string combining

[logantr]

#include <stdio.h>
#include <ctype.h>
#include <string.h>
 
void interpose(char s[], char a[], int pos)
{
    char b[80];
    int j=0;
    int l=0;
    for(int i=pos;s[i]!='\0';i++)
 
    {
        b[j]=s[i];
        j++;
    }
    b[j]='\0';
    for(int c=0;a[c]!='\0';c++)
    {
        s[pos]=a[c];
        pos++;
    }
    for (int k=pos;b[l]!='\0';pos++)
    {
        s[pos]=b[l];
        l++;
    }
    s[pos]='\0';
}
 
int main()
{
    char str1[80];
    char str2[80];
    int posi;
    printf("String1:\n");
    gets(str1);
    printf("String2:\n");
    gets(str2);
    printf("Sayıyı giriniz");
    scanf("%d", &posi);
    interpose(str1,str2,posi);
    puts(str1);
 
 
    return 0;
}

The program should take 2 string parameter and 1 int parameter for combining. (without any str functions, the functions should be written by coder)

my problem is when i enter int value program crashes.
For example,
str1='Good';
str2=' Morning';
int x= 5;
The output will be 'Good Morning'

[donbock]

If you call interpose() with these arguments:
interpose("Good", "Morning", 5);

then from line 5, s points at "Good" and line 10 is equivalent to

for (i=5; s[i]!='\0'; i++)
 
s[0] == 'G'
s[1] == 'o'
s[2] == 'o'
s[3] == 'd'
s[4] == '\0'

s[5] is beyond the end of the array, and this access thus causes undefined behavior.

[logantr]

Done.Thank to you for satisfying answer.