Difference between data section and the bss section in C

In C programs, the Data Segment and BSS Segment are two memory areas used to store program variables, but they serve distinct purposes and store different types of content.

Data Segment

The Data Segment is primarily used for storing initialized global and static variables. These variables are initialized at compile time. The Data Segment is created when the program is loaded into memory and is typically located at a fixed memory address.

Example:

c
int globalVar = 5;  // Initialized global variable stored in the Data Segment
static int staticVar = 10; // Initialized static variable also stored in the Data Segment

BSS Segment

The BSS Segment is used for storing uninitialized global and static variables. In memory, variables in the BSS Segment are initialized to zero or NULL. Unlike the Data Segment, the BSS Segment does not occupy storage space in the program file; it is created when the program is loaded into memory and allocates the necessary memory space.

Example:

c
int globalVar;  // Uninitialized global variable stored in the BSS Segment
static int staticVar; // Uninitialized static variable also stored in the BSS Segment

Summary

In summary, the main difference between the Data Segment and BSS Segment is that the Data Segment stores initialized variables, while the BSS Segment stores uninitialized variables. Variables in the Data Segment are initialized at compile time, whereas variables in the BSS Segment are automatically initialized to 0 or NULL when the program starts. This distinction helps optimize memory usage and loading time because variables in the BSS Segment do not require specific storage space in the program file.