Using sizeof() on malloc'd memory [duplicate]

Answer 1

You cannot. As pointed out, you can get the size of the void * mallocated, but that does not tell you much.

You cannot get the size of the malloed *mallocated. That is to say, there is no function call to return 100 (in your example) - unless you write your own memory management routines.

Simplest is just to remember it somewhere ... maybe ....

stuct {
  void *data;
  unsigned int size
 } myStructureWhichRemebersItsSize;

myStructureWhichRemebersItsSize *someData;
someData.data = malloc(100);  // and check for NULL
someData.size = 100;