CODEWITHSUNDEEP
c++cwinapi
hzh
hzh

Reputation: 351

How can I modify a char* in DLL?

When I pass a char* as a parameter to a DLL, I can get a new char* when I "return" a char* from DLL, but I can't get it done when I just use that char* parameter and return nothing.

This DLL function is OK, seems I can pass a char* to DLL.

void getStr(char *str){
   print("%s\n", str);
}

This is OK, too. It seems I can modify a pointer inside DLL.

void setInt(int a, int b, int *sum){
   *sum = a + b;
}

But there is no any luck when the pointer points to a char array.

void setStr(char* str) {
    char tmp[] = "From DLL";
    int len = strlen(tmp) + 1;
    str = (char*)malloc(len);
    memcpy(str, tmp, len);
}

I try to call this DLL function from a c++ program like this:

int main()
{
    char tmp[] = "Hello World!";
    setStr(tmp);  // I hope I can get "From DLL" here.
    std::cout << tmp;
}

The output is still "Hello World!", nothing happened after I try to get a new char*. If I set the return value of DLL function to char* and get the return value in c++ program, everything is fine, but I can't just use parameter to do it.

Did I miss something?

Upvotes: 0

Views: 484

Answers (1)

MikeCAT
MikeCAT

Reputation: 75062

To modify given string buffer, modify given string buffer.

void setStr(char* str) {
    char tmp[] = "From DLL";
    int len = strlen(tmp) + 1;
    memcpy(str, tmp, len);
}

int main()
{
    char tmp[] = "Hello World!";
    setStr(tmp);  // I hope I can get "From DLL" here.
    std::cout << tmp;
}

To modify the pointer, pass a pointer to the pointer.

void setStr(char** str) {
    char tmp[] = "From DLL";
    int len = strlen(tmp) + 1;
    *str = (char*)malloc(len);
    memcpy(*str, tmp, len);
}

int main()
{
    char tmp[] = "Hello World!";
    char* ptmp = tmp; // character array is NOT a pointer, so add a pointer to modify
    setStr(&ptmp);  // I hope I can get "From DLL" here.
    std::cout << ptmp;
}

Upvotes: 3

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