C++ Library - <cstdlib>

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The <cstdlib> library in C++ provides a variety of functions for performing common operations, such as memory management, random number management and string conversion. It is a part of the C standard library, that is useful when dealing with the low level operations.

In C++ <cstdlib> is used when interacting with C-style operations or dealing with low-level memory management. While C++ has its own alternatives for memory allocations and type conversions, the functions provided by the <cstdlib> are still widely used in certain contexts.

Including <cstdlib> Header

To include the <cstdlib> header in your C++ program, you can use the following syntax.

#include <cstdlib>

Functions of <cstdlib> Header

Below is list of all functions from <cstdlib> header.

String Conversion

These functions are used for converting strings (C-style strings) to numerical values such as integers and floating numbers or to convert numbers to strings. The commonly used C++ <cstdlib> string conversion functions are listed below along with their description.

Sr.No	Functions & Description
1	atof It converts a string to a floating point value.
2	atoi & atol & atoll It converts a string to an integer value.
3	strtoul & strtoull It converts a string to a unsigned integer value.
4	strtof & strtod & strtold It converts a string to a floating point value.

String to Integer Conversion

In the following example, we are going to use the atoi to convert the string to an integer.

#include <iostream>
#include <cstdlib>
int main() {
   const char * x = "11123";
   int y = std::atoi(x);
   std::cout << "Result : " << y << std::endl;
   return 0;
}

Output

Output of the above code is as follows −

Result : 11123

Process Control

These functions are used to manage and control program executions. They include operations such as program termination, environment manipulation, and process handling. The commonly used C++ <cstdlib> process control functions are listed below along with their description.

Sr.No	Functions & Description
1	abort It causes the program termination.
2	exit It causes the program termination with cleaning up.
3	atexit It registers a function to be called on the exit().
4	quick_exit It causes the program termination without completely cleaning up.
5	at_quick_exit It registers a function to be called on the quick_exit().
6	_Exit It is used to terminate the calling process.

Terminating Program

Let's look at the following example, where we are going to terminate the program immediately.

#include <cstdlib>
#include <iostream>
int main() {
   std::cout << "Program Started" << std::endl;
   if (true) {
      std::cout << "Exiting Program..." << std::endl;
      std::exit(0);
   }
   std::cout << "Hello" << std::endl;
   return 0;
}

Output

Following is the output of the above code −

Program Started
Exiting Program..

Memory Management

Memory management refers to the process of allocating and deallocating of the memory. The commonly used C++ <cstdlib> memory management functions are listed below along with their description.

Sr.No	Functions & Description
1	calloc It is used to allocates memory for an array and initializes all to zero.
2	free It is used to deallocates the previously allocated memory .
3	malloc It is used to allocate the memory.
4	realloc It is used to expands or shrinks previously allocated memory.

Memory Allocation

Consider the following example, where we are going to allocate a block a memory dynamically.

#include <iostream>
#include <cstdlib>
int main() {
   int * x = (int * ) std::malloc(4 * sizeof(int));
   if (x == nullptr) {
      std::cerr << "Memory Allocation Failed." << std::endl;
      return 1;
   }
   for (int a = 0; a < 4; ++a) {
      x[a] = a * 2;
      std::cout << x[a] << " ";
   }
   std::cout << std::endl;
   std::free(x);
   return 0;
}

Output

If we run the above code it will generate the following output −

0 2 4 6