How to use malloc in functions with dynamic strings and not get symbols added at the end

Question

I am trying to create a function fib(n) that takes the two characters a and b as initial arguments then prints every single term of the Fibonacci sequence for a given index n

the sequence is described as:

S0=" "
S1="b"
S2="a"
Sn="S(n-1)S(n-2)"

meaning for example the result for n=6 should be :

"b","a","ab","aba",abaab","abaababa"...

My issue is when I run the code the strings get random symbols added to them which if removed would give the desired result and I can't find the reason for that anywhere.

When I run the code and give n the value 6 This is the returned result

here is the code :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void combine(char**s,char**ch1,char**ch2)
{
    int size=strlen(*ch1)+strlen(*ch2);
    free(*s);
    *s=(char*)malloc(size*sizeof(char)+1);
    strcpy(*s,*ch1);
    strcat(*s,*ch2);
}
void replace(char**ch1,char**ch2)
{
    free(*ch1);
    *ch1=(char*)malloc(strlen(*ch2)*sizeof(char)+1);
    strcpy(*ch1,*ch2);
}
void fib(int n)
{
    char*a,*b,*s;
    int i;
    printf("S0 = ' '\n");
    if(n>=1)
    {   
        printf("S1 = 'b'\n");
        if(n>=2)
        {
            printf("S2 = 'a'\n");
            if(n>2)
            {
                s=(char*)malloc(sizeof(char)+1);
                b=(char*)malloc(sizeof(char)+1);b[0]='b';
                a=(char*)malloc(sizeof(char)+1);a[0]='a';
                for(i=3;i<=n;i++)
                {
                    combine(&s,&a,&b);
                    printf("S%d = '%s'\n",i,s);
                    replace(&b,&a);
                    replace(&a,&s);
                }
                
            }
        }
    }
}
int main()
{
    int n;
    char *a,*b,*s;
    printf("Give the index at which to stop : ");
    scanf("%d",&n);
    fib(n);
}

Answer 1

Doing this in proper C means dumping the & references of C++ and adding one level of pointer indirection. This only needs to be done for parameters whose pointers need to be modified.

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

void combine(char **ps, const char *ch1, const char *ch2)
{
    int size=strlen(ch1) + strlen(ch2);

    free(*ps);

    *ps = malloc(size + 1);

    strcpy(*ps, ch1);
    strcat(*ps, ch2);
}

void replace(char **pch1, const char *ch2)
{
    free(*pch1);

    *pch1 = malloc(strlen(ch2) + 1);

    strcpy(*pch1, ch2);
}

void fib(int n)
{
    printf("S0 = ' '\n");

    if (n >= 1)
    {
        printf("S1 = 'b'\n");

        if (n >= 2)
        {
            printf("S2 = 'a'\n");

            if (n > 2)
            {
                char *s = (char*)malloc(2);
                char *b = (char*)malloc(2); b[0]='b'; b[1] = 0;
                char *a = (char*)malloc(2); a[0]='a'; a[1] = 0;

                int i;
                for (i = 3; i <= n; i++)
                {
                    combine(&s, a, b);

                    printf("S%d = '%s'\n", i, s);

                    replace(&b, a);
                    replace(&a, s);
                }

            }
        }
    }
}

int main()
{
    int n;
    printf("Give the index at which to stop : ");
    scanf("%d",&n);
    fib(n);
}

Looking at combine(), the only pointer that needs to be modified is the first one, so I've changed char &*s to char **ps, and we dereference it with *ps throughout. The other parameters are just "regular" pointers because the pointers themselves don't need to be modified. I made them const char * for good measure.

Also of note are the memory allocations of a and b:

char *b = (char*)malloc(2); b[0]='b'; b[1] = 0;
char *a = (char*)malloc(2); a[0]='a'; a[1] = 0;

We need two bytes each, and the NUL byte after; without this your strings run off into crazy-town. This is the main thing that broke the program.

I think it's not necessary to multiply all the allocation counts by sizeof(char) but maybe that's your personal style.

EDIT:The reason for the appearance of the bizzard symbols is in Function fib(int n) is the not initialising a[1] and b[1] meaning that the dynamic string did not have an end mark \0 thus resulting in weird symbols being attached to the end of every string passed to a function weather it being combine() or replace() this issue is fixed by initialising a[1]=0;b[1]=0;

EDIT: The filename needs to end in .c, not .cpp to compile as C and not C++

EDIT: When I compile the above code, I get:

$ ./a.out
Give the index at which to stop : 6
S0 = ' '
S1 = 'b'
S2 = 'a'
S3 = 'ab'
S4 = 'aba'
S5 = 'abaab'
S6 = 'abaababa'