WIndows System Programming use of standard C library [duplicate]

Question

I'm reading Johnson M. Hart Windows System programming. He has a method that makes use of the va_start standard c method. In the below sample can someone explain why the Handle hOut is being passed to va_start?

BOOL PrintStrings (HANDLE hOut, ...)

/* Write the messages to the output handle. Frequently hOut
    will be standard out or error, but this is not required.
    Use WriteConsole (to handle Unicode) first, as the
    output will normally be the console. If that fails, use WriteFile.

    hOut:   Handle for output file. 
    ... :   Variable argument list containing TCHAR strings.
        The list must be terminated with NULL. */
{
    DWORD msgLen, count;
    LPCTSTR pMsg;
    va_list pMsgList;   /* Current message string. */
    va_start (pMsgList, hOut);  /* Start processing msgs. */
    while ((pMsg = va_arg (pMsgList, LPCTSTR)) != NULL) {
        msgLen = lstrlen (pMsg);
        if (!WriteConsole (hOut, pMsg, msgLen, &count, NULL)
                && !WriteFile (hOut, pMsg, msgLen * sizeof (TCHAR), &count, NULL)) {
            va_end (pMsgList);
            return FALSE;
        }
    }
    va_end (pMsgList);
    return TRUE;
}

void va_start (va_list ap, paramN); Initialize a variable argument list Initializes ap to retrieve the additional arguments after parameter paramN. — dcrearer
– dcrearer, Commented Nov 8, 2015 at 22:14
Because the C standard requires it: the second argument to va_start must be the last named parameter of the function. — Kninnug
– Kninnug, Commented Nov 8, 2015 at 22:15
The ... in the function signature means that the function can accept any number of arguments following hOut, and va_start and va_arg are how the function is able to access those arguments. — Jonathan Potter
– Jonathan Potter, Commented Nov 8, 2015 at 22:24

Community · Accepted Answer · 2017-05-23 12:23:10Z

1

va_start is a macro that pulls of variadic parameters. It works by being given the parameter immediately before the first variadic parameter. The point is that va_start needs to know where the variadic parameters can be found. They are found immediately after the last named parameters. Hence the use of hOut.

Some details on how variadic parameters are typically implemented:

edited May 23, 2017 at 12:23

CommunityBot

11 silver badge

answered Nov 8, 2015 at 22:15

David Heffernan

616k46 gold badges1.1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

dcrearer Over a year ago

can you please expound a little on this statement "va_start is a macro that pulls of variadic parameters"? I very new to C and seeking some insight.

Jonathan Potter Over a year ago

@DavidHeffernan I'm not sure that dupe really explains what varargs are which seems to be the OP's problem.

David Heffernan Over a year ago

@JonathanPotter The question can only really be interpreted as though asker already knows what varargs are.

dcrearer Over a year ago

I understand variadics. Its just unclear why the HANDLE is used.

David Heffernan Over a year ago

Did you read the dupe question?

|

Collectives™ on Stack Overflow

WIndows System Programming use of standard C library [duplicate]

1 Answer 1

9 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

9 Comments

Linked

Related