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.Dd October 17, 2013 .Dt __CONCAT 3 .Os .Sh NAME .Nm __CONCAT , .Nm __STRING .Nd argument substitution .Sh SYNOPSIS n sys/cdefs.h .Ft xy .Fn __CONCAT "x" "y" .Ft const char * .Fn __STRING "x" .Sh DESCRIPTION The .Nm macro makes use of the .Xr cpp 1 preprocessor to concatenate two tokens. When the macro is expanded, .Fa x and .Fa y are combined into a single token, provided that the result forms a valid token; two tokens that together do not form a valid token can not be concatenated. This is known as .Dq token concatenation or .Dq token pasting .
p The .Fn __STRING macro uses the conventional .Sq # preprocessing operator to replace the argument .Fa x with a string literal. This is also known as .Dq stringification . .Sh EXAMPLES The following two .Xr printf 3 calls produce the same output: d -literal -offset indent #define Net 0x01 #define BSD 0x02 #define NetBSD "NetBSD" (void)printf("%s\en", __CONCAT(Net, BSD)); (void)printf("%s%s\en", __STRING(Net), __STRING(BSD)); .Ed .Sh SEE ALSO .Xr cpp 1 , .Xr cdefs 3 .Sh HISTORY The .Fn __CONCAT and .Fn __STRING macros first appeared in .Nx 1.3 . .Sh CAVEATS Many small details direct the proper use of the macros. For example, while all leading and trailing whitespace is ignored when .Fn __STRING is used, it is undefined whether .Xr cpp 1 puts white space between the tokens when .Fn __CONCAT is used. It can be also noted that the C preprocessor converts all comments to whitespace before any macros are even considered. The use of either macro is discouraged in complex constructs.
p Use of this macro is non-portable; this is part of the implementation namespace and should only be used in .Nx code.