memset (Memory Set Utility)
Description

The memset command allows you to fill a block of memory with a specified value in the debugged process. This feature is useful for initializing memory regions, clearing data, or setting up specific memory patterns for testing and debugging purposes.

Syntax

memset <dst> <val> <size>
Parameters
  • <dst>: Destination address where the memory will be set.
  • <val>: The value to set (0x00 - 0xFF). This is treated as a byte value.
  • <size>: Number of bytes to set starting from the destination address.
Usage Notes
  1. The command uses the current process context in the debugger.
  2. The destination address can be specified in various formats:
    • Hexadecimal: Prefixed with ‘0x’ (e.g., 0x00656789)
    • Decimal: Plain number (e.g., 6789456)
    • Symbol name: If symbols are loaded (e.g., &myVariable)
  3. The value parameter is interpreted as a byte (0-255 or 0x00-0xFF).
  4. Size can use expressions for convenience (e.g., 1024*1024 for 1 MB).
  5. Ensure that the destination memory range is within the process’s address space and is writable.
Examples

  1. Basic usage:

    memset 0x00656589 0xFF 200

    Sets 200 bytes starting at address 0x00656589 to the value 0xFF.

  2. Using a symbol name:

    memset &myBuffer 0x00 1024

    Clears (sets to zero) 1024 bytes starting at the address of myBuffer.

  3. Setting a larger region:

    memset 0x20000000 0xAA 1024*1024

    Sets 1 MB of memory starting at address 0x20000000 to the value 0xAA.

Error Handling
  • Invalid or inaccessible address: Shows an error if the destination address is invalid or not accessible.
  • Destination not writable: Displays an error message if the destination range is not writable.
  • Invalid value: Reports an error if the specified value is not within the valid range (0-255).
  • Size exceeds available memory: Sets as much as possible and reports actual bytes set.
  • Memory write issues: Shows appropriate error messages if unable to write to the specified memory range.
Example Code and Usage

Sample code 

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

int main()
{
    // Buffer size
    int bufferSize = 48;
    
    // Create buffer
    char* buffer = (char*)malloc(bufferSize);
    
    // Initialize buffer with a pattern
    for (int i = 0; i < bufferSize; i++) {
        buffer[i] = 'A' + (i % 26);
    }
    
    printf("Before memset:\n");
    printf("Buffer: %.*s\n", bufferSize, buffer);
    
    // Use memset here to fill part of the buffer
    
    printf("\nAfter memset:\n");
    printf("Buffer: %.*s\n", bufferSize, buffer);
    
    // Release memory
    free(buffer);
    return 0;
}

After initializing the buffer and before the second print statement, you can use memset to fill part of the buffer with a specific value:

memset buffer+16 0x58 16

This command will set 16 bytes starting at buffer+16 to the value 0x58 (ASCII ‘X’).

When you run the program after using memset, it will output:

Before memset:
Buffer: ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUV

After memset:
Buffer: ABCDEFGHIJKLMNOPXXXXXXXXXXXXXXXXGHIJKLMNOPQRSTUV

This example demonstrates how memset can be used to fill a specific region of memory with a given value. This is particularly useful for initializing data structures, clearing sensitive information, or creating specific memory patterns for testing and debugging purposes.