memory allocation

[cs32]

I might not be fully understanding what's going on here, but when I allocate two segments of memory for two variables, the address difference between the two doesn't seem to account for the amount of space that I'm allocating.

int                                                                          
main(int argc, char* argv[]) {                                               
                                                                             
  int *foo = malloc(4*sizeof(int));                                          
  int *bar = malloc(sizeof(int));                                            
                                                                                                                                      
  printf("foo: %X\n", (unsigned int) foo);                                
  printf("bar: %X\n", (unsigned int) bar);                                        
                                                            
  return 0;                                                                  

}

Output:

foo: 100120
bar: 100130

Given that I'm allocating 4 x sizeof(int) for foo (16 bytes), and assuming that allocation is contiguous, I'm expecting there to be a separation of at least 16 between the output for foo and bar (i.e. 100120 and 100136). What am I missing? Thanks in advance!

[tabstop]

The difference between 100136 and 100120 is twenty-two bytes. The difference between 100130 and 100120 is sixteen bytes. (Look at your format specifier.)

[cs32]

The difference between 100136 and 100120 is twenty-two bytes. The difference between 100130 and 100120 is sixteen bytes. (Look at your format specifier.)

Thanks for the tip on the format specifier. Changing it to %d gave me the information that I had expected to see.

I do have another question, though. How did you come to the conclusion that there were sixteen bytes between 100130 and 100120 (apologies for being dense).

[MacGyver]

Who says that the two blocks must be right next to each other? Theoretically, malloc() can allocate anywhere from the heap, giving the appearance of a huge gap between memory.

[xuftugulus]

It's always easy to make such a mistake. Your two Hexadecimal values converted to decimal differ by 16.

[tabstop]

That's why I said look at your format specifier: %x means "print this unsigned integer as a hexadecimal integer." (That's also why you got a warning about passing a pointer as an integer without a cast.) The format specifier for pointers is %p, and is system-specific (but is also often hexadecimal).

[cs32]

Who says that the two blocks must be right next to each other? Theoretically, malloc() can allocate anywhere from the heap, giving the appearance of a huge gap between memory.

Yes, I understand that. But in this case, the gap seemed pretty close, and I was trying to understand why I wasn't seeing 16 bytes in between the two numbers.