brk v.s. sbrk

brk() and sbrk() are the system calls that is used for memory management. They are often used by the memory allocation like malloc and free in the C standard library.^[1]

They change the location of the program break, which defines the end of the process's heap start address and end address.

• brk: This function sets the end of the data segment to the value specified by the argument.
• sbrk: This function increments (or decrements if the number is negative) the program's data spec by specified amount. It changes the program break value and returns the previous program break. If the increment is 0, this function simply returns the current location of the program break.

Usage:

#include "unistd.h"
int brk(void *addr);
void *sbrk(intptr_t increment);

How to Get Current .BSS Location

int bssvar;

int main() {
	printf("end of bss section: %p\n", (long)&bssvar + 4);
	// ...
}

Example

#include <stdio.h> // for printf
#include <stdlib.h> // for exit
#include <unistd.h> // for brk and sbrk

int bssvar; // define a uninitialized variable which will be put in .bss segment

int main(void) {
	char *pmem;
	long heap_gap_bss;
	printf("end of bss section: %p\n", (void *)&bssvar + 4);

	pmem = (char *)malloc(32);
	if (pmem == NULL) {
		perror("malloc");
		exit(EXIT_FAILURE);
	}
	printf("pmem: %p\n", pmem);
	heap_gap_bss = (long)pmem - (long)&bssvar - 4;
	printf("1-gap between heap and bss: %lu\n", heap_gap_bss);
	free(pmem);
	sbrk(32); // adjust program break position.
	pmem = (char*)malloc(32);
	if (pmem == NULL) {
		perror("malloc");
		exit(EXIT_FAILURE);
	}
	printf("pmem: %p\n", pmem);
	heap_gap_bss = (long)pmem - (long)&bssvar - 4;
	printf("2-gap between heap and bss:%lu\n", heap_gap_bss);
	free(pmem);
	return 0;
}

This program showed the following output:

end of bss section: 0x5619096ea018
pmem: 0x56190a5552b0
1-gap between heap and bss: 15119000
pmem: 0x56190a5552b0
2-gap between heap and bss:15119000