Variadic macro in the C preprocessor

The declaration syntax is similar to that of variadic functions: a sequence of three full stops "{{var|...}}" is used to indicate that one or more arguments must be passed. During macro expansion each occurrence of the special identifier {{var|__VA_ARGS__}} in the macro replacement list is replaced by the passed arguments.

Additionally, regular macro arguments may be listed before the ..., but regular arguments may not be listed after the ....

No means is provided to access individual arguments in the variable argument list, nor to find out how many were passed. However, macros can be written to count the number of arguments that have been passed.{{cite newsgroup

|author= Laurent Deniau

|title= __VA_NARG__

|date= 2006-01-16

|newsgroup= comp.std.c

|message-id= dqgm2f$ije$1@sunnews.cern.ch

|url= http://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5

}}

Both the C99 and C++11 standards require at least one argument, but since C++20 and C23 this limitation has been lifted through the {{var|__VA_OPT__}} functional macro. The {{var|__VA_OPT__}} macro is replaced by its argument when arguments are present, and omitted otherwise. Common compilers also permit passing zero arguments before this addition, however.[https://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html Variadic Macros – Using the GNU Compiler Collection (GCC)][https://msdn.microsoft.com/en-us/library/ms177415(v=vs.140).aspx Variadic Macros (C++)]

The C preprocessor rules prevent macro names in the argument of {{var|__VA_OPT__}} from expanding recursively. It is possible to work around this limitation up to an arbitrary fixed number of recursive expansions, however.[https://www.scs.stanford.edu/~dm/blog/va-opt.html Recursive macros with C++20 __VA_OPT__]

Several compilers support variable-argument macros when compiling C and C++ code: the GNU Compiler Collection 3.0, Clang (all versions),Clang source code change that mentions __VA_ARGS__ support (2006-07-29), note that Clang was open-sourced in 2007. http://llvm.org/viewvc/llvm-project?view=revision&revision=38770 Visual Studio 2005, C++Builder 2006, and Oracle Solaris Studio (formerly Sun Studio) Forte Developer 6 update 2 (C++ version 5.3).Sun Studio feature comparison – http://developers.sun.com/sunstudio/support/CCcompare.html GCC also supports such macros when compiling Objective-C.

Support for the {{var|__VA_OPT__}} macro to support zero arguments has been added in GNU Compiler Collection 8,{{cite web|url=https://gcc.gnu.org/projects/cxx-status.html#cxx2a|title=C++2a Support in GCC|access-date=June 14, 2018}} Clang 6,{{cite web|url=https://clang.llvm.org/cxx_status.html|title=C++ Support in Clang|access-date=June 14, 2018}} and Visual Studio 2019.{{cite web|url=https://docs.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview|title=MSVC new preprocessor overview|date=September 10, 2020|access-date=December 8, 2020}}

If a printf-like function {{code|dbgprintf()}} were desired, which would take the file and line number from which it was called as arguments, the following solution applies.

{{olist

|Our implemented function

void realdbgprintf (const char *SourceFilename,

int SourceLineno,

const char *CFormatString,

...);

|

Due to limitations of the variadic macro support in C++11 the following

straightforward solution can fail and should thus be avoided:

#define dbgprintf(cformat, ...) \

realdbgprintf (__FILE__, __LINE__, cformat, __VA_ARGS__)

The reason is that

dbgprintf("Hallo")

gets expanded to

realdbgprintf (__FILE__, __LINE__, "Hallo", )

where the comma before the closing brace will result in a syntax error.

| GNU C++ supports a non-portable extension which solves this.

#define dbgprintf(cformat, ...) \

realdbgprintf (__FILE__, __LINE__, cformat, ##__VA_ARGS__)

| C++20 eventually supports the following syntax.

#define dbgprintf(cformat, ...) \

realdbgprintf (__FILE__, __LINE__, cformat __VA_OPT__(,) __VA_ARGS__)

| By using the 'cformat' string as part of the variadic arguments we can

circumvent the abovementioned incompatibilities. This is tricky but

portable.

1. define dbgprintf(...) realdbgprintf (__FILE__, __LINE__, __VA_ARGS__)

}}

{{code|dbgprintf()}} could then be called as

dbgprintf ("Hello, world");

which expands to

realdbgprintf (__FILE__, __LINE__, "Hello, world");

Another example is

dbgprintf("%d + %d = %d", 2, 2, 5);

which expands to

realdbgprintf(__FILE__, __LINE__, "%d + %d = %d", 2, 2, 5);

Without variadic macros, writing wrappers to printf is not directly possible. The standard workaround is to use the stdargs functionality of C/C++, and have the function call vprintf instead.

There is a portability issue with generating a trailing comma with empty args for variadic macros in C99. Some compilers (e.g., Visual Studio when not using the new standard-conformant preprocessor) will silently eliminate the trailing comma. Other compilers (e.g.: GCC) support putting {{code|##}} in front of {{var|__VA_ARGS__}}.

1. define MYLOG(FormatLiteral, ...) fprintf (stderr, "%s(%u): " FormatLiteral "\n", __FILE__, __LINE__, __VA_ARGS__)

The following application works

MYLOG("Too many balloons %u", 42);

which expands to

fprintf (stderr, "%s(%u): " "Too many balloons %u" "\n", __FILE__, __LINE__, 42);

which is equivalent to

fprintf (stderr, "%s(%u): Too many balloons %u\n", __FILE__, __LINE__, 42);

But look at this application:

MYLOG("Attention!");

which expands to

fprintf (stderr, "%s(%u): " "Attention!" "\n", __FILE__, __LINE__, );

which generates a syntax error with GCC.

GCC supports the following (non-portable) extension:

1. define MYLOG(FormatLiteral, ...) fprintf (stderr, "%s(%u): " FormatLiteral "\n", __FILE__, __LINE__, ##__VA_ARGS__)

which removes the trailing comma when {{var|__VA_ARGS__}} is empty.

C23 solves this problem by introducing {{var|__VA_OPT__}} like C++.{{cite web|url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3033.htm|title=WG14 - N3033 : Comma omission and comma deletion|date=2022-07-20}}

Before the existence of variable-arguments in C99, it was quite common to use doubly nested parentheses to exploit the variable number of arguments that could be supplied to the {{code|printf()}} function:

1. define dbgprintf(x) realdbgprintf x

{{code|dbgprintf()}} could then be called as:

dbgprintf (("Hello, world %d", 27));

which expands to:

realdbgprintf ("Hello, world %d", 27);

{{Reflist}}

• Variadic function
• Variadic template

Category:C (programming language)

Category:C++

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