I was investigating a compile and link issue within my program when I came across the following macro that was defined in a header and source file:
/* file_A.c */
#ifndef _NVSize
#define _NVSize 1
#endif
/* file_B.c */
#include "My_Header.h"
#ifndef _NVSize
#define _NVSize 1
#endif
/* My_Header.h */
#define _NVSize 1024
Nothing out of the ordinary yet, until I saw the following information in the GCC output map file:
/* My Map File */
...
.rodata 0x08015694 _NVSize
...
My understanding of the map file is that if you see a symbol in the .rodata section of the map file, this symbol is being treated as a global variable by the compiler. But, this shouldn't be the case because macros should be handled by the preprocessor before the compiler even parses the file. This macro should be replaced with it's defined value before compiling.
Is this the standard way that GCC handles macros or is there some implementation specific reason that GCC would treat this as a global (debug setting maybe)? Also, what does this mean if my macro gets redefined in a different source file? Did I just redefine it for a single source file or did I modify a global variable, thereby changing _NVSize everywhere it's used within my program?
I think the compiler is free to assign your macro to a global variable as long as it ensures that this produces the exact same result as if it did a textual replacement.
During the compilation the compiler can mark this global specially to denote that it is a macro constant value, so no re-assignment is possible, no address can be taken, etc.
If you redefine the macro in your sorce, the compiler might not perform this transformation (and treat it as you'd expect: a pre-compier textual replacement), perform it on one of the different values (or on all of them say, using different names for each occurrance), or do domething else :)
Macros are substituted in the preprocessor step, the compiler only sees the substituted result. Thus if it sees the macro name, then my bet is that the macro wasn't defined at the point of usage. It is defined between the specific #define _NVSize and an #undef _NVSize. Redefining an existing macro without using an #undef first should result in a preprocessor error, AFAIR.
BTW, you shouldn't start your macro names with an underscore. These are reserved for the implementation.
Related
I have often seen code like
#ifndef HEADERFILE_H
#define HEADERFILE_H
// some declarations in
// the header file.
#endif
I want to know what #define HEADERFILE_H define HEADERFILE_H to?
I tried doing
cout<<HEADERFILE_H<<endl;
but I am getting
error: expected expression
A define preprocessing directive has the form # define identifier preprocessing-tokens, ending with a new-line character. The preprocessing-tokens is a list of zero or more preprocessing tokens. It may be empty, that is, it may have zero tokens. This means, that when the identifier is encountered in a place where macro replacement occurs, it will be replaced with nothing.1
Tests of the form #ifdef identifier, #ifndef identifier, or define identifier in a #if or #elif directive test whether identifier is defined or not. If it was not defined (or its definition was removed with the #undef directive), then the test indicates it is not defined. If it was defined, then the test indicates it was defined, even if the definition is for zero tokens.
A definition with zero tokens is different from no definition at all, and defined identifier will evaluate as true for the former and false for the latter.
Footnote
1 If the list does have tokens, then identifier will be replaced with those tokens and # and ## operators among them will be applied. A preprocessing token is largely an identifier (like foo34), a constant (like 3, 4u, or 1.8e4), one of the C operators or special characters (like * or +=), or certain other components of the C language.
It actually defines "nothing else than itself". That is: you may define a macro without assigning it a specific value. Since you can check if a given macro is defined or not, you therefore can ask for the simple "existence" of a given macro.
This is useful to indicate a context (for example, if you're compiling for a given OS) and/or the availability of some resources.
In this particular example, this is a called a "guard": it will define itself if this hasn't been done first before, as well as including the rest of the file, which is totally embedded in the #ifdef … #endif clause.
This is used to implement a kind of require_once, that is something that will be included if needed, but not multiple times. This is required when you are defining functions or declaring variables at a global scope, for instance.
This is a language idiom (I will comment it):
#ifndef HEADERFILE_H
Between this, and the last #endif everything is included in compilation, but only if HEADERFILE_H has not been defined before.
#define HEADERFILE_H
The first thing we do in the block is to #define the identifier, so next time we find this fragment again later, the contents between #ifndef and #endif will not be #included again (because of the identifier declaration).
// some declarations in
// the header file.
this block will be included only once, even if you #include this file several times.
#endif
and this marks the end of the protected block.
It is common to include some file that, indeed, #includes another, and that file includes another, leading to a point in which you don't know which files have been included and which don't. This phrasing allows you to be protected, and to be able to #include the same file several times (normally you cannot, as some definitions cannot be repeated in the same compilation unit, e.g. declarations) the lines above will include the contents and define the identifier, making next inclussions (that are effectively done) not to include the contents, as the identifier appears as #definen in second and ulterior times.
