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Algorithms in a Nutshell by Gary Pollice, Stanley Selkow, George T. Heineman

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Manual Memory Allocation

Most modern programming languages allow the programmer to allocate dynamic memory from the Heap (as opposed to the Stack) during the execution of a program. Consider the C program in Example 3-1.

Example 3-1. Sample program that allocates memory

#include <stdlib.h>
#include <string.h>
void f(char *inner) {
  char temp[11];
  strcpy (temp, "algorithms");
  int i;
  for (i=0; i<11; i++) {
    inner[i] = temp[i];
  }
}
int main (int argc, char **argv) {
  char *ar1  = malloc(132);
  char *ar2  = malloc(132);
  int i = 17, j;
  f (ar2);
  return 0;
}

When the program executes, variables that are local to a function (such as argc, and argv for main) are assigned to locations on the Execution Stack, which stores the progress of the program execution. Dynamically allocated memory (such as ar1 in main) is instead stored on the Heap, a separate region of memory available for allocation. The address of the variable determines where the memory can be found. In the example, one possible assignment of variables (on a Linux i686) is shown in Table 3-5.

Table 3-5. Addresses for symbols and variables

f variables

Addresses

main variables

Addresses

Global symbols

Addresses

f.inner

3221222728

main.argc

3221222796

&f

4195576

f.temp

3221222704

main.argv

3221222784

&main

4195648

f.i

3221222700

main.ar1

3221222776

  
  

main.ar2

3221222768

  
  

main.i

3221222764

  
  

main.j

3221222760

  
  

*main.ar1

5246992

  
  

*main.ar2

5247136

  

Note how the address values are all near one another, ...

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