Thread: Memory allocation for an array in a struct pointer

Hello there,
this might be a silly question but I am a bit confised about the case and I wanted to ask around. I need to allocate memory for an object that looks like this:

Code:

typedef struct{
	uint32_t		len;
	uint8_t* 		data;
} cdcRaw_t;

It contains pointer to data, with should be also alocated with length of value len.

So my aproach looks like this (test program)

Code:

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


typedef struct
{
	uint32_t		len;
	uint8_t* 		data;
} cdcRaw_t;


int main(void)
{
	uint8_t data[] = "abcdefghijklmn";
	uint32_t len = 14;


	// first allocate memory for the struct
	cdcRaw_t* item = malloc(sizeof(cdcRaw_t));
	
	item->len = len;
	
	// then allocate memory for data within struct
	item->data = malloc(sizeof(item->len));


	// copy the data
	memcpy(item->data, data, item->len);


	int i;
	for (i = 0; i < item->len; i++)
	{
		printf("item %c at index %i \n", item->data[i], i);
	}


	// double free
	free(item->data);
	free(item);


	return 0;
}

:

In this aproach I have to malloc twice and free twice... Is there a more efficient way to allocate and free memory for this struct? Important note- for each struct object, the length of data varies.

Apart from using

Code:

uint32_t len = 14;

You might consider

Code:

uint32_t len = sizeof(data) / sizeof(data[0]);

But apart from that your general approach looks correct (ignoring that you should be making sure that malloc() does not return NULL before using the pointers).

uint32_t len = sizeof(data) / sizeof(data[0]);

But does the compiller know the size of allocated data like that? If yes, then I dont need to store length in variable len.

I see. Eventually it wont be so len variable has to stay. So is there no other way to allocate memory for the scruct (and the containing vector) using only 1 malloc?

Ok, i understand everything. Thank you for help .