Beyond std::find: Leveraging std::memchr for Efficient Character Search in C++

Function Definition

const void* memchr( const void* ptr, int ch, std::size_t count );

Parameters

• count: This is the number of bytes (characters) to search within the memory block pointed to by ptr.
• ch: This is an integer representing the character you're looking for. Internally, it gets converted to an unsigned char before the search.
• ptr: This is a pointer to the memory block you want to search. Since std::memchr treats the memory as a sequence of bytes, it takes a void* pointer.

Return Value

• If the character is not found within the specified number of bytes, the function returns a nullptr.
• If std::memchr successfully finds the character within the specified number of bytes, it returns a pointer to the location of that character within the memory block.

Key Points

• Unlike some string searching functions that rely on null terminators, std::memchr works with a defined search length (count) making it more flexible.
• It searches sequentially through the memory block, stopping once it finds a match or reaches the end of the specified number of bytes.
• std::memchr searches for the character value, not considering its interpretation as an actual character. This makes it suitable for binary data searches as well.

Example

#include <cstring>

int main() {
  const char* str = "Hello, world!";
  char target = 'o';

  // Find the first occurrence of 'o' in the string
  const void* result = std::memchr(str, target, strlen(str));

  if (result != nullptr) {
    // Character found, convert the pointer back to char* and print the index
    int index = reinterpret_cast<const char*>(result) - str;
    std::cout << "The first '" << target << "' is at index: " << index << std::endl;
  } else {
    std::cout << "Character '" << target << "' not found in the string." << std::endl;
  }

  return 0;
}

In this example, std::memchr searches for the character 'o' within the string "Hello, world!". If found, it returns a pointer to the location of 'o'. We then calculate the index by subtracting the original string pointer from the returned pointer.

---

Finding a null terminator within a memory block

#include <cstring>

int main() {
  char buffer[100] = "This is a test string";

  // Search for the null terminator
  const void* null_ptr = std::memchr(buffer, '\0', sizeof(buffer));

  if (null_ptr != nullptr) {
    // Null terminator found, calculate the length of the string
    size_t string_length = reinterpret_cast<const char*>(null_ptr) - buffer;
    std::cout << "String length: " << string_length << std::endl;
  } else {
    std::cout << "Buffer might not be a null-terminated string." << std::endl;
  }

  return 0;
}

In this example, we search for the null terminator (\0) within a character buffer. If found, we calculate the string length based on the position of the null terminator.

#include <iostream>

int main() {
  unsigned char data[] = {0xAB, 0xCD, 0xEF, 0x12, 0x34, 0xAB};
  size_t data_size = sizeof(data);
  unsigned char pattern[] = {0xCD, 0xEF};
  size_t pattern_size = sizeof(pattern);

  // Search for the pattern within the binary data
  const void* found_pattern = std::memchr(data, pattern[0], data_size - pattern_size + 1);

  if (found_pattern != nullptr) {
    // Check if the following bytes match the pattern
    if (memcmp(found_pattern, pattern, pattern_size) == 0) {
      std::cout << "Pattern found at offset: " 
                << reinterpret_cast<const char*>(found_pattern) - data << std::endl;
    } else {
      std::cout << "Found a single byte match, but not the complete pattern." << std::endl;
    }
  } else {
    std::cout << "Pattern not found in the data." << std::endl;
  }

  return 0;
}

---

1. std::find with std::string
   

• Example:
• It searches for a specific character or substring within the string and returns an iterator pointing to the first occurrence.
• If you're working with std::string objects, the find member function is a safer and more convenient alternative.

#include <string>

int main() {
  std::string str = "Hello, world!";
  char target = 'o';

  std::string::iterator it = str.find(target);

  if (it != str.end()) {
    // Character found, print the index
    int index = std::distance(str.begin(), it);
    std::cout << "The first '" << target << "' is at index: " << index << std::endl;
  } else {
    std::cout << "Character '" << target << "' not found in the string." << std::endl;
  }

  return 0;
}

1. Loop with manual comparison
   

• This approach is less efficient than std::memchr but offers more control over the search logic.
• For simple cases, a basic loop that iterates through the memory block and compares each byte with the target character can be used.

1. SIMD (Single Instruction Multiple Data) Instructions (advanced)
   

• This approach requires knowledge of assembly language or compiler intrinsics and is generally for performance-critical scenarios.
• On modern processors, specialized SIMD instructions can be used for very efficient byte-level searches.

• Readability
  Do you prioritize clear and concise code over ultimate performance?
• Performance
  How critical is search speed for your application?
• Data type
  Are you working with std::string objects or raw memory?