c - Adding CHAR16 variable to String

Question

I am a novice in C and trying to amend and compile an EDK2 EFI program.

The program portion I want to change has a function MsgLog which takes a Char16 * variable and uses this to write to a log file.

The current code has this

MsgLog("SomeText ...%r\n", Status);

Status is an EFI_STATUS which can be "Success" or "Not Found". I.E., you can get the following in a log file:

SomeText ...Success

or

SomeText ...Not Found

I will like to change this to:

SomeText ...Success : ABC

or

SomeText ...Not Found : XYZ

I have loaded : ABC or : XYZ into a Char16 * variable (has to be Char16 * to match other restrictions by the function used to set this)

I have then tried various options to append it to the string such as

MsgLog("SomeText ...%r%s\n", Status, myVariable);

and

MsgLog("SomeText ...%r%r\n", Status, myVariable);

but I end up with

SomeText ...Success<null string>

and

SomeText ...Not Found<null string>

I am not sure what formatting placeholder I am supposed to use or whether or how I should cast myVariable to some other appropriate format and would appreciate some pointers.

Please note that this is a wider program of which I am changing a small portion and I don't have scope to define different variable types.

EDIT: Added Context

ORIGINAL WORKING CODE

EFI_STATUS Funct_A()
{
    EFI_STATUS Status;

    //Funct_B returns EFI_SUCCESS or EFI_NOT_FOUND
    Status = Funct_B();

    MsgLog("SomeText ...%r\n", Status);

PROBLEM CODE

EFI_STATUS Funct_A()
{
    EFI_STATUS Status;
    CHAR16     *myVariable = NULL;

    //Funct_B returns EFI_SUCCESS or EFI_NOT_FOUND
    Status = Funct_B();


    // From some header file, I see "#define SPrint UnicodeSPrint". Not 100% sure it is the relevant one
    // From other code implementations, I know SPrint takes "CHAR16" as first variable. 
    if (!EFI_ERROR (Status)) {
        SPrint (myVariable,  255, L" : ABC");
    } else {
        SPrint (myVariable,  255, L" : XYZ");
    }

    MsgLog("SomeText ...%r%r\n", Status, myVariable);
    // MsgLog is a bit of a rabbit's warren and I can't provide all the background but it expects "CHAR16".

Could you post some more context? Some more code? What is myVariable? How is it defined? How is it initialized? How is Status defined and initialized? — KamilCuk
Thanks @KamilCuk, Status is working fine and is not part of the issue. It returns Success or Not Found as needed and this is in the log. myVariable is what I have added and what I need to see how to add to the string. It is set in a different function which I don't have scope to amend. The question is whether a CHAR16 Variable can be added to a string as shown or not. — Dayo
It returns A variable can't "return". Sure, that's great it "works", just more context would be helpful. myVariable is what I have added How and where and what exactly have you added? (ie. how it's defined and initialized or assigned) whether a CHAR16 It's CHAR16 or Char16? It just would be really helpful if you would create an minimal reproducible example, even with your current not working code. — KamilCuk
I used "it returns" in a loose sense ... I meant I don't have an issue with it. I can add more context but worried it will start to get too long as several things are pulled from multiple header files etc. Will see what can be done. Thanks — Dayo

KamilCuk KamilCuk · Accepted Answer · 2020-09-06T07:42:06

SPrint (myVariable, 255, L" : ABC"); is wrong. myVariable is NULL - you can't write to a NULL pointer. You have to actually allocate memory for the string if you want to use SPrint. For more information review your knowledge about pointers and about snprintf standard C function. The second argument to SPrint is actually the size of allocated memory - you allocated no memory, so the 255 is just invalid.

CHAR16 myVariable[255];
SPrint(myVariable, sizeof(myVariable), L" : ABC");

But in your case, there is no point in that. First, there is no need to use SPrint - you do not use the formatting string. A simple StrCpy (ie. the alternative to standard wcscpy/strcpy) would just suffice. But that said, you do not need any memory at all, just use the pointer to point to string literal.

const CHAR16 *myVariable = NULL;
if (!EFI_ERROR (Status)) {
    myVariable = L" : ABC";
} else {
    myVariable = L" : XYZ";
}
// or simpler
myVariable = !EFI_ERROR (Status) ? L" : ABC" : L" : XYZ";

c - Adding CHAR16 variable to String

1 Answers