C: Problem in comparing two strings in a function

Question

Good morning everyone, I have to simulate the operation of the strstr() function with a function written by me.

In the code I slide the original string in a temporary string and then the comparison with the string to look for, if they are equal it should return 1.

But even if the strings are equal and of the same length the code never enters the if cycle and therefore never returns 1.

My code:

int *strstr_new(char *s7, char *s8) {

int length_s7 = strlen(s7);
int length_s8 = strlen(s8);
char search_string[length_s8];
printf("%d\n", length_s8);

for(int i=0; i<length_s7; i++) {
    for(int j=0; j<length_s8; j++) {
        search_string[j] = s7[i+j];
        search_string[j+1] = '\0';
    }
    printf("%s\n", s8);
    printf("%s\n", search_string);
    printf("%d\n", length_s8);
    printf("%d\n", strlen(search_string));
    //search_string[length_s8+1] = '\0';
    if(search_string == s8) {
        return(1);
    }
}
if(search_string != s8) {
    return(NULL);
}}

Does someone have an idea of where I'm wrong?

Thanks!

Answer 1

The function declaration

int *strstr_new(char *s7, char *s8);

looks very strange.

For example why is the return type int *? Why are function parameters named s7 and s8 instead of for example s1 and s2? Why are not the function parameters qualified with const?

Creating a variable length array within the function is inefficient and redundant and can lead to stack exhaustion.

char search_string[length_s8];

This loops

for(int j=0; j<length_s8; j++) {
    search_string[j] = s7[i+j];
    search_string[j+1] = '\0';
}

invokes undefined behavior because this statement

search_string[j+1] = '\0';

writes beyond the array when j is equal to length_s8 - 1.

In this statement

if(search_string == s8) {

there are compared two pointers and it is evident that they are unequal because they point to different arrays.

Without using standard C functions except the function strlen (that could be also defined explicitly) the function can be declared and defined the following way

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

char * strstr_new( const char *s1, const char *s2 )
{
    char *p = NULL;
    
    size_t n1 = strlen( s1 );
    size_t n2 = strlen( s2 );
    
    if ( !( n1 < n2 ) )
    {
        for ( size_t i = 0, n = n1 - n2 + 1; p == NULL && i < n; i++ )
        {
            size_t j = 0;
        
            while ( j < n2 && s1[i + j] == s2[j] ) ++j;
        
            if ( j == n2 ) p = ( char * )( s1 + i );
        }
    }
    
    return p;
}

int main( void ) 
{
    const char *s1 = "strstr_new";
    const char *s2 = "str";
    
    for ( const char *p = s1; ( p  = strstr_new( p, s2 ) ) != NULL; ++p )
    {
        puts( p );
    }
}   

The program output is

strstr_new
str_new

If you are allowed to use standard string functions along with strlen then the loop within the function strstr_new can be simplified the following way

for ( size_t i = 0, n = n1 - n2 + 1; p == NULL && i < n; i++ )
{
    if ( memcmp( s1 + i, s2, n2 ) == 0 ) p = ( char * )( s1 + i );
}