Passing parameter to a function

[shoonya]

what does this declaration mean ??

void Foo( int a, int b, ... ){
..
}

Shoonya

[r035198x]

Google "variable number of arguments in c".

[curiously enough]

A function with no return value, and integer variables passed as parameters to the function called a and b in the function.

[donbock]

Are you familiar with the printf() function? Its first argument is a format string that encodes the number and meaning of the following arguments. Notice that you can pass a different number of arguments each time you call printf() without provoking any compiler warnings about violating the function prototype.

The function prototype for printf follows:
int printf(const char *fmt, ...);

The normal job of a function prototype is to inform the compiler of the function's calling protocol so that compile-time checks can be performed to verify that the function was called properly.

The "..." construct in a prototype informs the compiler not to concern itself with which arguments appear beyond this point. It is up to the programmer to get it right. You're performing without a net!

This variable argument list feature provides great flexibility; but at the cost of reduced compile-time checking and increased complexity if you try to write a such a function.

[shoonya]

hi donbock,

Thanks a lot

Shoonya

[Flugeldorph]

Actually, this methodology is not difficult to use, and is extrenely powerful. An example is the best way to show how it works. The following routine is used as a graceful exit from a program when things don't go right. It is used like printf in that you can send a variable amount of data to the function

1st the code:
make sure that you include <stdarg.h>

#include <stdarg.h>
 
void SysAbort(char *String, int NumArgs, ...)
{
   va_list       ap;
   va_start(ap, NumArgs);
 
   vfprintf(stderr, String, ap);
 
   va_end(ap);
 
   // Check if n\memory needs to be cleaned up
   MyCleanupRoutine();
   exit(1);
}

This can be called as follows

SysAbort("Unable to open file %s from path %s", 2, FileName, Path);

The '2' in the call tells the SysAbort function that there are two arguments following, the results would (assuming FileName = "InFile.txt" and Path = ""C:\Data\NewData" look like:

'Unable to open InFile.txt from path C:\Data\NewData'

The same as printf.