Equivalent for the C language NULL value

[Marcel Kollenaar]

Hi,

In the C language there is special value NULL. However A5 doesn't know this value. Is it appropriate to use the function NULL_VALUE() insteat or do I have to use 0 - zero?

Regards,

Marcel

[Selwyn Rabins]

RE: Equivalent for the C language NULL value

what is the context for this question?

in a5 you generally don't have to concern yourself as to how strings are terminated, as you would in a C program.

[Marcel Kollenaar]

RE: Equivalent for the C language NULL value

I use API calls. They want a in certain situations a NULL value.

Regards,

Marcel

[Jos Schaars]

RE: Equivalent for the C language NULL value

I was to post a question regarding numeric fields and saw this one, they are related:

Can I test in Xbasic that a numeric field is filled in?
So this test should return false when any value (including 0!) is filled in and true if the field haS no default value and was skipped over, or any previuous value was deleted.

Found ISBLANK() for character fields and REMSPECIAL(CDATE())=="" for dates in the documentation, but nothing about numeric feilds.

[Marcel Kollenaar]

RE: Equivalent for the C language NULL value

NULL Values

Alpha Five does not really have the concept of a NULL value for a field value. If you want to test if the field is empty, then you use these expressions:

Character

"fieldname" = ""


Date

"fieldname" = ctod("")


Numeric

You cannot distinguish between records that have no value, and those that have a 0 in them.

"fieldname" = 0


Logical

You cannot distinguish between records that have no value, and those that have a .f. value.

"fieldname" = .f.


Supported By

Alpha Five Version 5 and Above

[csda1]

RE: Equivalent for the C language NULL value

Hi Marcel,

You can test for the fields of a record being empty as such

IF(ISBLANK("fieldname"),0,1)

This works with number and logical fields (but not variables).

I have found that

IF(TRIM(CDATE(datefield_or_var_or_value))=="",0,1) works most reliably across all versions of A5, both fields, values and variables

For numbers, it used to be that the numeric constant MATH_NAN use to evaluate to a blank field or undefined variable, but doesn't seem to work in the latest a5v5 (although it use to work in a5v4 and I think some versions of a5v5). I have not tried this on 6. For example

IF(fieldname=MATH_NAN,0,1)

Character fields can not be distinguished between a null string and "empty" value.

Regards,

Ira

[Marcel Kollenaar]

RE: Equivalent for the C language NULL value

Hi all,

I appreciate your efforts. But this isn't what I mean. It has to do with API functions. Some of these functions need a pointer to a buffer or structure. But in certain circumstances you does't have to pass a structure. In that case there is a NULL pointer. That is pointer provided by the system that points to a save "noware". Now I use 0.

What I also can do is use 0 as a NULL value until it hits the dangeling pointer problem. It's just as playing Russian Roulette but it is only deadly for your program.

Marcel

[AaronBBrown]

RE: Equivalent for the C language NULL value

Marcel,

I believe you are overthinking this problem. I have not ever run into an API function that requires me to allocate buffers in Alpha Five.

I've also found that many API functions have a COM/OLE interface through the Windows Scripting objects. You might want to investigate this as a cleaner alternative to using declare and declarestruct.

[Marcel Kollenaar]

RE: Equivalent for the C language NULL value

Aaron Brown wrote:
-------------------------------
Marcel,

"I Believe you are overthinking this problem. I have not ever run into an API function that requires me to allocate buffers in Alpha Five."

'That's very good possible. But making a declarestruct as:

declarestruct _MyBuff 1024CMyVarBuff

and use this in a API declare where it collects a string value is such a buffer? Or am I wrong. I'm still a novice in Xbasic.

In the following API ReadFile function the member lpBuffer is such buffer. In this case I don't have to pass a NULL value just the buffers memory address in this case (_MyBuff).

But the member lpOverlapped must be or an overlapped structure or it must be passed a NULL value. The only question for me is can I just simply put a 0 -zero- into this position in Xbasic? I know I can experiment with this and surely it will work but is it correct in Xbasic terms and is the 0 interpreted as a NULL value pointer.

BOOL ReadFile(
  HANDLE hFile,
  LPVOID [i]lpBuffer[/i],
  DWORD nNumberOfBytesToRead,
  LPDWORD lpNumberOfBytesRead,
  LPOVERLAPPED [b]lpOverlapped[/b]
);

"I've also found that many API functions have a COM/OLE interface through the Windows Scripting objects. You might want to investigate this as a cleaner alternative to using declare and declarestruct."

I'll investige this tip.

Thanks for your information Aaron.

Marcel