Thread: Can not copy a string from index to index?

Hello, I have this piece of code, which does not actually work, it does not copy the desired symbols to the needed destination:

Code:

static char* aux_cutStringFromTo(const char* src, int from, int to)
{
    if ( from > to ) return 0;
    if ( aux_strlen(src) < 0) return 0;
    int chars = to - from;
    printf("SIZE IS: [%d] \n", chars);
    char* str = (char*) malloc( sizeof(char)* chars+1);
    if ( !str ) return 0;
    char* begin = &src[from];
    while ( from++ < to )
    {
        printf("[%c] >> [%c] \n", *begin, *str);
        *str++ = *begin++;
    }
    *str++ = 0 ;
    puts("\n\n");
    return str;
}

Assuming that while [from] is less than [to] a simple strcpy is performed with no 0 check.

The print message shows that str is empty, no chars are assigned to it.

Code:

[1] >> [] 
[2] >> [] 
[3] >> [] 
[4] >> [] 
[5] >> [] 
[6] >> [] 
[7] >> [] 
[8] >> [] 
[9] >> [] 
[1] >> [] 
[0] >> []

Even if the print is below the assignment, it`s the same. And the returned [str] is empty.

The function could be written without any assistance from other functions, while allowing for really long strings:

Code:

#include <stdlib.h>
#include <errno.h>

/* Return a dynamically allocated duplicate of src[first .. limit-1],
 * or NULL if nothing to copy (never returns an empty string).
*/
char *dup_substr(const char *const src,
                 const size_t first, const size_t limit)
{
    if (src != NULL && limit > first) {
        const char *const s0 = src + first;
        const char *const s1 = src + limit;
        const char *s = src;
        char *dst, *d;

        /* Verify src is at least first characters long. */
        while (s < s0 && *s != '\0')
            s++;

        /* Premature end of string? */
        if (s < s0 || *s == '\0') {
            errno = 0;
            return NULL;
        }

        /* Allocate a dynamic string large enough
         * to hold (limit - first) characters, plus a '\0'. */
        dst = malloc(limit - first + 1);
        if (dst == NULL) {
            errno = ENOMEM;
            return NULL;
        }
        d = dst;

        /* Copy s to d, until s reaches end of string or
         * limit, whichever occurs first. */
        while (s < s1 && *s != '\0')
            *(d++) = *(s++);

#ifdef WISH_TO_BE_CAREFUL
        /* Clear the rest of the allocated buffer,
         * also adding the end-of-string '\0'. */
        {
            char *const dend = dst + limit - first;
            while (d <= dend)
                *(d++) = '\0';
        }
#else
        /* Add the end-of-string mark. */
        *d = '\0';
#endif        

        errno = 0;
        return dst;

    } else {
        errno = EINVAL;
        return NULL;
    }
}

The above always sets errno: To zero if success, to an error code otherwise. It never returns an empty string; in that case, it returns NULL with errno==0.

Oh, I`ve completely forgot the begin ptr. My mistake . . . Thanks for hinging.

Originally Posted by heatblazer

Thanks for hinging.

I definitely did not!