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Minix/CD-ROM-2.0/MINIX/MANUALS/CAT3/STDARG.3

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+STDARG(3)                 Minix Programmer's Manual                  STDARG(3)
+
+
+NAME
+     stdarg - variable argument list
+
+SYNOPSIS
+     #include <stdarg.h>
+
+     void va_start(va_list ap, argtypeN parmN)
+     type va_arg(va_list ap, type)
+     void va_end(va_list ap)
+
+DESCRIPTION
+     This set of macros provides a means of writing portable  procedures  that
+     accept  variable argument lists.  Routines having variable argument lists
+     (such as printf(3)) that do not use stdarg  are  inherently  nonportable,
+     since different machines use different argument passing conventions.
+
+     A function that accepts a variable argument list is declared  with  "..."
+     at  the  end  of  its  parameter  list.  It must have at least one normal
+     argument before the "...".  For example:
+
+          int printf(const char *format, ...) { /* code */ }
+          int fprintf(FILE *stream, const char *format, ...) { /* code */ }
+
+     va_list is a type which is used for the variable ap within the body of  a
+     variable argument function which is used to traverse the list.
+
+     va_start(ap, parmN) is called to initialize ap to the  beginning  of  the
+     list.   The  last true parameter of the function, parmN, must be supplied
+     to allow va_start to compute the address of the first variable parameter.
+
+     va_arg(ap, type) will return the next argument in the list pointed to  by
+     ap.   Type  is  the type to which the expected argument will be converted
+     when passed as an argument.
+
+     Different types can be mixed, but it is up to the routine  to  know  what
+     type of argument is expected, since it cannot be determined at runtime.
+
+     va_end(ap) must be used to finish up.
+
+     Multiple  traversals,  each  bracketed  by  va_start  ...   va_end,   are
+     possible.
+
+EXAMPLE
+         #include <stdarg.h>
+
+         execl(const char *path, ...)
+         {
+             va_list ap;
+             char *args[100];
+             int argno = 0;
+
+
+                                May 15, 1986                                 1
+
+
+
+STDARG(3)                 Minix Programmer's Manual                  STDARG(3)
+
+
+             va_start(ap, path);
+             while ((args[argno++] = va_arg(ap, char *)) != NULL) {}
+             va_end(ap);
+             return execv(path, args);
+         }
+
+NOTES
+     It is up to the calling routine to determine  how  many  arguments  there
+     are,  since  it  is  not possible to determine this from the stack frame.
+     For example, execl passes a null pointer to signal the end of  the  list.
+     Printf  can  tell  how  many  arguments  are  supposed to be there by the
+     format.
+
+     The macros va_start and va_end may be arbitrarily complex;  for  example,
+     va_start  might  contain  an opening brace, which is closed by a matching
+     brace in va_end.  Thus, they should only be  used  where  they  could  be
+     placed within a single complex statement.
+
+BUGS
+     It is impossible to properly show the macros as C declarations as is done
+     in  the  synopsis.   They  can never be coded as C functions, because all
+     three macros use their arguments  by  address,  and  the  type  field  is
+     certainly impossible.  Just look at them as being part of the C language,
+     like sizeof.
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+                                May 15, 1986                                 2
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