problem in getting os version in use

i made this function today that get os version so far its good but see problem is that i need also to get service pack i want a char * return so i cant strcat or calloc or anyother so i used temp var then i concatenate it but it worked inside the function itself but it didnt work when i printed it in main ?? so here is the code

Code:

#include <stdio.h>
#include <windows.h>
#define DELIT(P,N) P[N]=0
char *windows[]=
{
    "windows 7","Windows Xp","Windows 2000","Windows Vista"
};
char * GetWindows(char *Cstring,int *BL)
{
    char *buffer;
    OSVERSIONINFO Str;
    ZeroMemory(&Str, sizeof(OSVERSIONINFO));
    Str.dwOSVersionInfoSize = sizeof(OSVERSIONINFO);
    GetVersionEx(&Str);
    if(Str.dwMinorVersion)
        switch(Str.dwMajorVersion) {
            case 6:
                buffer=windows[0];
                break;
            case 5:
            {
                int len,len1;
                Cstring=Str.szCSDVersion;
                len=strlen(Cstring);
                len1=strlen(windows[1]);
                if(len<0)
                    buffer=windows[1];
                else {
                    char temp[len+1];
                    strcpy(temp,Cstring);
                    DELIT(Cstring,0);
                    buffer=windows[1];
                    strcat(Cstring,buffer);
                    strcat(Cstring,temp);
                    *BL=1;
                }

            }
            break;
        }
    else {
        if(Str.dwMajorVersion==5)
            buffer="Windows 2000";
        else
            buffer="Windows Vista";
    }

    return buffer;
}
int main(void)
{
    char *Windowstype;
    int num;
    char ServicePack[50];
    Windowstype=GetWindows(ServicePack,&num);
    if(num)
        puts(ServicePack);
    else
        puts(Windowstype);
    return 0;
}

Why not use the safe variant and make both windows version and service pack parameters to be filled in ? What is num for anyway? Note the extra parameters for size, so your function won't write more into the buffer than it should be. It's up to you to implement it that way

Code:

void GetWindows( char* version, size_t version_size, char* servicepack, size_t servicepack_size );

int main(void)
{
    const size_t string_size = 50;
    char version[ string_size ];
    char servicepack[ string_size ];

    GetWindows( version, string_size, servicepack, string_size );

    puts( version );
    puts( servicepack );

    return 0;
}

hey but i want to make it in one i also tried that but it got corrupt 2 the string so i dunno why the hell it corrupt it :S?