how to make a not null-terminated c string?

Creating Non-Null-Terminated C Strings: Advanced Memory Management

Explore the techniques and implications of working with C strings that do not adhere to the traditional null-termination convention, focusing on scenarios where explicit length tracking is necessary.

In C and C++, strings are conventionally null-terminated character arrays. This means a special null character (\0) marks the end of the string, allowing functions like strlen() and strcpy() to determine their length and boundaries. However, there are scenarios where you might encounter or need to create strings that are not null-terminated. This article delves into why and how to handle such strings, emphasizing the critical role of explicit length management.

Why Non-Null-Terminated Strings?

While less common in general-purpose programming, non-null-terminated strings appear in specific contexts, often for performance or interoperability reasons. Understanding these use cases is key to appreciating their necessity.

1. Fixed-Size Buffers/Protocols: Many network protocols, file formats, or hardware interfaces define fields as fixed-length character arrays. The length is known implicitly by the field's definition, not by a terminator.
2. Performance Optimization: In highly performance-critical applications, avoiding the overhead of scanning for a null terminator (e.g., strlen()) can be beneficial if the length is already known. This is especially true when dealing with very long strings or frequent string operations.
3. Binary Data Handling: When treating a sequence of bytes as a 'string' but those bytes might legitimately contain null characters (e.g., encrypted data, image data), a null terminator would prematurely truncate the data.
4. Substrings/Views: When working with a portion of a larger buffer, it's often more efficient to represent the substring as a pointer and a length, rather than copying it or modifying the original buffer to add a null terminator.

flowchart TD
    A[Start] --> B{String Type?}
    B -->|Null-Terminated| C[Implicit Length (Scan for '\0')]
    B -->|Non-Null-Terminated| D[Explicit Length (Store Length Separately)]
    C --> E[Standard C String Functions (strlen, strcpy)]
    D --> F[Custom Functions (memcpy, memcmp) with Length Parameter]
    E --> G[End]
    F --> G[End]

Decision flow for handling string types based on termination

Creating and Managing Non-Null-Terminated Strings

The fundamental principle when dealing with non-null-terminated strings is that you must always explicitly track their length. This typically involves storing the string's starting address (a pointer) and its length (an integer) together. C++ offers std::string_view and std::span as modern, safer ways to handle such scenarios.

In C, you'll often see this represented as a struct or by passing both a char* and a size_t to functions.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

// A common way to represent a non-null-terminated string in C
typedef struct {
    const char* data;
    size_t length;
} NonNullTerminatedString;

// Function to print a non-null-terminated string
void print_nn_string(NonNullTerminatedString s) {
    for (size_t i = 0; i < s.length; ++i) {
        putchar(s.data[i]);
    }
    putchar('\n');
}

int main() {
    char buffer[20];
    const char* source = "Hello, World!";
    size_t source_len = strlen(source);

    // Example 1: Copying a portion without null-termination
    // We only copy a part of the string
    memcpy(buffer, source, 5); // Copy "Hello"
    // buffer is now {'H', 'e', 'l', 'l', 'o', ?, ?, ...}
    // It is NOT null-terminated at index 5

    NonNullTerminatedString s1 = {buffer, 5};
    printf("NN String 1: ");
    print_nn_string(s1);

    // Example 2: Using a fixed-size buffer that might not be filled
    char fixed_buffer[10];
    const char* short_data = "short";
    size_t short_data_len = strlen(short_data);

    memcpy(fixed_buffer, short_data, short_data_len);
    // fixed_buffer is {'s', 'h', 'o', 'r', 't', ?, ?, ?, ?, ?}
    // It is NOT null-terminated unless we explicitly add it, or if the source was shorter

    NonNullTerminatedString s2 = {fixed_buffer, short_data_len};
    printf("NN String 2: ");
    print_nn_string(s2);

    // Example 3: A string that *could* contain nulls (binary data)
    char binary_data[] = {'A', 'B', '\0', 'C', 'D'};
    NonNullTerminatedString s3 = {binary_data, sizeof(binary_data)};
    printf("NN String 3 (binary): ");
    print_nn_string(s3);

    return 0;
}

C example demonstrating the creation and handling of non-null-terminated strings using a struct and memcpy.

C++ Alternatives: std::string_view and std::span

C++17 introduced std::string_view, a lightweight, non-owning view into a sequence of characters. It stores a pointer and a length, making it ideal for representing non-null-terminated strings or substrings without copying data. C++20's std::span offers similar functionality for any contiguous sequence of objects.

These types provide type safety and convenience, reducing the risk of errors associated with raw pointers and lengths.

#include <iostream>
#include <string>
#include <string_view>
#include <span> // C++20

void process_data(std::string_view sv) {
    std::cout << "Processing string_view: '" << sv << "' (length: " << sv.length() << ")\n";
}

void process_bytes(std::span<const char> s) {
    std::cout << "Processing span of bytes: ";
    for (char c : s) {
        if (c == '\0') {
            std::cout << "\\0";
        } else {
            std::cout << c;
        }
    }
    std::cout << " (length: " << s.size() << ")\n";
}

int main() {
    std::string full_string = "This is a longer string.";

    // Create a string_view from a substring without copying
    std::string_view sv1(full_string.data() + 5, 7); // "is a lo"
    process_data(sv1);

    // Create a string_view from a C-style array that isn't null-terminated
    char buffer[] = {'A', 'B', 'C', 'D', 'E'};
    std::string_view sv2(buffer, sizeof(buffer));
    process_data(sv2);

    // Example with binary data (containing nulls)
    char binary_payload[] = {'H', 'E', 'L', 'L', 'O', '\0', 'W', 'O', 'R', 'L', 'D'};
    std::span<const char> binary_span(binary_payload, sizeof(binary_payload));
    process_bytes(binary_span);

    return 0;
}

C++ example using std::string_view and std::span for safe handling of non-null-terminated character sequences.

Common Pitfalls and Best Practices

Working with non-null-terminated strings requires meticulous attention to detail. A single mistake can lead to severe bugs.

Pitfalls:

• Off-by-one errors: Incorrectly calculating or passing the length can lead to reading past the buffer or truncating data.
• Forgetting the length: If the length is lost or not passed along, the data becomes unusable.
• Mixing with null-terminated functions: Using printf("%s") or strlen() on a non-null-terminated string is a classic error.
• Lifetime issues: std::string_view and std::span do not own the data they point to. Ensure the underlying buffer outlives the view/span.

Best Practices:

• Encapsulate: Use structs (C) or classes/std::string_view/std::span (C++) to keep the pointer and length together.
• Clear documentation: Explicitly state in comments or API documentation whether a string parameter is expected to be null-terminated or requires an explicit length.
• Use memcpy/memcmp: For operations on raw byte sequences, memcpy, memcmp, memchr, etc., are the correct functions, as they operate on a specified number of bytes rather than searching for a terminator.
• Validate lengths: Always validate that provided lengths are within reasonable bounds, especially when receiving data from external sources.