c - What is the difference in these stack and heap memory addresses? -

i making example stack , heap allocation on ubuntu 14.04 vm (linux 3.13.0-55-generic i686) , confused memory addresses heap allocations.

the below c code allocates 3 32-bit unsigned ints on stack , 3 allocation on heap of diminishing sizes, 32 bits, 16 bits , 8 bits.

in output below can see memory addresses 3 32 bit ints on stack 4 bits apart. uint32_t @ 0xbffd4818 , 4 addresses later @ 0xbffd481c uint32_t j. can see here each individual byte of memory addressable , each 4 byte memory block 4 memory addresses apart.

looking @ heap allocations though can see uint32_t i_ptr points 0x99ae008 , malloc requested 4 bytes of space, expect uint16_t j_ptr start @ 0x99ae00c starts @ 0x99ae018. third heap allocation uint8_t k_ptr starts 16 bytes after uint16_t i_ptr starts 16 bytes after uint32_t i_ptr.

1. is default os setting each heap allocation 16 bytes apart?
2. why happening irrelevant of size passed malloc?
3. how can fit 4 bytes of information between 0x99ae008 , 0x99ae018?

c source:

#include <stdint.h> #include <stdlib.h> #include <stdio.h>  int main () {      register uint32_t ebp asm ("ebp");     printf("0x%x\n", ebp);      register uint32_t esp asm ("esp");     printf("0x%x\n", esp);      uint32_t i;     printf("%p\n", &i);      uint32_t j;     printf("%p\n", &j);      uint32_t k;     printf("%p\n", &k);      uint32_t *i_ptr = malloc(4);     printf("%p\n", i_ptr);      uint16_t *j_ptr = malloc(2);     printf("%p\n", j_ptr);      uint8_t *k_ptr = malloc(1);     printf("%p\n", k_ptr);      free(i_ptr);     free(j_ptr);     free(k_ptr);      return 0;  } 

cli output:

$ gcc -o heap2 heap2.c $ ./heap2 0xbffd4838  // ebp 0xbffd4800  // esp 0xbffd4818  // uint32_t 0xbffd481c  // uint32_t j 0xbffd4820  // uint32_t k 0x99ae008   // uint32_t i_ptr 0x99ae018   // uint16_t j_ptr 0x99ae028   // uint8_t  k_ptr 

malloc returns pointer of type void * may casted pointer of other type. malloc provides alignment satisfies requirement of type.

usually malloc returns address aligned paragraph (in systems equal 16 bytes). , malloc allocates extents have minimal size of paragraph. if write example

char *p = malloc( 1 ); 

then in fact malloc reserves extent of 16 bytes.