How to write and read dynamic arrays from binary file - c

Question

I have this struct
struct _recipe
{
char name[50];
char** ingredients;
char diff[12];
int time;
int calories;
char** procedure;
} recipe;
And I want to copy all the data inside a binary file.
First of all I've allocated dynamically the memory for both the ingredients and procedure and I have written all I needed. But then I need to write all in a binary file. I know that they are both pointers, so this mean that if I use
fwrite(&recipe,sizeof(recipe),1,fbr);
I'll write inside the file the address and not the actually value which is I need. I've tried writing every single field of the struct inside the file this way
fwrite(recipe.name,sizeof(recipe.name),1,fbr);
fgets(recipe.ingredients[j],30,stdin);
strcpy(buff,recipe.ingredients[j]);
len = strlen(buff);
fwrite(buff,sizeof(recipe.ingredients[0]),len,fbr);
fwrite(recipe.diff,sizeof(recipe.diff),1,fbr);
fwrite(&recipe.time,sizeof(recipe.time),1,fbr);
fwrite(&recipe.calories,sizeof(recipe.calories),1,fbr);
fgets(recipe.procedure[i],1000,stdin);
strcpy(buff,recipe.procedure[i]);
len = strlen(buff);
fwrite(buff,sizeof(recipe.procedure[0]),len,fbr);
I'm not sure it's the right way but I tried putting the string inside an other one and then copying it on the file. The problem is, I'm not sure it worked because I don't know what kind of command should I put for reading all the values I stored. With the name of course it worked, I didn't have problems with that but then when I was about to read the ingredients I blocked myself because I wrote the value inside an other string and I don't know what lenght should I put in reading. Maybe I'm missing something, probably I'm messing with writing in the first place but I don't know what to do at this point.

Answer 1

You want to do something like this to write out your recipe:
fwrite(recipe.name,sizeof(recipe.name),1,fbr);
for (int i = 0; i < num_ingredients; i++) {
int len = strlen(recipe.ingredients[i]) + 1;
int num_elements = fwrite(recipe.ingredients[i],sizeof(char),len,fbr);
printf("wrote %d elements for %s\n", num_elements, recipe.ingredients[i]);
}
fwrite(recipe.diff,sizeof(recipe.diff),1,fbr);
for (int i = 0; i < num_procedures; i++) {
int len = strlen(recipe.procedure[i]) + 1;
int num_elements = fwrite(recipe.procedure[i],sizeof(char),len,fbr);
printf("wrote %d elements for %s\n", num_elements, recipe.procedure[i]);
}
You also need to write out the calories and time of course. I would also do this before reading anything into the struct:
memset(&recipe, 0, sizeof(struct _recipe));
If you use fprintf to write out the calories and time with a \n for the separator, your recipe file would look like this:
$ od -c recipe.bin
0000000 m u f f i n s \0 \0 \0 \0 \0 \0 \0 \0 \0
0000020 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
*
0000060 \0 \0 f l o u r \0 s u g a r \0 e g
0000100 g s \0 v a n i l l a \0 n u t s \0
0000120 n o d i f f \0 \0 \0 \0 \0 m i x \0
0000140 b a k e \0 r e s t \0 6 0 \n 2 5 0
0000160 \n

Answer 2

You can design a simple storage protocol for writing/reading.
For example:
| struct recipe | t l v | t l v | ... | t l v | struct recipe | t l v | ... | t l v |
|<- a complete struct recipe data ->|
| description | t(ype) | l(ength) | v(alue)|
| ingredient | i | strlen() | string |
| procedure | p | strlen() | string |
| data end | e | 0 | <null> |
Read a recipe data:
1. Read a struct recipe.
2. Read a t, t=='i' -- go (3.), t=='p' -- go (4.), t=='e' -- go (5.).
3. Read a l, then read a string whose length is l as an ingredient. Go (2.).
4. Read a l, then read a string whose length is l as a procedure. Go (2.).
5. Finish.
Write a recipe data:
1. Write a struct recipe.
2. Write ingredients to `i strlen(ingredient) ingredient` one by one.
3. Write procedures to `p strlen(procedure) procedure` one by one.
4. Write `e 0`.
5. Finish.
Pseudo code: (just a demo, please design your own protol-)
void write_recipe(recipe *r, FILE *fp)
{
fwrite(r, sizeof(recipe), 1, fp);
// ...
for ()
{
sprintf(buff, "i %5d %s", strlen(r->ingredients[i]), r->ingredients[i]);
fwrite(buff, strlen(buff), 1, fp);
}
sprintf(buff, "e 0");
fwrite(buff, strlen(buff), 1, fp);
//...
for ()
{
sprintf(buff, "p %5d %s", strlen(r->procedure[i]), r->procedure[i]);
fwrite(buff, strlen(buff), 1, fp);
}
sprintf(buff, "e 0");
fwrite(buff, strlen(buff), 1, fp);
// ...
}
void read_recipe(FILE *r, FILE *fp)
{
fread(r, sizeof(recipe), 1, fp);
// ...
while (true)
{
fread(&t, sizeof(char), 1, fp);
if (t == 'e')
{
// ...
break;
}
fread(buff, sizeof(char), 7, fp);
sscanf("%d", &len);
fread(buff, sizeof(char), len, fp);
buff[len] = 0;
ingredient = malloc(sizeof(char) * (len + 1));
strcpy(ingredient, buff);
// ...
}
// ...
while (true)
{
fread(&t, sizeof(char), 1, fp);
if (t == 'e')
{
// ...
break;
}
fread(buff, sizeof(char), 7, fp);
sscanf("%d", &len);
fread(buff, sizeof(char), len, fp);
buff[len] = 0;
procedure = malloc(sizeof(char) * (len + 1));
strcpy(procedure, buff);
// ...
}
// ...
}