Typecast:LPCTSTR to Char * for string concatenate operation

Question

Can u Give solution for this code of typecasting, LPCTSTR(here lpsubkey) to Char* for below code snippet ,

char* s="HKEY_CURRENT_USER\\";
strcat(s,(char*)lpSubKey); 
printf("%S",s);

here it makes error of access violation ,so what will be the solution for that?. ...thanks in advance

Answer 1

These are not the same thing. What are you trying to do?

The problem is you are trying to append to a string that you have not reserved memory for. Try:

char s[1024] = "HKEY_CURRENT_USER"; 
strcat(s,(char*)lpSubKey ); 
printf("%S",s);

Do be careful with the arbitrary size of 1024. If you expect your keys to be much longer your program will crash.

Also, look at strcat_s.

Answer 2

ATL and MFC has set of macros to such conversion, where used next letters:

• W - wide unicode string
• T - generic character string
• A - ANSI character string
• OLE - BSTR string,

so in your case you need T2A macros

Answer 3

There are several issues with your code that might well lead to the access violation. I don't think any have anything to do with the cast you mentioned.

You are assigning a pointer to the first element of a fixed size char array to a char * and then attempt to append to this using strcat. This is wrong as there is no additional space left in the implicitly allocated string array. You will need to allocate a buffer big enough to hold the resulting string and then copy the string constant in there before calling strcat. For example, like so:

char *s = (char*)malloc(1024 * sizeof(char));
strcpy(s, "HKEY_CURRENT_USER\\");
strcat(s, T2A(lpSubKey));
printf("%s", s);
free(s);

Please note that the fixed size array I'm allocating above is bad practise. In production code you should always determine the correct size of the array on the go to prevent buffer overflows or use functions like strncat and strncpy to ensure that you are not copying more data into the buffer than the buffer can hold.

Answer 4

strcat does not attempt to make room for the combination. You are overwriting memory that isn't part of the string. Off the top of my head:

char *strcat_with_alloc(char *s1, char *s2)
{
    if (!s1 || !s2) return NULL;

    size_t len1 = strlen(s1);
    size_t len2 = strlen(s2);

    char *dest = (char *)malloc(len1 + len2 + 1);
    if (!dest) return NULL;

    strcpy(dest, s1);
    strcat(dest, s2);

    return dest;
}

now try:

char* s="HKEY_CURRENT_USER\\";
char *fullKey = strcat_with_alloc(s,(char*)lpSubKey); 
if (!fullKey)
    printf("error no memory");
else {
    printf("%S",fullKey);
    free(fullKey);
}