finding substring and replacing it in C

Question

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

int main(int argc, char *argv[]) {     

char * str = "Testing replace text...\n\n";


        char* buffer = malloc(sizeof(char));
        char* insertPoint = &buffer[0];
        char* copy = str; 
        char* p = strstr(str, "epl");
        char* g = "gard";
        int size = 0; 

        size = p-copy; //p = 9, which is the number of elemts till the first element of the substring
                       //want to allocate this space, and then increment insertPoint, by that amt(it'll be pointing
                       // to nothing)
        buffer = realloc(buffer, size);
        printf("Size: %d\n", size);
        memcpy(insertPoint, copy, size);
        printf("COPY: %s\n", buffer);

        copy += size;
        buffer = realloc(buffer, size+strlen(g));
        insertPoint += size;
        printf("%c", *insertPoint);
        memcpy(insertPoint, g, strlen(g)); //insert after the 9 letters, the string the size of g
        size += strlen(g); //size if the size of the buffer
        printf("Size2: %d\n", size);
        printf("COPY2: %s\n", buffer);

  return EXIT_SUCCESS;
}

Just some quick experimental code; I am just trying to replace the substring epl in str with "gard" but when I print it out there are no changes to the string buffer I am printing, meaning the first string im printing works where it gets all the letters into buffer before the substring occurs, but when I try to replace with substring it doesn't work. I've testing the individual pointers and they all seem correct...not sure what is happening, any insight? Thanks...fully runnable program.

Possible duplicate of [What is the function to replace string in C?](https://stackoverflow.com/questions/779875/what-is-the-function-to-replace-string-in-c) — Suvam Roy, Dec 24 '17 at 05:58
no i've looked at ways to do a substring replacement in C and i based this small program on one of the answers on this site - but it's not working. besides this is a specific problem with this code @AndrewWatson — GCGSAUCE, Dec 24 '17 at 06:09
`malloc(sizeof(char))` is wrong. Rather, initialize to NULL. — Antti Haapala -- Слава Україні, Dec 24 '17 at 06:10
I don't know if it's your actual problem, but when you realloc "buffer," there's no guarantee that it starts at the same location as it did previously, so "insertPoint" is effectively random after that point. — EdmCoff, Dec 24 '17 at 06:11
You assign `insertPoint` to point to the first character of `buffer`, but the `realloc()` call reassigns `buffer` to a different location, at which point `insertPoint` no longer points to it. — John Gordon, Dec 24 '17 at 06:12
Also you don't appear to be adding a null terminator, which is required for strings. — John Gordon, Dec 24 '17 at 06:13
@GCGSAUCE you're using C99, so there is no reason to declare all variables in the beginning like that. Declare them as you need them. — Antti Haapala -- Слава Україні, Dec 24 '17 at 06:16
a good habit is to check return values of runtime functions when they have one e.g. realloc — AndersK, Dec 24 '17 at 06:22

Lalmani Dewangan · Answer 1 · 2017-12-24T17:54:02.260

You are not appending remaining text "ace text..." after replacing "epl".

Can you try code below

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

int main() {

    char *source_string = "Testing replace text...";
    char *search_string = "epl";
    char *replace_string = "gard";

    int search_length = strlen(search_string);
    int replace_length = strlen(replace_string);

    // Find start position of search string
    char *start = strstr(source_string, search_string); // start pointing to "eplace text..."
    int intial_length = start - source_string; // intial_length = 9

    // Get remaining text which should append after replace
    char *remaining_string = (start + search_length); // remaining_string pointing to "ace text..."
    int remaining_length = strlen(remaining_string); // remaining_length = 11

    // Find total length of string after replacing text
    int total_string_length = intial_length + replace_length + remaining_length; // 24

    char *buffer = (char *)malloc(total_string_length + 1); // +1 for null pointer
    char *current_index = buffer;

    // Add initial text
    memcpy(current_index, source_string, intial_length);
    current_index += intial_length;

    // Add replace text
    memcpy(current_index, replace_string, replace_length);
    current_index += replace_length;

    // Add remaining text
    memcpy(current_index, remaining_string, remaining_length);
    current_index += remaining_length;

    memcpy(current_index, "\0", 1); // add null pointer at last
    printf("Final Output: %s", buffer); // Final Output: Testing rgardace text...

    return 0;
}

score 0 · Accepted Answer · answered Dec 24 '17 at 12:30

I think the problem in your code arise because strlen does not include the terminating zero. I tried to fix your code but in the end I found it easier to re-write it anew (and using more sensible variable names).

The following simple four steps work. The continuous use of strlen may be replaced by variables, but I left them for clarity. (Also, a good compiler may very well optimize this code by leaving the calls out.)

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

int main(int argc, char *argv[])
{     
    char *str = "Testing replace text...\n\n";
    char* buffer;
    char* find_str = "epl";
    char* repl_str = "gard";

    char *find_str_pos = strstr (str, find_str);

    /* 1. create new buffer of the correct size */
    buffer = malloc (strlen(str) - strlen(find_str) + strlen(repl_str) + 1);
    /* 2. copy first part */
    memcpy (buffer, str, find_str_pos - str);
    /* 3. add new text */
    memcpy (buffer + (find_str_pos - str), repl_str, strlen(repl_str));
    /* 4. append original text */
    memcpy (buffer + (find_str_pos - str) + strlen(repl_str), find_str_pos + strlen(find_str), strlen(find_str_pos) - strlen(repl_str) + 1);

    printf ("-> [%s]\n", buffer);

    return EXIT_SUCCESS;

}

finding substring and replacing it in C

2 Answers2