GDB: Displaying incorrect values in struct - struct

Question

I'm trying to implement the malloc function and it looks like that gdb is giving me some weird values from this struct:
struct MemoryBlock {
struct MemoryBlock * next;
size_t size;
signed char is_free;
} startBlock;
And that's the function where I'm debugging it with gdb:
struct MemoryBlock * create_new_block(size_t size)
{
struct MemoryBlock * ret_block;
// add some space for the struct block
size += sizeof(struct MemoryBlock);
ret_block = (void *) sbrk(size);
// test first, if we can allocate that much of ram
if (ret_block == (void *) -1)
return NULL;
ret_block->size = size - sizeof(struct MemoryBlock);
ret_block->is_free = 0;
ret_block->next = NULL;
return ret_block; // HERE'S the breakpoint
}
So here's the issue (I'm at the breakpoint return ret_block):
If I want to see what kind of values are inside of the ret_block pointer, than I'm getting this:
(gdb) p (struct MemoryBlock) ret_block
$26 = {next = 0x555555559000, size = 140737488347680, is_free = -53 '\313'}
size is fine, because if I convert it into the decimal system than I'm getting 3 as expected. (the argument size from the function is currently 3)
But I'm surprised that next and is_free aren't 0 since the last three lines should set both to 0.
So I looked up what is in the memory:
As you can see each value is correctly stored in my heap. But why am I getting these values if I do p (struct MemoryBlock) ret_block?
If you need the whole code
#include <unistd.h>
#include <stdio.h>
/* ============
* Structs
* ============ */
struct MemoryBlock {
struct MemoryBlock * next;
size_t size;
signed char is_free;
} startBlock;
/* ==============
* Functions
* ============== */
struct MemoryBlock * create_new_block(size_t size);
void * malloc(size_t size);
/* ==================
* Main Programm
* ================== */
int main()
{
char * buffer;
char * b2;
unsigned short index;
// The start of my heap :D
startBlock.is_free = 0;
startBlock.size = 0;
buffer = malloc(3);
b2 = malloc(3);
// ----- ERROR -----
if (buffer == NULL || b2 == NULL)
return 1;
// ----- ERROR -----
// fill the buffers with random stuff
for (index=0; index<2; index++) {
buffer[index] = 'a';
b2[index] = 'b';
}
buffer[index] = '\0';
b2[index] = '\0';
puts(buffer);
puts(b2);
return 0;
}
struct MemoryBlock * create_new_block(size_t size)
{
struct MemoryBlock * ret_block;
// add some space for the struct block
size += sizeof(struct MemoryBlock);
ret_block = (void *) sbrk(size);
// test first, if we can allocate that much of ram
if (ret_block == (void *) -1)
return NULL;
ret_block->size = size - sizeof(struct MemoryBlock);
ret_block->is_free = 0;
ret_block->next = NULL;
return ret_block;
}
void * malloc (size_t size)
{
struct MemoryBlock * ret_block;
struct MemoryBlock * prev_block;
prev_block = &startBlock;
ret_block = startBlock.next;
// go through the linked lists and look if you can find a suitable block
while (ret_block != NULL && (ret_block->size < size || !ret_block->is_free))
{
prev_block = ret_block;
ret_block = ret_block->next;
}
// couldn't find a suitable block => create a new one
if (ret_block == NULL) {
ret_block = create_new_block(size);
if (ret_block == NULL)
return NULL;
}
prev_block->next = ret_block;
ret_block->is_free = 0;
return ret_block;
}

Answer 1

Ok, one of my friends told me my issue... The casting was wrong! Here's the solution:
(gdb) p * ret_block
$57 = {next = 0x0, size = 3, is_free = 0 '\000'}
A star was enough to get the desired output...