Memory allocation

Memory is another resource that an application must request using a system call. Most applications are given global and stack memory resources when the application is first executed, along with a small heap of memory that the application can use when calls to functions such as malloc() and free() are made.

If the application only uses the memory that it is initially given in this heap, no extra memory needs to be requested by the application. If, however, heap memory runs out, the application's malloc() or free() engine will have to ask the operating system (via a system call) for more memory.

To do this, the operating system will extend the end of the application by adding more physical memory to the application. The malloc() or free() engine is then able to make use of this additional memory, until more is needed. 

On systems with limited RAM, when a request for additional memory is made, the operating system has to take memory from other applications that aren't currently executing. It does this by swapping these applications to disk, an operation that is expensive to perform.

For this reason, on resource-constrained systems, calls to malloc() or free() should not be made in time-critical code, as the time it takes to execute these functions can vary greatly.

We will go into further detail on memory management in Chapter 7, A Comprehensive Look at Memory Management.