I use macros like #DEBUG to print some additional debugging info and even possibly do something differently to help me with debugging. For example:
in header a.h:
#define DEBUG 1
in src a.c:
#include "a.h"
int func_a () {
/*some code*/
#if DEBUG
//do this
#endif
}
What will happen if I use a macro with the same name in another file ?
header b.h
#define DEBUG 1
#if DEBUG
# define PRINT 1
#elif
#define PRINT 0
#endif
src b.c
#include "a.h"
#include "b.h"
int func_b () {
/*some code*/
#if PRINT
//do this
#endif
/*some code*/
#if DEBUG
//do this
#endif
}
What will happen if I change the value of #DEBUG in one of the headers? I saw in some other answers that redefining a macro is not allowed in the C standard. But when I compile with GCC using the -Wall flag I see no errors or warnings.
What will happen if I use a macro with the same name in another file ?
It depends. C does not allow an identifier that is already defined as a macro name at some point in a translation unit to be defined again at that point, unless the redefinition specifies an identical replacement list. This is a language constraint, so conforming implementations will emit a diagnostic about violations they perceive. Compilers may reject code that contains violations, and if they nevertheless accept such code then the resulting behavior is undefined as far as C is concerned.
In practice, implementations that do accept such violations have two reasonable choices (and a universe of unreasonable ones):
ignore the redefinition, or
process the redefinition as if it were proceeded by an #undefine directive specifying the affected macro name.
Implementations of which I am aware accept such redefinitions and implement the latter option, at least by default.
If your headers are defining macros solely for their own internal use then you may be able to address the issue by exercising some discipline:
Each header puts all its #include directives at the beginning, before any definition of the possibly-conflicting macro(s).
Each header #undefines the possibly-conflicting macro at the end, under all conditional-compilation scenarios in which the macro may be defined in the first place.
On the other hand, if the macro is intended to be referenced by files that use the header(s) where it is defined then undefining it within the header would defeat the purpose. Under some circumstances, probably including yours, you can address that by defining the macro only conditionally in each header:
#if !defined(DEBUG)
#define DEBUG 1
#endif
That will avoid redefinition, instead using (only) the first definition encountered, which may even come from compiler command-line arguments. If you do this, however, it is essential that all the default definitions specified in that way be the same, else changing your headers' inclusion order will have unexpected effects code that depends on which definition is used.
I am new to C, so please help me with an answer before downvoting, it will help me a lot.
The definition of header file states that-
A header file consists of C function declaration and macro definitions to be shared between different files.
And the definition of C Preprocessor states that-
The C preprocessor is a macro preprocessor that transforms your program before it is compiled.All preprocessor directives begin with the # symbol.
My question is the macro declaration are done using # symbol in a program, does it depend on inclusion or exclusion of any header file, also how to find whether a particular file has a pre-defined macro declaration in it.
For example-
Say a file 'ani.h' has a macro declaration,
#define anii 2
So, once I include this file, I am allowed to use the CNAME i.e. aniidirectly?
It is easier if you look at this form the view of what the pre-processor actually does.
It reads a file and replaces text and then output a new file. The new file is sent to the compiler. The pre-proc knows nothing about C code, its just a text manipulation engine
#include xxxx
says, 'replace this line by the contents of that file'
#define FOO BAR
says, 'whenever you see FOO replace it by BAR
There are also some simple conditionals: #if etc
A macro is simply a FOO BAR replacement, usually FOO is small and BAR is large.
Although they can be used to do some nice metaprogramming tricks as well as conditional programming, macros most basic use is to make programmers lives easier via syntactic sugaring. Before compiling your code, the preprocessor module will substitute all the macros by the terms they represent, so everything you define must have been declared somewhere above the point where the macro is first used. So if you do:
#include <stdio.h>
#include <math.h>
#define MY_MACRO(x) for(int i=0; i<(x); ++i) printf("%d\n", func((x),i);
int func(int n, int m) {return pow(n, m);}
int main()
{
int a = 10;
MY_MACRO(a)
return 0;
}
The preprocessor will substitute all occurrences of MY_MACRO in your code by the loop that was defined after it. The code will then be sent to compilation. When the compiler reaches the line where the macro is used, it is necessary that printf and func are declared somewhere above this line, just as if you were writing the code without the macro. (Note that printf and pow are declared in the headers included before the definition and func is declared after the definition, but them three are declared before the first use of the macro.)
In what concerns to knowing which macros are declared inside some lib and which aren't, I believe the only way is to check the header files you are using or to read their documentation. But if you know that some specific macro may or may not be declared, you can test it using the code below:
#ifdef SOME_MACRO
printf("SOME_MACRO defined!\n");
#else
printf("SOME_MACRO not defined!\n");
#endif
Per this question, it seems there is some flexibility to how you can write that--
#ifndef _HEADER_H
or:
#ifndef __HEADER___H__
etc. It's not set in stone.
But I don't understand why we're using underscores at all in the first place. Why can't I just write:
#ifndef header.h
What's wrong with that? Why are we placing underscores everywhere and capitalizing everything? What does the preprocessor do with underscores?
header.h is not a valid identifier. You cannot have a period in a macro name.
That said, the name you pick for your include guard macros is completely arbitrary. After all, it's just another variable. It is purely convention (and reasonable in order to avoid clashes) to name them after the file.
