when printing a char* in C only the first character is being printed

Question

I'm trying to write a function that will print out a substring of string however when printing it, only the first character in the array is printed.

As you can see in the code I've put a printf statement in the function after the substring is created and it displays properly. However when the function is passed into a printf function in the main function it only prints the first character.

Thanks for any help people are able to provide.

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

char *ft_substr (const char *s, unsigned int start, size_t len)
{
    char        *str;
    unsigned int    i;
    unsigned int    j;

    str = malloc(len * sizeof(char) + 1);
    i = start;
    j = 0;
    while (i < len + start)
    {
        str[j] = s[i];
        j++;
        i++;
    }
    str[j + 1] = '\0';
    printf("%s\n", str);
    free(str);
    return (str);
}

int main (void)
{
    char hello[] = "Hello World";
    char sub = *ft_substr(hello, 1, 4);

    printf("%s\n", &sub);
    return (0);

}

Answer 1

Before returning the pointer str from the function you freed all the allocated memory

free(str);
return (str);

So the returned pointer is invalid.

Remove the statement

free(str);

Another problem in the function is using an incorrect index in this statement

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

Just write

str[j] = '\0';

Also this declaration is incorrect

char sub = *ft_substr(hello, 1, 4);

It declares a single character while you need to declare a pointer that will point to the dynamically allocated string in the function. So write

char *sub = ft_substr(hello, 1, 4);

and then write

printf("%s\n", sub);

free( sub );

And if you are using the type size_t for the length of a string then use indices also of the type size_t.

Here is a demonstrative program.

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

char * ft_substr( const char *s, size_t start, size_t len )
{
    char *str = malloc( len + 1 );
    
    if ( str != NULL )
    {
        size_t i = 0;
        for ( ; i < len; i++ )
        {
            str[i] = s[start + i];
        }
        
        str[i] = '\0';
    }
    
    return str;
}

int main(void) 
{
    const char *hello= "Hello World";
    
    char *sub = ft_substr( hello, 1, 4 );

    if ( sub != NULL ) puts( sub );
    
    free( sub );
    
    return 0;
}

Its output is

ello

The function will be more safer if it will check whether the starting index and the length are specified correctly. For example

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

char * ft_substr( const char *s, size_t start, size_t len )
{
    char *str = NULL;
    
    size_t n = strlen( s );
    
    if ( start < n )
    {
        if ( n - start < len ) len = n - start;
    
        str = malloc( len + 1 );
    
        if ( str )
        {
            memcpy( str, s + start, len );
            str[len] = '\0';
        }
    }
    
    return str;
}

int main(void) 
{
    const char *hello= "Hello World";
    
    char *sub = ft_substr( hello, 1, 4 );

    if ( sub != NULL ) puts( sub );
    
    free( sub );
    
    return 0;
}