I'm trying to enable debugging options in MuPDF. For some reason they have used #ifndef NDEBUG and #endif greying out code I want to use. I searched throughout the library but couldn't find any traces of NDEBUG defined anywhere. I've managed to work around this by adding #undef NDEBUG in a header, but I would like to know if there is a more non-intrusive method to do so.
SO, can you enable "#ifndef/#endif" blocks from the makefile?
Also, why would you use #ifndef to grey out code? Isn't it supposed to be #ifdef NDEBUG?
You can add -DNDEBUG to the following 3 variables - CFLAGS, CPPFLAGS and CXXFLAGS in your Makefile to define NDEBUG.
Which is equivalent to adding #define NDEBUG
There are other variations too:
-DNBDEBUG=1
is equivalent to
#define NDEBUG 1
And to answer the question of why would someone use #ifndef instead of #ifdef is because it highlights your modifications to the original code much clearly.
For example consider the following code as the original version:
int a = 123;
int b = 346;
int c = a + b;
And you need to add a macro DO_MULT which will multiply instead - there are 2 ways to do this.
First Variation:
int a = 123;
int b = 346;
#ifdef DO_MULT
int c = a *b;
#else
int c = a + b;
#endif
Second variation:
int a = 123;
int b = 346;
#ifndef DO_MULT
int c = a + b;
#else
int c = a *b;
#endif
If you use difftools to see the changes - the second variation will show the diff more clearly compared to the first one.
One more reason why you would use a #ifndef is to DO something in CATCH-ALL-EXCEPT scenarios.
#ifndef won't omit the code if the flag is defined, hence it's usage. You've managed to include the code using #undef.
Both #ifdef and #ifndef are useful, as justification consider this contrived example: you have a bunch of debug printf code that you only want to compile into a Debug build, using #ifdef DEBUG conditions. In the same executable you also have code that you want to leave out of a Debug build. In this case using #ifndef DEBUG is your only choice.
You can define such flags in the makefile, and you can also try to remove previous definitions by using -U. Look for a CFLAGS variable in the makefile and add -UNDEBUG, or add it directly to the compiler call in the compilation target.
If the flag is being defined somewhere in the source tree then this won't help.
Related
I'm writing a C program that uses a custom logging function to debug my program. Whenever I compile my program as a release version, I want all of my logging functions to be stripped from the code so it won't show up if someone tries to disassemble it.
Take the following example:
#include <stdio.h>
void custom_logging_function(char* message)
{
// Do something here
}
int main()
{
custom_logging_function("Hello world"); // This call should be removed.
return 0;
}
How could I make it so that the custom_logging_function and it's arguments aren't compiled into my program without having to write include guards everywhere throughout my code? Thank you
You can use pre-processor flags, for example:
#include <stdio.h>
#ifdef DEBUG
void custom_logging_function(char* message)
{
// Do something here
}
#else
#define custom_logging_function(x) ((void) 0)
#endif
int main()
{
custom_logging_function("Hello world"); // This call should be removed.
return 0;
}
With this code you will have to tell the "debug" target to define DEBUG, if you want to define something specifically for the "release" target you can replace #ifdef DEBUG with #ifndef NDEBUG and add the NDEBUG flag to the "release" definitions.
Edit:
Changed #define custom_logging_function(x) 0 to #define custom_logging_function(x) ((void) 0) inspired by #JoachimPileborg his answer.
Assuming you only want the logging calls to happen in a debug-build of your application, and not the release build you send to customers, you can still use the preprocessor and conditional compilation for it. It can be made vert simple though by using macros instead of having checks at every call.
Something like this in a heder file:
#ifdef _DEBUG
void custom_logging_function(char* message);
#else
# define custom_logging_function(message) ((void) 0)
#endif
You could use an empty macro body for the release-macro, but that can cause some compilers to give "empty statement" warnings. Instead I use an expression casted to void (to tell the compiler that the result of the expression will not be used). Any smart compiler will not include the expression after optimization.
I'm writing a function that should have two versions: a debug version and non-debug version. Which one of the two functions is used should be decided by the caller.
I want something like this:
caller.c
// comment out the following line when not necessary anymore
#define MY_FUNC_DEBUG
#include "my_func.h"
// some code that calls my_func()
my_func.h
void my_func(void);
my_func.c
void my_func()
{
// lots of code
#ifdef MY_FUNC_DEBUG
// debug code
#endif
// more code
}
This obviously won't work, because my_func.c is compiled separately from caller.c, therefore it can't know what macros it defined.
How can I make this work easily? I wouldn't want to write the two versions of my_func separately, because they share most of their code.
Assuming that you are using gcc, this problem can be easily solved by defining the macro at compile time via the -D option in both files.
In your example you could compile both files using -D MY_FUNC_DEBUG when you want the debug code to be activated and nothing otherwise. There is not need for defining MY_FUNC_DEBUG in caller.c.
