4.3BSD-UWisc/man/cat3/varargs.3

VARARGS(3)          UNIX Programmer's Manual           VARARGS(3)



NAME
     varargs - variable argument list

SYNOPSIS
     #include <varargs.h>

     _f_u_n_c_t_i_o_n(va_alist)
     va_dcl
     va_list _p_v_a_r;
     va_start(_p_v_a_r);
     f = va_arg(_p_v_a_r, _t_y_p_e);
     va_end(_p_v_a_r);

DESCRIPTION
     This set of macros provides a means of writing portable pro-
     cedures that accept variable argument lists.  Routines hav-
     ing variable argument lists (such as _p_r_i_n_t_f(3)) that do not
     use varargs are inherently nonportable, since different
     machines use different argument passing conventions.

     va_alist is used in a function header to declare a variable
     argument list.

     va_dcl is a declaration for va_alist.  Note that there is no
     semicolon after va_dcl.

     va_list is a type which can be used for the variable _p_v_a_r,
     which is used to traverse the list.  One such variable must
     always be declared.

     va_start(pvar) is called to initialize _p_v_a_r to the beginning
     of the list.

     va_arg(_p_v_a_r, _t_y_p_e) will return the next argument in the list
     pointed to by _p_v_a_r.  _T_y_p_e is the type to which the expected
     argument will be converted when passed as an argument.  In
     standard C, arguments that are char or short should be
     accessed as int, unsigned char or unsigned short are con-
     verted to unsigned int, and float arguments are converted to
     double.  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(_p_v_a_r) is used to finish up.

     Multiple traversals, each bracketed by va_start ...  va_end,
     are possible.

EXAMPLE
          #include <varargs.h>
          execl(va_alist)
          va_dcl



Printed 12/27/86          May 15, 1986                          1






VARARGS(3)          UNIX Programmer's Manual           VARARGS(3)



          {
               va_list ap;
               char *file;
               char *args[100];
               int argno = 0;

               va_start(ap);
               file = va_arg(ap, char *);
               while (args[argno++] = va_arg(ap, char *))
                    ;
               va_end(ap);
               return execv(file, args);
          }

BUGS
     It is up to the calling routine to determine how many argu-
     ments there are, since it is not possible to determine this
     from the stack frame.  For example, _e_x_e_c_l passes a 0 to sig-
     nal the end of the list.  _P_r_i_n_t_f can tell how many arguments
     are supposed to be there by the format.

     The macros _v_a__s_t_a_r_t and _v_a__e_n_d may be arbitrarily complex;
     for example, _v_a__s_t_a_r_t might contain an opening brace, which
     is closed by a matching brace in _v_a__e_n_d.  Thus, they should
     only be used where they could be placed within a single com-
     plex statement.





























Printed 12/27/86          May 15, 1986                          2