C语言字符串替换

void exchg(char * str)
{
    if(str == NULL) return;

    int len = strlen(str);
    char tmp;
    
    for(int i=0,j=len-1;i<j;)
    {   
        tmp = str[i];
        str[i++] = str[j];
        str[j--] = tmp;
    }
}