Make the debugging code in my_func() switchable at run-time.
my_func.h
#ifndef MY_FUNC_H_INCLUDED
#define MY_FUNC_H_INCLUDED
extern int my_func_debug(int level);
extern void my_func(void);
#endif
my_func.c
#include "my_func.h"
static int debug = 0;
int my_func_debug(int level)
{
int rv = debug;
debug = level;
return rv;
}
void my_func(void)
{
...
#ifdef MY_FUNC_DEBUG
if (debug)
...debug...
#endif
...
}
caller.c
void consumer(void)
{
int old = my_func_debug(9);
my_func();
my_func_debug(old);
}
Discussion
The outline code means that you can have one copy of the source for my_func.c, but it can be compiled with debug included, or with it excluded. The consumer code (caller.c) can request the level of debugging it wants, but whether that does anything useful depends on whether the copy of my_func.o (my_func.obj on Windows) was compiled with debug included. You get one source file; you get to choose which variant of the object file is included in the program with caller.o. And at runtime you can request debugging.
Note that my_func_debug() is unconditionally defined; it just doesn't do anything very useful if the my_func.c code is not compiled with -DMY_FUNC_DEBUG.
I need to temporarily overwrite a macro and then restore it. Like:
#define FOO X
#save FOO
#define FOO Y
...
#restore FOO
Is it possible in standard C preprocessor? In GCC?
ADDED. About real world example. I use a global macro for error exception. It acts like assert, but for persistent usage, not only for debug versions; so, for example, I usually call functions (with side-effect) inside the macro. It's defined once, but the definition isn't persistent; therefore I don't know it a-priori. For some piece of code I need its own, modified version of the macro, but I want to save general style of code. It's looks ugly when one part of code uses the one macro, other part uses other macro -- both macros have the same purpose, but slightly different implementation.
So, it's good for me to save original macro temporarily, use different version for a part of code, after that restore original macro.
This is possible with #pragma push_macro and #pragma pop_macro. These are not standard C—they're originally an MSVC extension—but clang supports them, and so does GCC.
Example usage:
int main() {
#define SOME_MACRO 1
printf("SOME_MACRO = %d\n", SOME_MACRO);
#pragma push_macro("SOME_MACRO")
#define SOME_MACRO 2
printf("SOME_MACRO = %d\n", SOME_MACRO);
#pragma pop_macro("SOME_MACRO")
printf("SOME_MACRO = %d\n", SOME_MACRO);
return 0;
}
prints:
SOME_MACRO = 1
SOME_MACRO = 2
SOME_MACRO = 1
You can also #undef a macro inside a push_macro / pop_macro pair, and the pop_macro call will redefine it.
As already said, it is not really possible. Depending on the situation, this might be a workaround:
#include "generalmacrodefs.h" // put them in here or include them indirectly
#undef macro1
#define macro1 "specialized temporary value"
#undef macro1
#include "generalmacrodefs.h" // restores
This requires that generalmacrodefs.h uses a pattern like this at least for the definitions you might temporarily overwrite:
#ifndef macro1
#define macro1 "original value"
#endif
The closest you can come in C is the #undef directive, which simply undefines the macro, allowing it to be replaced:
#define FOO X
...
#undef FOO
#define FOO Y
...
#undef FOO
#define FOO X
The problem is that you cannot know the 'old' value of FOO once you redefine it - so your values must be hard-coded in one place.
You cannot create a macro to save the values for you either, as it isn't possible to have a macro that creates other preprocessor directives in standard C.
I have a structure defined in a header file called data.h.
I am including data.h in myfile.c.
In the structure, I have part of the variables blocked off with:
#ifndef TEST
int x;
#endif
and in myfile.c I have:
#ifdef TEST
localx++;
#else
mystruct.x++; //<-compiler complains on this line when compiling
#endif
When I try to compile with -DTEST I get a compiler complaining that mystruct type does not containing a field called x. What is up with this?
I don't have a C compiler handy, so here is what I just typed up:
in data.h
typdef struct {
#ifndef TEST
int x;
#endif
int y;
} coords;
in myfile.c
#include "data.h"
static coords coord1;
int localx;
int main( )
{
#ifdef TEST
localx = 1;
#else
coord1.x = 1;
#endif
coord1.y = 2;
printf("%i\n", coord1.x);
printf("%i\n", coord1.y);
printf("%i\n", localx);
return 0;
}
This compiles when I type cc myfile.c but not with cc myfile.c -DTEST
I am using the MIPSPro C compiler referenced here.
You most recent edit (which may well be different by the time anyone reads this) will have a problem in the section that has a bunch of printf() statements. The line:
printf("%i\n", coord1.x);
is referencing the x member of the struct regardless of the setting of the TEST preprocessor macro. It needs to be inside a conditional compilation section too in order to compile correctly (rather not compile at all) when the x member doesn't exist.
