malloc(sizeof(int)) vs malloc(sizeof(int *)) vs (int *)malloc(sizeof(int))

c memory allocation malloc
sabergeek picture sabergeek · Mar 5, 2013 · Viewed 77.4k times · Source

I acknowledge that all three of these have a different meaning. But, I don't understand on what particular instances would each of these apply. Can anyone share an example for each of these? Thank you.

       malloc(sizeof(int))
       malloc(sizeof(int *))
(int *)malloc(sizeof(int))

Answer

Gort the Robot picture Gort the Robot · Mar 5, 2013

malloc(sizeof(int)) means you are allocating space off the heap to store an int. You are reserving as many bytes as an int requires. This returns a value you should cast to int *. (A pointer to an int.) As some have noted, typical practice in C is to let implicit casting take care of this.

malloc(sizeof(int*)) means you are allocating space off the heap to store a pointer to an int. You are reserving as many bytes as a pointer requires. This returns a value you should cast to an int **. (A pointer to a pointer to an int.)

(int *)malloc(sizeof(int)) is exactly the same as the first call, but with the the result explicitly casted to a pointer to an int.

Note that on many architectures, an int is the same size as a pointer, so these will seem (incorrectly) to be all the same thing. In other words, you can accidentally do the wrong thing and have the resulting code still work.

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