Va First

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va_first

Returns a pointer to the first argument in a variable argument list

Syntax

pointer_variable = va_first()

Description

The va_first function provides an untyped pointer value that points to the first variable argument passed to a function.

Not supported when using -gen gcc. Use Cva_List variadic argument list type for cross platform compatibility.

Remarks

Dialect Differences

• Not available in the -lang qb dialect unless referenced with the alias __Va_first.

Differences from QB

• New to FreeBASIC

See also

• ... (Ellipsis)
• va_arg
• va_next

Examples

Function average cdecl(count As Integer, ... ) As Double

    Dim arg As Any Ptr

    Dim sum As Double = 0

    Dim i As Integer

    arg = va_first()

    For i = 1 To count

        sum += va_arg(arg, Double)

        arg = va_next(arg, Double)

    Next

    Return sum / count

End Function

Print average(4, 3.4,5.0,3.2,4.1)  '' all passed variable arguments must be of type double

Print average(2, 65.2,454.65481)   '' all passed variable arguments must be of type double

Sleep

The output would look like:

3.925
259.927405

'' Example of a simple custom printf

Sub myprintf cdecl(ByRef formatstring As String, ...)

    '' Get the pointer to the first var-arg

    Dim As Any Ptr arg = va_first()

    '' For each char in format string...

    Dim As UByte Ptr p = StrPtr(formatstring)

    Dim As Integer todo = Len(formatstring)

    While (todo > 0)

        Dim As Integer char = *p

        p += 1

        todo -= 1

        '' Is it a format char?

        If (char = Asc("%")) Then

            If (todo = 0) Then

                '' % at the end

                Print "%";

                Exit While

            End If

            '' The next char should tell the type

            char = *p

            p += 1

            todo -= 1

            '' Print var-arg, depending on the type

            Select Case char

            '' integer?

            Case Asc("i")

                Print Str(va_arg(arg, Integer));

                '' Note, different from C: va_next() must be

                '' used as va_arg() won't update the pointer.

                arg = va_next(arg, Integer)

            '' long integer? (64-bit)

            Case Asc("l")

                Print Str(va_arg(arg, LongInt));

                arg = va_next(arg, LongInt)

            '' single or double?

            '' Note: because the C ABI, all singles passed on

            '' var-args are converted to doubles.

            Case Asc( "f" ), Asc( "d" )

                Print Str(va_arg(arg, Double));

                arg = va_next(arg, Double)

            '' string?

            Case Asc("s")

                '' Strings are passed byval, so the length is unknown

                Print *va_arg(arg, ZString Ptr);

                arg = va_next(arg, ZString Ptr)

            End Select

        '' Ordinary char, just print as-is

        Else

            Print Chr( char );

        End If

    Wend

End Sub

    Dim As String s = "bar"

    myprintf(!"integer=%i, longint=%l single=%f, double=%d, string=%s, string=%s\n", _

             1, 1ll Shl 32, 2.2, 3.3, "foo", s)

    Sleep

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Category: Procedures
Kind: keyword

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Reproduced from the FreeBASIC Wiki Manual under the GNU Free Documentation License 1.3 — with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. Copyright © the FreeBASIC project and wiki contributors. See License.