how can I do it properly? I have found some solutions for C++ but I need to stay in C.

EDIT: Later on, I want to call a function which looks like this:

void work_with_struct(MyStruct s){
   char ** xone = s.one;
   mytype **xtwo = s.two;
   mytype2 *xthree = s.three;
}

This post is related to this post, but as I could specify my mistake now, asking in a new post makes more sense to me.