program

difference between stack and heap

Difference

  • stack
    • allocate memory on stack by operate system
    • Have limit size, and less than 1MB always
  • heap
    • allocate memeory on heap
    • using malloc or new to allocate memory
    • user and defined the size, and the limit depend on virtual memory size
    • prone to memory fragments

What is memory fragments

  • Internal fragmentation
    • memory allocation provided memory to program in chunks divisible by 4, 8 or 16
    • if application request 23 bytes, it actually get a chunk of 32 bytes.
    • Really? How to prove it? And what happen if I using memory not belong to me?
  • External fragmentation
    • cause by free called
    • An example, first, you allocate 9 bytes memory, then allocate another 5 bytes,
      and later you free the first 9 bytes, but now, you want to allocate 10 bytes,
      this time the first 9 bytes memory can’t be used, because it is not enough.
      Allocation must using continues memory, so it would allocate another 10 bytes
      behind the 5 bytes position. Now the first block memory is memory fragments.

How to avoid memory fragments

Why memory overflow but not error occur?

  • sample code

    #include <stdio.h>
#include <stdlib.h>
#include <string.h>

void main()
{
    // allocate 13 bytes memory on heap
    char* p = (char*)malloc(13);
    char* c_12 = "012345678912";
    char* c_15 = "012345678912345";
    char* c_19 = "0123456789123456789";
    char* c_99 = "aaaaaaaaaaaaaaaaaaaa"
    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
    "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
    "aaaaaaaaaaaaa";

    printf("do copy 12 bytes to p\n");
    strncpy(p, c_12, strlen(c_12)+1);
    p[12] = '\0'; printf("p=%s\n", p);

    printf("do copy 15 bytes to p\n");
    strncpy(p, c_15, strlen(c_15)+1);
    p[16] = '\0'; printf("p=%s\n", p);

    printf("do copy 19 bytes to p\n");
    strncpy(p, c_19, strlen(c_19)+1);
    p[20] = '\0'; printf("p=%s\n", p);

    printf("do copy 99 bytes to p\n");
    strncpy(p, c_99, strlen(c_99)+1);
    p[100] = '\0'; printf("p=%s\n", p);

    printf("Hello world\n");
}

Reference