Allocate memory for an array of strings that is in a struct-CodePudding

so I'm trying to allocate memory for an array of strings thats in a struct: This is the struct:

typedef struct{
    int aisleNumber;
    char **aisleProducts;
}Aisle;

And this is how I allocate the memory:

Aisle.aisleProducts = (aisleProducts*)malloc( sizeof(aisleProducts) );

For now, I only need space for one string in the array, hence why I'm not multiplying the size. Still doesn't work and I don't know why...

Any help would be appreciated.

CodePudding user response：

The malloc call is senseless. For an array of pointers you should allocate n * sizeof(char*) and since malloc returns a pointer to the first allocated element (which is a pointer), you should not cast the result to char* but treat it as a char**.

Example:

#include <stdlib.h>
aisles.aisleProducts = malloc( n * sizeof(*Aisle.aisleProducts) );

Or equivalent:

#include <stdlib.h>
aisles.aisleProducts = malloc( n * sizeof(char*) );

Or equivalent:

#include <stdlib.h>
aisles.aisleProducts = malloc( sizeof(char* [n]) );

CodePudding user response：

It's not working because sizeof(...) returns a compile-time constant value. Your struct is exactly 12 bytes (at least when targetting a 64-bit platform). The int value is 4 bytes, while the pointer (like all other pointers) is 8 bytes. If you want to allocate more memory for your strings, then you'll have to allocate and assign your pointers manually, maybe in a loop. Let's say you have 4 strings:

// Allocate space for the starting pointers
Aisle.aisleProducts = (char**) malloc(sizeof(char*) * 4);
// Then more space for each individual string (for convenience, let's give each one of them 64 bytes)
for (int i = 0; i < 4; i  )
{
    Aisle.aisleProducts[i] = (char*) malloc(sizeof(char) * 64);
}
// Assign your strings...

Edit: as other people have noted, 12 bytes is the theoretical size of the struct, but in practice, it will be aligned to an arbitrary value.

CodePudding user response：

You can try

Aisle* pAisle = (Aisle*)malloc(sizeof(Aisle));
pAisle->aisleProducts = (char**)malloc(sizeof(char*));