malloc (Memory Allocation Utility)

Syntax

malloc <size>

Description

The malloc command (also known as MemAlloc in VSDebugPro) allows you to allocate memory in the debugged process’s heap. This feature is useful for dynamically creating memory blocks during debugging sessions, which can be used for testing, data injection, or simulating memory allocation scenarios.

Parameters

  • <size>: The number of bytes to allocate.

Usage Notes

  1. The command uses the current process context in the debugger.
  2. The size can be specified as a decimal number or a hexadecimal number (prefixed with 0x).
  3. The command returns the address of the newly allocated memory block.
  4. The allocated memory is not initialized and may contain arbitrary data.
  5. Remember to free the allocated memory using the free command when it’s no longer needed to prevent memory leaks.
  6. The allocation is performed in the debugged process’s heap, not in the debugger’s memory space.

Examples

  1. Basic usage:
    malloc 200
    

    Allocates 200 bytes of memory and returns the address of the allocated block.

  2. Allocating a larger block:
    malloc 0x100000
    

    Allocates 1 MB (1,048,576 bytes) of memory.

  3. Using an expression:
    malloc 1024*1024
    

    Also allocates 1 MB of memory, using an expression to calculate the size.

Error Handling

  • Insufficient memory: Displays an error message if the process doesn’t have enough available memory for the requested allocation.
  • Invalid size: Shows an error if the specified size is invalid (e.g., negative or too large).
  • Allocation failure: Reports an error if the memory allocation fails for any other reason.
  • free: Releases memory previously allocated with malloc.
  • memset: Fills a memory region with a specified value.
  • memcpy: Copies data between memory locations.
  • dumpmem: Dumps memory contents to a file.

Example Code and Usage

#include <cstdlib>
#include <cstring>
#include <cstdio>

int main()
{
    // We'll use malloc to allocate memory during debugging
    char* buffer = nullptr;
    int bufferSize = 48;
    
    // Normally, we would allocate here:
    // buffer = (char*)malloc(bufferSize);
    
    // Instead, we'll use the debugger's malloc command
    
    printf("Buffer address: %p\n", (void*)buffer);
    
    // Use memset to initialize the buffer (if allocation was successful)
    if (buffer != nullptr) {
        memset(buffer, 'A', bufferSize);
        printf("Buffer content: %.*s\n", bufferSize, buffer);
        
        // Don't forget to free the allocated memory
        free(buffer);
    }
    
    return 0;
}

During debugging, after the buffer variable is declared but before the printf statement, you can use the malloc command to allocate memory:

malloc 48

This command will allocate 48 bytes of memory and return an address. You can then use the debugger to assign this address to the buffer variable. For example, if the returned address is 0x00A23CF0, you might use a debugger command like:

buffer = (char*)0x00A23CF0

When you continue running the program, it will output something like:

Buffer address: 00A23CF0
Buffer content: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

This example demonstrates how malloc can be used during a debugging session to dynamically allocate memory, which can be particularly useful for testing memory allocation scenarios, injecting data structures, or simulating out-of-memory conditions.

Remember to use the free command in the debugger to release the allocated memory if you stop debugging before the free(buffer) line is executed, to prevent memory leaks in the debugged process.