`malloc()`

If you don't know C, the remaining pages of this chapter can be skipped. However, they should be readable by everyone. The ideas are similar to other modern languages.

The malloc() function from the C standard library requests a block of memory.

void *malloc(size_t size)

The function allocates a block of memory of size number of bytes, and returns the address of the first byte of that block. (The data type of size is an unsigned integer. The return type of the function is void * which is the way ANSI C describes a memory address.)

Here is the start of our example assembly program:

main:    
        # create the first struct 
        li      $v0,9             # allocate memory
        li      $a0,12            # 12 bytes
        syscall                   # $v0 <-- address
        move    $s1,$v0           # $s1 first struct
        . . . .

And here is how this would be done in C:

#include <stdlib.h>    /* include a header file of standard definitions */

struct EMPLOYEE              /* Declaration of a type of struct */
{                            /* This does not allocate any memory */
  int age;                   
  int pay;
  int class;
};

main()
{
  struct EMPLOYEE *empA;     /* declaration of the pointer variable empA */

  empA = (struct EMPLOYEE *)malloc( sizeof( struct EMPLOYEE) );

  . . . .

The source code starts by including a file of standard definitions that the compiler may use in compiling this source program. Then the EMPLOYEE structure is declared. This describes a layout of a memory block but does not itself request memory.

The main() routine of C is equivalent to the main entry point of an assembly language program. It is where the operating system first sends control when it starts the program running.

The declaration of empA describes it as a variable that is expected to contain an address of a block of memory suitable for an EMPLOYEE structure. In C, this type of variable is called a pointer.

The final statement (in this fragment) requests a block of memory. The number of bytes is the size needed the structure. The compiler determines this using the sizeof operator. (Or we could explicitly put "12", but this is poor coding technique.)

The address of the dynamically allocated block of memory is returned by malloc() and assigned to the variable empA.

Answer:

`malloc()`

QUESTION 15: