C string replace, string swap

strf.c

#include <stdlib.h>
#include <assert.h>
#include <string.h>
#include <wchar.h>

/*
 * Swap string
 * Returns copy of the given string in the reverse order.
 */
char *
strswp(const char *str)
{
    register int i = 0;
    register int j = 0;
    char *chr = NULL;
    size_t len = 0;

    assert(str != NULL);
	
    len = strlen(str);
    chr = (char *)malloc((len + 1) * sizeof(char));
    if(NULL == chr) return NULL;

    j = (int)len -1;
    for(i = 0; i < (int)len; ++i)
    {
        chr[i] = str[j--];
    }
    chr[i] = '\0';

    return chr;
}

wchar_t *
wcsswp(const wchar_t *wstr)
{
    register int i = 0;
    register int j = 0;
    wchar_t *wchr = NULL;
    size_t len = 0;

    assert(wstr != NULL);
	
    len = wcslen(wstr);
    wchr = (wchar_t *)malloc((len + 1) * sizeof(wchar_t));
    if(NULL == wchr) return NULL;

    j = (int)len - 1;
    for(i = 0; i < (int)len; ++i)
    {
        wchr[i] = wstr[j--];
    }
    wchr[i] = L'\0';
    return wchr;
}



/*
 * Replace substring.
 * Replaces given substring by another substring.
 * Both substrings have to be the same length.
 * Returns the number of replaced instances of the character or substring. 
 * Zero if the string is not changed.
 */
int
strrpl(const char *str, const char *substr, const char *repl)
{
    register int i = 0;
    char *chr = NULL;
    size_t sub_len = 0;
    size_t repl_len = 0;

    assert(str != NULL);
    assert(substr != NULL);
    assert(repl != NULL);
	
    chr = strstr(str, substr);
    if(chr == NULL) return 0;

    sub_len = strlen(substr);
    repl_len = strlen(repl);

    for(i = 0; i < (int)sub_len && i < (int)repl_len; ++i)
    {
        *chr = repl[i];
        chr++;
        if(chr == '\0') break;
    }

    return i;
}


int
wcsrpl(const wchar_t *wstr, const wchar_t *wsubstr, const wchar_t *wrepl)
{
    register int i = 0;
    wchar_t *chr = NULL;
    size_t sub_len = 0;
    size_t repl_len = 0;

    assert(wstr != NULL);
    assert(wsubstr != NULL);
    assert(wrepl != NULL);
	
    chr = wcsstr(wstr, wsubstr);
    if(chr == NULL) return 0;

    sub_len = wcslen(wsubstr);
    repl_len = wcslen(wrepl);

    for(i = 0; i < (int)sub_len && i < (int)repl_len; i++)
    {
        *chr = wrepl[i];
        chr++;
        if(chr == '\0') break;
    }

    return i;
}