03 Memory Layout of a C program

Typical memory layout of a C program has the following sections:

1. Text or code segment - Text segment contains the program i.e., the executable instructions.
2. Data segments - Data segments contain initialised and uninitialized global and static variables respectively.
3. Stack segment - Stack segment is used to store all local or automatic variables. When we pass arguments to a function, they are kept in stack.
4. Heap segment - Heap segment is used to store dynamically allocated variables.

Static variables

• Static variables are stored in the data segment, not in the stack.
• The data segment is active during the entire life-time of program.
• So, static variables preserve their values even after they are out of their scope.

Global variables

• A global variable is declared outside all functions.
• It can be read or updated by all functions.

Dynamic memory allocation functions

C provides dynamic memory management functions in stdlib.h

malloc()

1. Allocated requested number of bytes of contiguous memory from the Heap.
2. Returns a pointer to the first byte of the allocated space.
3. If the memory allocation fails, then malloc() returns a NULL pointer.

Memory leak

If memory allocated using malloc() is not free()-ed, then the system will “leak memory”. Consequences of memory leak would be:

• Memory leak slowdown system performance by reducing the amount of available memory.
• In modern operating systems, memory used by an application is released when the application terminates.
• Memory leak will cause serious issues on resource-constrained embedded devices.