String appending function doesn't work - c

Question

I tried to create a function that easily appends two strings. The source string has to be previously allocated dynamically. I used my knowledge but this resulted in ub and any sort of leaks.
void strapp (char *source_offset, int position, char *appendage)
{
size_t appendage_size = strlen(appendage);
source_offset = realloc(source_offset, strlen(source_offset) + appendage_size);
sprintf( &source_offset[position], "%s%s", appendage, source_offset + (position + appendage_size) );
}
What am I doing wrong?

Answer 1

source_offset + (position + appendage_size) is somehow strange. It seems that you tried to catenate the second string with a substring of the first copying the result in the first string.
source_offset + (position + appendage_size) is the suffix of the source string starting at offset position+appendage_size which is a non-sense as it is past the end of the source string...
May be you wanted something like this?
If you want to catenate the two string then the following is correct:
size_t appendage_size = strlen(appendage);
source_offset = realloc(source_offset, position + appendage_size + 1);
sprintf( &source_offset[position], "%s", appendage );
Which appends appendage to source_offset starting at position.
Now if you want to insert appendage in the middle this can be a little more tricky:
size_t appendage_size = strlen(appendage);
char *result = malloc(strlen(source_offset) + appendage_size + 1);
char cut = source_offset[position];
source_offset[position] = '\0';
sprintf( result, "%s%s%c%s", source_offset,appendage,cut,source_offset+position+1);
// do hat you want with result
Beware that realloc may change the base address of the initial memory, so you can't do things like this as the parameter value source_offset will be changed only locally.

Answer 2

size_t appendage_size = strlen(appendage) + 1; /* To hold a \0 character */
The new array allocated should be able to hold both the null characters in each string
strlen(string) + 1

Answer 3

actually your coes does not append string B to string A, it does append string B at position n of string A along with string A, with some out of memory offset of String A
char *
strapp(char *dest, const char *src)
{
size_t i,j;
for (i = 0; dest[i] != '\0'; i++)
;
for (j = 0; src[j] != '\0'; j++)
dest[i+j] = src[j];
dest[i+j] = '\0';
return dest;
}
the above code appends string B to String A and returns pointer to string a A;
char* strapp (char *source_offset, const char *appendage, int position)
{
char* temp = NULL:
size_t appendage_size = strlen(appendage);
if (( temp = realloc(source_offset, strlen(source_offset) + appendage_size + 1)) == NULL )
return NULL;
else
source_offset = temp;
sprintf( &source_offset[position], "%s", appendage);
return source_offset;
}

Answer 4

If you want to fix your function try this:
void strapp (char **source_offset, int position, char *appendage)
{
size_t appendage_size = strlen(appendage);
*source_offset = (char*)realloc(*source_offset, strlen(*source_offset) + appendage_size);
sprintf( *source_offset + position, "%s%s", appendage, *source_offset + (position + appendage_size) );
}
And call this like strapp(&str, 10, "INSERTION"); // &str - pointer to pointer.
The problem was in the following: when you sent pointer to function you are able to change data pointed by this pointer, but cannot change the pointer, so memory allocation made inside function do mot change address of initial string.

Answer 5

Since I was asked to provide an example of function usage and respectively the output. I converted this code to something compilable to mingw and it works like expected.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void strapp (char *source_offset, int position, char *appendage)
{
size_t appendage_size = strlen(appendage) + 1;
source_offset = realloc(source_offset, strlen(source_offset) + appendage_size);
sprintf( &source_offset[position], "%s%s", appendage, source_offset + (position + appendage_size) );
}
int main(void)
{
char *str1 = malloc(11 + 1);
sprintf(str1, "Hello World");
strapp(str1, 4, " Horrible ");
printf(str1);
free(str1);
return 0;
}
Output: Hell Horrible

Answer 6

Partial fix of the problem I found while investigating.
If I add a substring to a location of the source string like that:
sprintf(&source[4], "new");
the source string will be terminated by the "new" and the rest of the string will not be shown.
But if I declare the substring "new" and use
memcpy(&str1[4], appendage, 3); it does the job.
There, this is the working function:
void strapp (char *source_offset, int position, char *appendage)
{
size_t appendage_size = strlen(appendage) + 1;
char copy[strlen(source_offset) + 1];
strcpy(copy, source_offset);
source_offset = realloc(source_offset, strlen(source_offset) + appendage_size);
memcpy(&source_offset[position], appendage, strlen(appendage));
sprintf(&source_offset[position + strlen(appendage)], &copy[position]);
}