I encourage you to phrase the header structure out aloud to see what the preprocessor does.
#ifndef MY_HEADER_H /* If the macro MY_HEADER_H is not defined (yet)... */
#define MY_HEADER_H /* ... then define it now ... */
... /* ... and deal with all this stuff ... */
#endif /* ... otherwise, skip all over it and go here. */
You see that this mechanism works equally well if you substitute MY_HEADER_H with I_REALLY_LIKE_BANANAS or whatever. The only requirement is that it be a valid macro identifier and not clash with the name of any other include guard.
In the above example, the macro is defined empty. That's fine, but it is not the only option. The second line could equally well read
#define MY_HEADER_H 1
which would then define the macro to 1. Some people do this but it doesn't really add anything and the value 1 is rather arbitrary. I generally don't do this. The only advantage is that if you define it to 1, you can also use #if in addition to #ifdef.
A final word of caution: Identifiers that start with an underscore or contain two or more consecutive underscore characters are reserved for the implementation and should not be used in user-code. Hence, _MY_HEADER_H and __MY_HEADER__H__ are both unfortunate choices.
The logic by which the preprocessor finds the correct header file if you say
#include <myheader.h>
is completely unrelated. Here, myheader.h names a file and the preprocessor will search for it in a number of directories (that usually can e configured via the -I command line option). Only after it has found and opened the file it will go ahead parsing it and thereby, it will eventually find the include guards that will cause it to essentially skip over the file if it has already parsed it before (and the include guard macro is therefore already defined so the first check evaluates to false).
Because #ifdef or #ifndef requires a preprocessor symbol after it, and these symbols cannot contain dots.
In the C11 (latest draft) spec n1570 (§6.10.1):
Preprocessing directives of the forms
# ifdef identifier new-line group opt
# ifndef identifier new-line group opt
check whether the identifier is or is not currently defined as a macro name.
and identifiers cannot contain dots (§6.4.2.1)
BTW, include guards are not required to have #ifdef symbols related to the file name. You can have a header file foo.h guarded with a #ifndef JESUISCHARLIEHEBDO or by #ifndef I_LOVE_PINK_ROSES_BUT_NOT_YELLOW_ONES preprocessor directive if you want so. But by human convention, the names are often related.
Notice that identifiers starting with an underscore are implementation defined, so you should rather avoid #ifndef _FOO_INCLUDED but prefer #ifndef FOO_INCLUDED
I'm somewhat confused by #define statements. In libraries, they seem to be a means of communication across different files, with lots of #ifdefs and #ifndefs.
Having said that, I now have two files file1.c and file2.c compiled together, with #define TEST 10 inside file2.c. Yet, when I use TEST inside file2.c the compiler gives the following error message:
'TEST' undeclared (first use in this function)
Are #define directives global?
#defines are not global, they are just a substitution where ever they are used (if declared in the same compile unit)
They are not globals, they are not symbols, they are irrelevant at linkage, they are only relevant at pre-compilation.
#defined macros are global in that they do not follow normal C scoping rules. The textual substitution from the macro will be applied (almost) anywhere the macro name appears after its #define. (Notable exceptions are if the macro name is part of a comment or part of a string literal.)
If you define a macro in a header file, any file that #includes that header file will inherit that macro (whether desired or not), unless the file explicitly undefines it afterward with #undef.
In your example, file2.c does not know about the TEST macro. How would it know to pick up the #define from file1.c? By magic? Since macros perform textual substitution on the source code, there is no representation of them in the generated object files. file2.c therefore needs to know that substitution rule itself, and if you want that shared across multiple files, that #define needs to live in a common header file that your .c files #include.
If you're asking specifically about how many of the #ifdefs that you see in libraries work, many of them are likely checking against pre-defined macro names provided by the compilation environment. For example, a C99 compiler defines a __STDC_VERSION__ macro that specifies the language version; a Microsoft compiler defines an _MSC_VER macro. (Often these predefined macros start with leading underscores since those names are reserved for the compiler.)
Additionally, most compilers allow defining simple macros as command-line arguments. For example, you might compile your code via gcc -DNDEBUG file1.c to compile file.c with NDEBUG defined to disable asserts.
In case somebody reads this later, and to add some practical information:
Some environments like atmel, vs, or iar, allow you to define global #define directives. They basically pass these defined values to the precompiler in some commandline format.
You can do the same in batch commands or makefiles, etc.
Arduino always adds a board variant (usually located at hardware\arduino\variants) to all compilations. At that point you can create a new board that contains your global define directives, and use it that way. For example, you can define a mega2560(debug) board out of the original mega2560 that contains some debug directives. You will add a reference to that variant in "boards.txt", by copy pasting some text, and properly modifying it.
At the end of the day, you will have to give that global hfile or global directive to the compiler in one way or another.
you should make a file1.h and put your defines there. Then in file2.c
#include "file1.h"
easy as a pie :)