Since you are using ifndef for the field x, it is only there to use if TEST is not defined!!
#ifdef allows a section of a program to be compiled only if the macro that is specified as the parameter has been defined, no matter which its value is. For example:
#ifdef TABLE_SIZE
int table[TABLE_SIZE];
#endif
In this case, the line of code int table[TABLE_SIZE]; is only compiled if TABLE_SIZE was previously defined with #define, independently of its value. If it was not defined, that line will not be included in the program compilation.
#ifndef serves for the exact opposite: the code between #ifndef and #endif directives is only compiled if the specified identifier has not been previously defined. For example:
#ifndef TABLE_SIZE
#define TABLE_SIZE 100
#endif
int table[TABLE_SIZE];
In this case, if when arriving at this piece of code, the TABLE_SIZE macro has not been defined yet, it would be defined to a value of 100. If it already existed it would keep its previous value since the #define directive would not be executed.
From: http://www.cplusplus.com/doc/tutorial/preprocessor/
Except for the typo (typdef), your example compiles fine for me using gcc.
Edit:
The new example shouldn't compile. You need to wrap every reference to "x" in #ifdef directives.
Also, gcc accepts the -D flag before the file list, but I don't have access to MIPSpro. The docs say you have the command line out of order.
I'm writing firmware in C for an embedded processor. I want to have all the configuration information in one header file called config.h. This is causing problems with the ADC initialization, where simple #defines won't do the trick. Right now the code is like so:
config.h
#define NUMBER_OF_POTS 1
#define POT_1_CHANNEL 27
adc.c
#define MAKE_CSS(channel) _CSS##channel
#define CALL_MAKE_CSS(channel) MAKE_CSS(channel)
void initialize_adc() {
CALL_MAKE_CSS(POT_1_CHANNEL);
}
What I want to do is not have to touch adc.c if I change config.h to:
#define NUMBER_OF_POTS 2
#define POT_1_CHANNEL 27
#define POT_2_CHANNEL 29
adc.c should just automatically add a second CALL_MAKE_CSS with some macro trickery.
I guess the question is: is there a trick that gives you for loop capability with a macro?
Thanks,
Steve.
I didn't test this:
// config.h
#define NUMBER_OF_POTS 2
extern int pots[];
// config.c
int pots[NUMBER_OF_POTS] = {
27,
29
};
// adc.c
void initialize_adc() {
for (int i = 0; i < NUMBER_OF_POTS; i++) {
CALL_MAKE_CSS(pots[i]);
}
}
You don't have to rely entirely on macros. Just define your 'magic numbers' as #defines.
For example:
In config.h:
#define NUMBER_OF_POTS 2
#define POT_1_CHANNEL 27
#define POT_2_CHANNEL 29
unsigned int PotChannelList[NUMBER_OF_POTS] = {POT_1_CHANNEL, POT_2_CHANNEL};
In adc.c:
for(i = 0; i < NUMBER_OF_CHANNELS; i++)
{
initialize_adc(PotChannelList[i]);
}
You still define the setup in config.h and don't have to change adc.c when you add a channel. You just add it to the list. The list order also defines the order of initialization.
EDIT: Sorry about the formatting mess...
Have a look at boost.preprocessor. Although boost is usually for C++, the preprocessor metaprogramming lib works, well, just with the CPP, so it may do what you want. It provides a few datastructures (lists, tuples) and iteration macros.
Sorry, I can't give you any example if it really does what you want, or at least provides another way, because I seldom needed it, and it's too long ago.
Note Just saw Schroeder's answer. Not relying on the PP if it is not necessary is still the best option...
The C preprocessor cannot do loops. You'll either have to do the looping in C code, or if you really need to do something loop-like at compile time, you can write your own preprocessor (which can just be a simple shell script, e.g.) that generates the necessary code.
Although you can't do loops with the preprocessor, you can do unrolled loops. So if you know you're never going to have more than 4 pots you could do this;
void initialize_adc() {
#if NUMBER_OF_POTS > 0
CALL_MAKE_CSS(POT_1_CHANNEL);
#endif
#if NUMBER_OF_POTS > 1
CALL_MAKE_CSS(POT_2_CHANNEL);
#endif
#if NUMBER_OF_POTS > 2
CALL_MAKE_CSS(POT_3_CHANNEL);
#endif
#if NUMBER_OF_POTS > 3
CALL_MAKE_CSS(POT_4_CHANNEL);
#endif
}
The only benefit of this compared to other solutions here is that there is no runtime overhead at all. Extra inline code "magically" appears if and only if another channel is added, just as the questioner wanted. To extract the ugliness from within the function call (at the cost of putting it earlier in your code instead), define 4 new macros each using the same #if NUMBER_OF_POTS > x technique. Then you'd be able to go simply;
void initialize_adc() {
INIT_CSS_1();
INIT_CSS_2();
INIT_CSS_3();
INIT_CSS_4();
}