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#undef getchar using .net togenerate ecc200 on web,windows application RM4SCC Formal para visual .net data matrix barcodes meters are not replaced within quoted strings. If, however, a parameter name is preceded by a # in the replacement text, the combination will be expanded into a quoted string with the parameter replaced by the actual argument.

This can be combined with string concatenation to make, for example, a debugging print macro: When this is invoked, as in the macro is expanded into and the strings are concatenated, so the effect is Within the actual argument, each " is replaced by \" and each \ by \\, so the result is a legal string constant. The preprocessor operator ## provides a way to concatenate actual arguments during macro expansion. If a parameter in the replacement text is adjacent to a ##, the parameter is replaced by the actual argument, the ## and surrounding white space are removed, and the result is rescanned.

For example, the macro paste concatenates its two arguments: so paste(name, 1) creates the token name1.. #define pas te(front, back) front ## back printf("x/y = &g\n", x/y); printf("x/y" " = &g\n", x/y); dprint(x/y) #define dprint(expr) printf(#expr " = %g\n", expr). int getchar(void) { ...

}. The rules f or nested uses of ## are arcane; further details may be found in Appendix A. Exercise 4-14. Define a macro swap(t,x,y) that interchanges two arguments of type t.

(Block structure will help.). 4.11.3 Conditional Inclusion 77 It is po ssible to control preprocessing itself with conditional statements that are evaluated during preprocessing. This provides a way to include code selectively, depending on the value of conditions evaluated during compilation. The #if line evaluates a constant integer expression (which may not include sizeof, casts, or enum constants).

If the expression is non-zero, subsequent lines until an #endif or #elif or #else are included. (The preprocessor statement #elif is like else-if.) The expression defined(name) in a #if is 1 if the name has been defined, and 0 otherwise.

For example, to make sure that the contents of a file hdr.h are included only once, the contents of the file are surrounded with a conditional like this:. #if !define VS .NET gs1 datamatrix barcode d(HDR) #define HDR /* contents of hdr.h go here */.

The first i nclusion of hdr.h defines the name HDR; subsequent inclusions will find the name defined and skip down to the #endif. A similar style can be used to avoid including files multiple times.

If this style is used consistently, then each header can itself include any other headers on which it depends, without the user of the header having to deal with the interdependence. This sequence tests the name SYSTEM to decide which version of a header to include:. #if SYSTEM Data Matrix for .NET == SYSV #define HDR "sysv.h" #elif SYSTEM == BSD #define HDR "bsd.

h" #elif SYSTEM == MSDOS #define HDR "msdos.h" #else #define HDR "default.h" #endif #include HDR The #ifdef and #ifndef lines are specialized forms first example of #if above could have been written #ifndef HDR #define HDR /* contents of hdr.

h go here */ #endif. #endif that test whether a name is defined. The 5 - Pointers and Arrays A pointer i data matrix barcodes for .NET s a variable that contains the address of a variable. Pointers are much used in C, partly because they are sometimes the only way to express a computation, and partly because they usually lead to more compact and efficient code than can be obtained in other ways.

Pointers and arrays are closely related; this chapter also explores this relationship and shows how to exploit it. Pointers have been lumped with the goto statement as a marvelous way to create impossibleto-understand programs. This is certainly true when they are used carelessly, and it is easy to create pointers that point somewhere unexpected.

With discipline, however, pointers can also be used to achieve clarity and simplicity. This is the aspect that we will try to illustrate. The main change in ANSI C is to make explicit the rules about how pointers can be manipulated, in effect mandating what good programmers already practice and good compilers already enforce.

In addition, the type void * (pointer to void) replaces char * as the proper type for a generic pointer..
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