man/stdarg.3

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  38 .\"     @(#)stdarg.3    8.1 (Berkeley) 6/5/93
  39 .\"
  40 .Dd June 5, 1993
  41 .Dt STDARG 3
  42 .Os
  43 .Sh NAME
  44 .Nm stdarg
  45 .Nd variable argument lists
  46 .Sh SYNOPSIS
  47 .Fd #include <stdarg.h>
  48 .Ft void
  49 .Fn va_start "va_list ap" last
  50 .Ft type
  51 .Fn va_arg "va_list ap" type
  52 .Ft void
  53 .Fn va_end "va_list ap"
  54 .Sh DESCRIPTION
  55 A function may be called with a varying number of arguments of varying
  56 types.
  57 The include file
  58 .Aq Pa stdarg.h
  59 declares a type
  60 .Pq Em va_list
  61 and defines three macros for stepping
  62 through a list of arguments whose number and types are not known to
  63 the called function.
  64 .Pp
  65 The called function must declare an object of type
  66 .Em va_list
  67 which is used by the macros
  68 .Fn va_start ,
  69 .Fn va_arg ,
  70 and
  71 .Fn va_end .
  72 .Pp
  73 The
  74 .Fn va_start
  75 macro initializes
  76 .Fa ap
  77 for subsequent use by
  78 .Fn va_arg
  79 and
  80 .Fn va_end ,
  81 and must be called first.
  82 .Pp
  83 The parameter
  84 .Fa last
  85 is the name of the last parameter before the variable argument list,
  86 i.e. the last parameter of which the calling function knows the type.
  87 .Pp
  88 Because the address of this parameter is used in the
  89 .Fn va_start
  90 macro, it should not be declared as a register variable, or as a
  91 function or an array type.
  92 .Pp
  93 The
  94 .Fn va_start
  95 macro returns no value.
  96 .Pp
  97 The
  98 .Fn va_arg
  99 macro expands to an expression that has the type and value of the next
 100 argument in the call.
 101 The parameter
 102 .Fa ap
 103 is the
 104 .Em va_list Fa ap
 105 initialized by
 106 .Fn va_start .
 107 Each call to
 108 .Fn va_arg
 109 modifies
 110 .Fa ap
 111 so that the next call returns the next argument.
 112 The parameter
 113 .Fa type
 114 is a type name specified so that the type of a pointer to an
 115 object that has the specified type can be obtained simply by
 116 adding a *
 117 to
 118 .Fa type .
 119 .Pp
 120 If there is no next argument, or if
 121 .Fa type
 122 is not compatible with the type of the actual next argument
 123 (as promoted according to the default argument promotions),
 124 random errors will occur.
 125 .Pp
 126 The first use of the
 127 .Fn va_arg
 128 macro after that of the
 129 .Fn va_start
 130 macro returns the argument after
 131 .Fa last .
 132 Successive invocations return the values of the remaining
 133 arguments.
 134 .Pp
 135 The
 136 .Fn va_end
 137 macro handles a normal return from the function whose variable argument
 138 list was initialized by
 139 .Fn va_start .
 140 .Pp
 141 The
 142 .Fn va_end
 143 macro returns no value.
 144 .Sh EXAMPLES
 145 The function
 146 .Em foo
 147 takes a string of format characters and prints out the argument
 148 associated with each format character based on the type.
 149 .Bd -literal -offset indent
 150 void foo(char *fmt, ...)
 151 {
 152         va_list ap;
 153         int d;
 154         char c, *p, *s;
 155
 156         va_start(ap, fmt);
 157         while (*fmt)
 158                 switch(*fmt++) {
 159                 case 's':                       /* string */
 160                         s = va_arg(ap, char *);
 161                         printf("string %s\en", s);
 162                         break;
 163                 case 'd':                       /* int */
 164                         d = va_arg(ap, int);
 165                         printf("int %d\en", d);
 166                         break;
 167                 case 'c':                       /* char */
 168                         c = va_arg(ap, char);
 169                         printf("char %c\en", c);
 170                         break;
 171                 }
 172         va_end(ap);
 173 }
 174 .Ed
 175 .Sh STANDARDS
 176 The
 177 .Fn va_start ,
 178 .Fn va_arg ,
 179 and
 180 .Fn va_end
 181 macros conform to
 182 .St -ansiC .
 183 .Sh COMPATIBILITY
 184 These macros are
 185 .Em not
 186 compatible with the historic macros they replace.
 187 A backward compatible version can be found in the include
 188 file
 189 .Aq Pa varargs.h .
 190 .Sh BUGS
 191 Unlike the
 192 .Em varargs
 193 macros, the
 194 .Nm stdarg
 195 macros do not permit programmers to
 196 code a function with no fixed arguments.
 197 This problem generates work mainly when converting
 198 .Em varargs
 199 code to
 200 .Nm stdarg
 201 code,
 202 but it also creates difficulties for variadic functions that
 203 wish to pass all of their arguments on to a function
 204 that takes a
 205 .Em va_list
 206 argument, such as
 207 .Xr vfprintf 3 .