Question

This is the strcat function that I have implemented, but I get a segmentation fault when I go to the line *dst++ = *src++;. I have incremented src till '\0' as I want to append the next string starting from there. Can you please tell me why it gives segmentation fault? Am I doing any logical mistake by doing *dst++ = *src++;?

char *strconcat(char *dst, char *src)
{
    char *fdst;
    fdst = dst;
    if (dst == '\0' || src == '\0')
    return fdst;

    while (*dst != '\0')
        dst++;
    while (*src != '\0')
        *dst++ = *src++;

    return fdst;
}

Hey i went through many solution given below and i made following changes but still i get segmentation problem when i start concatenate two strings, here is my entire code

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

char *strconcat(char *dst, const char *src);
void main()
{
    char *str1 = "Stack";
    char *str2 = "overflow";
    printf("MY CONCAT:%s\n",strconcat(str1, str2));
    printf("INBUILT CONCAT:%s\n",strcat(str1, str2));
}

char *strconcat(char *dst, const char *src)
{
    char *fdst;
    int dst_len = 0, src_len = 0;

    dst_len = strlen(dst);
    src_len = strlen(src);
    fdst = (char *) malloc(dst_len + src_len + 1);

    fdst = dst;
    if (src == NULL)
        return fdst;

    while(*dst)
    {
       dst++;
       fdst++;
    }
    while(*src)
       *fdst++ = *src++;
       *fdst = '\0';

    return fdst;
    }

Answer 1

Multiple problems here:

PROBLEM – I

The memory allocated with malloc

    fdst = (char *) malloc(dst_len + src_len + 1);

is lost as some lines later you are doing this:

    fdst = dst;

replacing the address returned by malloc in ‘fdst’ with the address of the target string..!! Hope you fix it on your own, its darn simple.

PROBLEM – II

After fixing that above problem, you’ll have to fix this:

while(*dst)
{
   dst++;
   fdst++;
}

don’t only increment, you’ll also have to copy your characters from dst to fdst as this will be your concatenated string.

PROBLEM – III

finally, you are doing this in the end..!!

return fdst;

you realize the mistake there right? Hope you can take care of that [ Hint: save the starting address and return it in the end, not the incremented pointer 😉 ]

Note: Not an optimised solution but, fixes your code.