C: macro produced warning "statement with no effect" - c

I have following code in C:
int do_something(void);
#ifdef SOMETHING
#define DO_SOMETHING() do_something()
#else
#define DO_SOMETHING() 0
#endif
This code produced warning "statement with no effect" when compiled without SOMETHING defined. I am trying to fix it, but there is one problem - code which uses this macro sometimes checks that "return value" and sometimes ignores it. Because of this I cannot use the easiest solution - casting to void in macro itself.
Is it possible to write macro which allows to compare "returned value" and does not produce this warning when it is ignored?
I use gcc to compile my code.

Three possible solutions:
Define a do_nothing function, which will get optimized out by gcc:
int do_something(void);
int do_nothing(void) { return 0; }
#ifdef SOMETHING
#define DO_SOMETHING() do_something()
#else
#define DO_SOMETHING() do_nothing()
#endif
Or, modify the do_something implementation to move the #ifdef check there
int do_something(void)
{
#ifndef SOMETHING
return 0;
#endif
// Your implementation here
}
You can also ignore the warning using #pragma directives.
By the way, which version of gcc and with which flags are you compiling? gcc -Wall -pedantic with GCC 4.9 doesn't produce the warning.

#include <stdio.h>
int do_something(void);
#ifdef SOMETHING
#define DO_SOMETHING() do_something()
#define DO_SOMETHING_ASSIGN() do_something()
#else
#define DO_SOMETHING()
#define DO_SOMETHING_ASSIGN() 0
#endif
int do_something(void)
{
static int cnt=0;
/* code for do something here */
++cnt;
printf("do_something has been called %d %s!\n",cnt,
(cnt==1)?"time":"times");
return cnt;
}
void main(void)
{
int x;
DO_SOMETHING();
x=DO_SOMETHING_ASSIGN();
printf("%d %d\n",x,DO_SOMETHING_ASSIGN());
puts("end!!!");
}
I hope this is usefull to you!
If you save this code as main.c, you may compile it with:
gcc main.c -o main
when you run main, you obtain the following output:
0 0
end!!!
If you compile it with:
gcc main.c -DSOMETHING -o main
When you run main, you obtain the following output:
do_something has been called 1 time!
do_something has been called 2 times!
do_something has been called 3 times!
2 3
end!!!

Related

How come the preprocessor directives in my header are not seeing the macro I put above the #include in source? (C code)

I am attempting some conditional compilation for unit testing static functions in C. (roughly following the method outlined in this answer https://stackoverflow.com/a/593437/8347016)
I have it set up like so:
check_blah.c
#define UNIT_TEST 1
#include "blah.h"
#include "testframework.h"
...
blah.h
#if UNIT_TEST
#define u_static
#else
#define u_static static
#endif
...
#if UNIT_TEST
void foo(/* some parameters */)
#endif
blah.c
#include "blah.h"
...
u_static void foo(/*some parameters */) {
/* some definition */
}
...
And just to cover my bases, here is how the files are compiled in make
check_blah: check_blah.c blah.o testframework.o
gcc check_blah.c blah.o testframework.o -o check_blah
blah.o: blah.c blah.h
gcc -c blah.c -o blah.h
testframework.o: /* similar to above */
If I compile check_blah without it having any references to foo, and then run gdb info functions, I see that foo is considered static even though UNIT_TEST is defined. Even more confusing, the compiler somehow does recognize UNIT_TEST as defined. In an experiment I defined a macro, SPEEP to be 10 if UNIT_TEST was defined and 100 otherwise. Then in check_blah.c I set some int to SPEEP and then printed it. It would print 10! So the line #define ustatic MUST be being hit as well, but somehow it isn't since foo remains static.
If I take the line #define UNIT_TEST 1 and move it to the top of blah.h, everything seems to work (i.e, info functions claims foo is not static).
So does anyone know the reason for this awkward (and inconsistent looking) behavior with preprocessor macros and directives?

Check if Pre processor Directive is defined in C Programming [duplicate]

Based on this question How to catch empty defined macro with gcc? I have another problem. How to catch undefined macro in preprocessor #if condition? Example code:
#include <stdio.h>
int main()
{
#if ENABLE_SOMETHING == 1
... do something ..
#endif
return 0;
}
Is there a way to catch error/warning when ENABLE_SOMETHING is not set using gcc compiler(maybe some flag)? Or maybe there are external tools which I can use?
I know than i can write something like this :
#ifndef ENABLE_SOMETHING
#error "ENABLE_SOMETHING undefined!"
#endif
But I have a huge amount of different defines(ENABLE_STH1, ENABLE_STH2, ENALBE_STH3 ... etc.) in code and i don't want to fix this manually. I'm looking for some automatic solution for our project.
Is there a way to catch error/warning when ENABLE_SOMETHING is not set
using gcc compiler(maybe some flag)?
With GCC you can use the -Wundef flag.
From the official documentation
-Wundef
Warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero.
EDIT:
For example, this C-code:
#include <stdio.h>
int main(void)
{
#if UNDEFINED_MACRO
printf("hi mum!\n");
#endif
return 0;
}
... compiled with GCC and the -Wundef flag yields this:
$ gcc undef.c -Wundef
undef.c: In function 'main':
undef.c:5:5: warning: "UNDEFINED_MACRO" is not defined [-Wundef]
#if UNDEFINED_MACRO
^
Let's assume you have this code and it compiles, but you don't know if MAX_N_LENGTH is a macro, or if it's something else:
int main()
{
int a = MAX_N_LENGTH; // MAX_N_LENGTH could also be an int declared somewhere else
return 0;
}
You can check whether it actually is a macro like this:
#ifdef MAX_N_LENGTH
printf("MAX_N_LENGTH is a macro.\n");
#else
printf("MAX_N_LENGTH is NOT macro.\n");
#endif // MAX_N_LENGTH
Of course, if that ever is an issue, I'd rethink my naming conventions.
You could try something like the following:
#ifndef MAX_N_LENGTH
#warning "MAX_N_LENGTH is undefined"
int array[16];
#else
int array[MAX_N_LENGTH + 1];
#endif
You can test if a macro is defined in a #if preprocessor expression with defined(ENABLE_SOMETHING):
#if !defined(ENABLE_SOMETHING)
#error ENABLE_SOMETHING is not defined
#endif
You can handle macros with an empty definition this way:
#if ENABLE_SOMETHING + 0 == 1
/* ENABLE_SOMETHING is defined and not 0 or empty */
#endif

