From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/c/variadic%2Fva_start.html | 77 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 77 insertions(+) create mode 100644 devdocs/c/variadic%2Fva_start.html (limited to 'devdocs/c/variadic%2Fva_start.html') diff --git a/devdocs/c/variadic%2Fva_start.html b/devdocs/c/variadic%2Fva_start.html new file mode 100644 index 00000000..8dd8a365 --- /dev/null +++ b/devdocs/c/variadic%2Fva_start.html @@ -0,0 +1,77 @@ +

va_start

Defined in header <stdarg.h> 
void va_start( va_list ap, parmN );
 (until C23) 
void va_start( va_list ap, ... );
+		 (since C23)

The va_start macro enables access to the variable arguments following the named argument parmN(until C23).

+

va_start shall be invoked with an instance to a valid va_list object ap before any calls to va_arg.

+ + + + +

If parmN is declared with register storage class specifier, with an array type, with a function type, or with a type not compatible with the type that results from default argument promotions, the behavior is undefined.

+		 (until C23)

Only the first argument passed to va_start is evaluated. Any additional arguments are neither expanded nor used in any way.

+		 (since C23)

Parameters

+ + +
ap - an instance of the va_list type
parmN - the named parameter preceding the first variable parameter

Expanded value

(none)

+

Example

#include <stdio.h>
+#include <stdarg.h>
+ 
+int add_nums_C99(int count, ...)
+{
+    int result = 0;
+    va_list args;
+    va_start(args, count); // count can be omitted since C23
+ 
+    for (int i = 0; i < count; ++i) {
+        result += va_arg(args, int);
+    }
+ 
+    va_end(args);
+    return result;
+}
+ 
+#if __STDC_VERSION__ > 201710L
+// Same as above, valid since C23
+int add_nums_C23(...)
+{
+    int result = 0;
+    va_list args;
+    va_start(args);
+ 
+    int count = va_arg(args, int);
+    for (int i = 0; i < count; ++i) {
+        result += va_arg(args, int);
+    }
+ 
+    va_end(args);
+    return result;
+}
+#endif
+ 
+int main(void)
+{
+    printf("%d\n", add_nums_C99(4, 25, 25, 50, 50));
+#if __STDC_VERSION__ > 201710L
+    printf("%d\n", add_nums_C23(4, 25, 25, 50, 50));
+#endif
+}

Possible output:

+
150
+150

References

See also

+ + + + +
accesses the next variadic function argument
(function macro)
+
(C99)
 makes a copy of the variadic function arguments
(function macro)
ends traversal of the variadic function arguments
(function macro)
holds the information needed by va_start, va_arg, va_end, and va_copy
(typedef)
C++ documentation for va_start
+

+ © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
+ https://en.cppreference.com/w/c/variadic/va_start +

+
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