Safe String Merge in C

Issue Analysis

The provided C code has a few critical issues:

1. Incorrect Memory Allocation in merge_string function: The size calculation for the new string is incorrect.
2. Unsafe Memory Operations:
   • Incorrect use of memcpy which doesn't account for null-terminators.
3. Mistyped Function Return: The merge_string function should return a properly allocated string capable of holding concatenated data including the null-terminator.

Solution

Code Analysis

1. get_ln Function:

   • This function reads a line from stdin and removes the trailing newline character.

2. merge_string Function:

   • This implementation is incorrect because it allocates memory using the size of pointer (sizeof(str1)), which yields incorrect results when used with memcpy.

Revised Code

#include 
#include 
#include 

/* Function to get a line from stdin */
void get_ln(char* buffer, size_t max_len)
{
    fgets(buffer, max_len, stdin);
    char* ptr_n = strrchr(buffer, '\n');
    if(ptr_n != NULL)
        *ptr_n = '\0';
}

/* Merges two strings into a newly allocated string */
char* merge_string(const char* str1, const char* str2)
{
    // Compute the length of the new string
    size_t len1 = strlen(str1);
    size_t len2 = strlen(str2);
    size_t total_length = len1 + len2 + 1;

    // Allocate memory for the new string
    char* new_string = (char *)malloc(total_length * sizeof(char));
    if (new_string == NULL) {
        // Handle memory allocation failure
        printf("Memory allocation failed\n");
        exit(1);
    }

    // Copy the two strings into the new string
    strcpy(new_string, str1);
    strcat(new_string, str2);

    return new_string;
}

int main(void)
{
    char str_1[200] = {0};
    char str_2[200] = {0};

    // Get input strings from the user
    get_ln(str_1, sizeof(str_1));
    get_ln(str_2, sizeof(str_2));

    // Merge the strings
    char* final = merge_string(str_1, str_2);

    // Output the final, merged string
    puts(final);

    // Free the allocated memory to avoid memory leaks
    free(final);

    return 0;
}

Explanation and Validation

1. Fixed Memory Allocation:

   • Correctly compute the total length as strlen(str1) + strlen(str2) + 1 and use this size to allocate memory for new_string.

2. Safe Memory Operations:

   • Use strcpy and strcat for copying the C strings properly, ensuring the null-terminators are managed.

3. Memory Management:

   • Ensure that allocated memory for final is freed to avoid memory leaks.

Practical Example

To demonstrate the usage of this code:

1. Compile the C code using gcc:

   gcc -o string_merge merge_string.c

2. Run the executable:

   ./string_merge

3. Enter the input strings when prompted:

   Input 1: Hello
   Input 2: World

   The program will output:

   HelloWorld

This revised implementation ensures the memory is correctly managed and the functions operate safely within the constraints of C's memory handling.