Sbrk Function In C

The sbrk function adjusts the available memory space for the process that calls it. This adjustment involves increasing the process's break value by incr bytes and allocating the necessary space accordingly. The allocated space expands with a positive incr value and shrinks with a negative incr value. When incr is zero, sbrk provides the current program break value. This function is typically called when utilizing the malloc function from the Standard C Library.

The sbrk function increases the break value by adding incr characters, adjusting the allocated space accordingly. It is possible for the value of incr to be negative, resulting in a reduction of the allocated space.

The sbrk function is a system call that enables a process to adjust the size of its data segment, either expanding or reducing it. This adjustment involves manipulating the process's break value, which determines the address of the first location beyond the data segment, and allocating the necessary space accordingly. Increasing the break value results in a corresponding increase in the allocated space for the process.

Syntax:

It has the following syntax:

#include <unistd.h>

void* sbrk(intptr_t increment);

This parameter specifies the amount by which the data segment size should be adjusted, with the expected input being the number of bytes. A positive value is required to enlarge the heap size, while a negative value will reduce it.

How does the sbrk function work?

The heap area forms a part of the public address space within a process and is designated for the allocation of dynamic memory. By utilizing the sbrk function, the program gains the ability to adjust the size of this region by modifying its break point.

• ✅ Expanding Memory: When a positive increment value is provided to the sbrk function, the heap's size expands by the specified number of bytes. Consequently, this action moves the breakpoint to a higher memory address, thereby creating additional address space for the program.
• ✅ Reducing Memory: Conversely, when sbrk is invoked with a negative increment, the size of the heap decreases, causing the breakpoint to shift to a lower memory address. This functionality can be leveraged to release memory resources, similar to utilizing the ReleaseMemory method.

Return Value:

• ✅ Success: If the execution of sbrk is successful, it will provide the previous breakpoint address, that is the boundary of the data segment before the increment.
• ✅ Failure: In case of failure, sbrk returns (void *) -1 and sets the errno variable to indicate an error. Common errors include:

ENOMEM: There is insufficient memory available to fulfill the request.

EINVAL: The increment

Example:

Let's consider a scenario to demonstrate the implementation of the sbrk function in the C programming language.

#include <unistd.h>

#include <stdio.h>

int main() {

    // The current breakpoint value

    void* current_break_value = sbrk(0);

    printf("Current break: %p\n", current_break_value);

    // The break is then increased to 1024 bytes

    if (sbrk(1024) == (void*) -1) {

        perror("The sbrk method is failed");

        return 1;

    }

    // The new break pint value

    void* new_break_value = sbrk(0);

    printf("The New break value is : %p\n", new_break_value);

    // The differences in the break point value

    printf("The value is increased by: %ld bytes\n", (char*)new_break_value - (char*)current_break_value);

    return 0;

}

Output:

Current break: 0x1330000

The New break value is: 0x1351400

The value is increased by: 136192 bytes

Explanation:

This C program illustrates the functionality of the sbrk function in managing heap memory. The code utilizes sbrk(0) to retrieve the current breakpoint, representing the end address of the process's data segment, and saves this value in a variable named currentbreakvalue. Subsequently, it attempts to extend the breakpoint by 1024 bytes using sbrk(1024), thereby requesting more than 1KB of additional memory from the operating system. In case the allocation fails, sbrk returns (void*) -1, prompting the program to utilize perror for displaying an error message before exiting with a status code of 1.

Upon a successful allocation, the program employs sbrk(0) to acquire the new breakpoint and calculates the byte difference by subtracting the old breakpoint from the new one. This variance is then printed to the console to demonstrate the increment in heap size resulting from the current operation. Finally, the program concludes by exiting with a status code of 0, indicating successful execution.

Conclusion:

In summary, the sbrk function serves as a fundamental memory management function in Unix-like operating systems, enabling programs to directly control the heap. While not extensively utilized in modern application development, exploring sbrk provides valuable insights into the foundational aspects of memory management. Contemporary applications tend to favor higher-level memory management functions due to their advantages in terms of security, compatibility, and efficiency. Nonetheless, sbrk remains instrumental in illustrating the historical evolution of program memory management in the field of computer science.