Malloc Exercise

Depends on	Dynamic Memory Allocation • Pointers

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Sometimes, programs need to create variables at run time. This requires a request to the operating system for more of the machine’s resources. This request is typically contained in some language construct. In Java (for example) it is performed by the function “new”. In C the call is rather cruder and comes under the name “malloc” (from “memory allocate”); malloc() will allocate the requested number of (contiguous) bytes (if there are any spare) and return a pointer to their location.

Example:

int *p_new_int;
p_new_int = malloc( sizeof(int) ); // Allocate an int of memory
*p_new_int = 1234;                 // Assign a value to that memory - note the dereference

Oops: that example assumes malloc() will be successful; if the system rejects the request (unlikely!) then a NULL will be returned and the assignment will “seg. fault”. You can ignore the test for now, but your conscience should haunt you.

Whilst this works, it would be unusual to allocate a single integer; this mechanism is usually used for larger structures.

As we have no garbage collection, it is the programmer’s job to use the complementary call to dispose of the ‘litter’.

free(p_new_int);

This disposes of the memory allocated by malloc(). The pointer, created statically by the int *p_new_int; declaration is still there, of course. (Make a sketch showing the pointer and the temporarily allocated item.)

Don’t use that pointer value again after the free(); it is invalid.

Notice that free() used the only a pointer to identify the memory which was being released; this is because the system has kept track of the size of the blocks as they were allocated. The system is also keeping track of which program ‘owns’ which memory so that, when the program terminates, all its resources can be recovered anyway. This is an operating system function and will happen regardless of whether the program finished ‘properly’ or it crashed.

Practical

Compile and run allocate.c; look at the source code and understand what it is doing. You might like to add some printf()s to see the allocated address, too.

Puzzles

• What happens if you don’t allocate a big enough space for your string?
  (Alternatively, what if your string overflows your allocated space?)
• What problem (if any) might you have if you miss out the free() call?
  (You could try it but that might not show everything.)

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Articles on Exercises

Exceptions Exercise • Exercises • Files Exercise • Malloc Exercise • Pipes Exercise • Pointer Exercise • Processes Exercise • Shared memory Exercise • Structs Exercise • Synchronisation Exercise • Threads Exercise • Unix proc Exercise