Chapter 14: Advanced Topics
===========================

Variable-Length Argument Lists
------------------------------
* The function prototype for "printf" is
int printf(const char *format, ...);
* The ellipsis ("..."), which must be at the end of the parameter
list, indicates that the function receives a variable number of
arguments of any type.
* The macro and definitions of the variable arguments header
"stdarg.h" provide the capabilities necessary to build these
functions.
va_list A type suitable for holding information needed by
macros "va_start", "va_arg", and "va_end". To access
the arguments in a variable-length argument list, and
object type "va_list" must be declared.
va_start A macro that is invoked before the arguments of a
variable-length argument list can be accessed.
va_arg A macro that expands to an expression of the value and
type of the next argument in the variable-length
argument list.
va_end A macro that facilitates a normal return from a
function whose variable-length argument list was
referred to by the "va_start" macro.

* E.g.
#include <stdio.h>
#include <stdarg.h>

double average(int, ...);

main()
{
double w = 37.5, x = 22.5, y = 1.7, z = 10.2;

printf("%s%.1f\n%s%.1f\n%s%.1f\n%s%.1f\n\n",
"w = ", w, "x = ", x, "y = ", y, "z = ", z);
printf("%s%.3f\n%s%.3f\n%s%.3f\n",
"The average of w and x is ",
average(2, w, x),
"The average of w, x, and y is ",
average(3, w, x, y),
"The average of w, x, y, and z is ",
average(4, w, x, y, z));

return 0;
}


double average(int i, ...)
{
double total = 0;
int j;
va_list ap;

va_start(ap, i);

for (j = 1; j <= i; j++)
total += va_arg(ap, double);

va_end(ap);
return total / i;
}

Using Command-Line Arguments
----------------------------
* It is possible to pass arguments to "main" from a command line by
including parameter "int argc" and "char *argv[]" in the parameter
list of "main".

* Parameter "argc" receives the number of command-line arguments.
* Parameter "argv" is an array of string in which the actual command-
line arguments are stored.
* E.g.
#include <stdio.h>

main(int argc, char *argv[])
{
FILE *inFilePtr, *outFilePtr;
int c;

if (argc != 3)
printf("Usage: copy infile outfile\n");
else
if ((inFilePtr = fopen(argv[1], "r")) != NULL)

if ((outFilePtr = fopen(argv[2], "w")) != NULL)

while ((c = fgetc(inFilePtr)) != EOF)
fputc(c, outFilePtr);

else
printf("File \"%s\" could not be opened\n",
argv[2]);

else
printf("File \"%s\" could not be opened\n", argv[1]);

return 0;
}



Notes on Compiling Multiple-Source-File Programs
------------------------------------------------
* Global variables are accessible to functions in other files,
however, the global variables must be declared in each file in which
they are used.

* E.g., if integer variable "flag" is defined in one file, and refer
to it in a second file, the second file must contain the declaration
extern int flag;
prior to the variable's use in that file.

* The storage class specifier "extern" indicates to the compiler that
variable "flag" is defined either later in the same file or in a
different file.
* The compiler informs the linker that unresolved references to
variable "flag" appear in the file.

* The function prototype can be included in each file in which the
function is invoked, and compiling the files together.
* The function can be implementated in one of the files.
* E.g., "printf" and "scanf" in "stdio.h".

More on Files
-------------
* C provides capabilities for processing binary files, when the file
is opened in a binary file mode.
rb Open a binary file for reading.
wb Open a binary file for writing.
...