malloc / realloc对char **的限制？

Question

Say I have a char** "bob" which I malloc in the form of

char **bob = malloc(10 * sizeof(bob[0]));

And say for every element I store under bob, I malloc a certain amount of space in the form of

bob[num] = malloc(10);

My question is, can you only malloc elements under bob up to the amount you've malloced the char** itself?

Ie if bob[num] = malloc(10); is repeated 5 times, we've malloced 50 bytes in total - since that exceeds the original 10 * sizeof(bob[0]) (which mallocs 40), does this cause the program to screw up?

Or is the amount of memory you malloc to elements under the char** not restricted by the memory allocated to char** itself?

Answer 1

Or is the amount of memory you malloc to elements under the char** not restricted by the memory allocated to char** itself?

Yes, this is the case. When you allocate memory for your char** , you're making room for 10 char* 's. When you then allocate these char* 's, they end up pointing up to completely separate, new blocks of memory, unrelated to the block in which they themselves are stored.

Answer 2

You are creating a pointer-based look-up table. Don't confuse this for a 2D array.

The first malloc allocates an array of pointers, the actual table, which has no relation what-so-ever to the pointed at data. The memory layout will be like this:

bob[0] -> some segment of memory
bob[1] -> some other segment of memory
...

The sizes of the different segments could also be completely individual with no relation to each other.