[C]How to change value of input char* in a function-CodePudding

Below is my sample code.

void BTBasic_DLL_Call(char * functionName, char * parameters,  char * returnString, int * returnValue );

int _tmain(int argc, _TCHAR* argv[])
{
    char* fName = "connectProgrammer";
    char* parameter = "192.168.1.1";
    char* returnString ="0";
    int returnValue = 0;
    int n = 0;


    BTBasic_DLL_Call(fName,parameter,returnString,&returnValue);
    printf("%s \n", returnString);
    scanf("%d", n);

    return 0;
}

void BTBasic_DLL_Call(char * functionName, char * parameters,  char * returnString, int * returnValue )
{
    returnString = "DLL Test";
    *returnValue = 10;
}

In this code, I expect returnString would be changed as "DLL Test". But the printf result is still "0". Can anyone please help how to correctly read the changed returnString value?

CodePudding user response：

You can only set char * to a string literal at initialization. If you'd like to change this pointer to point to a different string literal, you'd have to pass it by pointer:

void BTBasic_DLL_Call(char * functionName, char * parameters, char ** returnString, int * returnValue) {
    *returnString = "DLL Test";
}

CodePudding user response：

Simple. The caller must pass the ADDRESS of the variable the function is to modify (just as you had done with the integer parameter).

// A function defined (ahead of use) is a function declared
// NB: Function receives address as "ptr to ptr"
void BTBasic_DLL_Call(char *functionName, char *parameters,  char **returnString, int *returnValue )
{
    functionName = functionName; // silence compiler warning
    parameters = parameters;

    *returnString = "DLL Test";
    *returnValue = 10;
}

//int _tmain(int argc, _TCHAR* argv[])
int main() // for demonstration 
{
    char *fName = "connectProgrammer";
    char *parameter = "192.168.1.1";
    char *returnString = NULL; // pointer initialised
    int returnValue = 0;

    // pass the address of the pointer
    BTBasic_DLL_Call( fName, parameter, &returnString, &returnValue );
    printf("%s \n", returnString);

    return 0;
}

DLL Test