How to catch undefined macro in preprocessor #if condition?

Based on this question How to catch empty defined macro with gcc? I have another problem. How to catch undefined macro in preprocessor #if condition? Example code:
#include <stdio.h>
int main()
{
#if ENABLE_SOMETHING == 1
... do something ..
#endif
return 0;
}
Is there a way to catch error/warning when ENABLE_SOMETHING is not set using gcc compiler(maybe some flag)? Or maybe there are external tools which I can use?
I know than i can write something like this :
#ifndef ENABLE_SOMETHING
#error "ENABLE_SOMETHING undefined!"
#endif
But I have a huge amount of different defines(ENABLE_STH1, ENABLE_STH2, ENALBE_STH3 ... etc.) in code and i don't want to fix this manually. I'm looking for some automatic solution for our project.
Is there a way to catch error/warning when ENABLE_SOMETHING is not set
using gcc compiler(maybe some flag)?
With GCC you can use the -Wundef flag.
From the official documentation
-Wundef
Warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero.
EDIT:
For example, this C-code:
#include <stdio.h>
int main(void)
{
#if UNDEFINED_MACRO
printf("hi mum!\n");
#endif
return 0;
}
... compiled with GCC and the -Wundef flag yields this:
$ gcc undef.c -Wundef
undef.c: In function 'main':
undef.c:5:5: warning: "UNDEFINED_MACRO" is not defined [-Wundef]
#if UNDEFINED_MACRO
^
Let's assume you have this code and it compiles, but you don't know if MAX_N_LENGTH is a macro, or if it's something else:
int main()
{
int a = MAX_N_LENGTH; // MAX_N_LENGTH could also be an int declared somewhere else
return 0;
}
You can check whether it actually is a macro like this:
#ifdef MAX_N_LENGTH
printf("MAX_N_LENGTH is a macro.\n");
#else
printf("MAX_N_LENGTH is NOT macro.\n");
#endif // MAX_N_LENGTH
Of course, if that ever is an issue, I'd rethink my naming conventions.
You could try something like the following:
#ifndef MAX_N_LENGTH
#warning "MAX_N_LENGTH is undefined"
int array[16];
#else
int array[MAX_N_LENGTH + 1];
#endif
You can test if a macro is defined in a #if preprocessor expression with defined(ENABLE_SOMETHING):
#if !defined(ENABLE_SOMETHING)
#error ENABLE_SOMETHING is not defined
#endif
You can handle macros with an empty definition this way:
#if ENABLE_SOMETHING + 0 == 1
/* ENABLE_SOMETHING is defined and not 0 or empty */
#endif

Function macro that evaluates to zero and can be used as a statement

We have a function macro #define FOO(arg) foo(arg) with int foo(const char* bar);. When NDEBUG is defined FOO is defined as #define FOO(arg) 0, however this causes many compiler warnings because in many cases FOO's return value is not used. The solution should work with with ANSI C compilers and cause no warnings. I've tried:
(void)0: can't be assigend to variable
static int foo(const char* bar) { return 0; } : causes unused static function warning in some modules
static inline int foo(const char* bar) { return 0; } : only works with C99 compilers
Thanks for your help!
edit1:
It's somewhat like a trace macro and used all over the project. Mostly it's just used as a statement like FOO("function x called");, but in a few cases I saw if (FOO("condition a")) { /* some more debug output */ }. With NDEBUG defined and optimization enabled nothing should be left of FOO. I didn't come up with this, but I have to clean up this mess :).
edit2: I should add that for gcc release builds these flags are used: -O3 -Wall -ansi
edit3: For now I'm going with __inline int dummy() { return 0; }. __inline works with both VisualC and GCC in ansi mode.
I guess it's a little bit compiler dependent but this should work:
#ifndef NDEBUG
#define FOO(arg) foo(arg)
#else
#define FOO(arg) ((int)0)
#endif
It prevents the "expression has no effect" warning, it does nothing and its value when used is still 0.
EDITED
It seems it's something not so portable so (now) you have these conditions:
(0) or ((int)0) work at least on VC 2010.
__noop should work on any version of VC after 2003.
VC6 is not a problem because it doesn't emit the C4555 warning at all. For other compilers you may use:
((void)0, 0) It may work on a lot of compilers (maybe it's the more portable one?).
inline int foo(const char* bar) { return 0; } works with any other C99 compiler (as you wrote you may need to declare it as static on gcc).
For any other prehistoric C compiler use the solution pointed by #Jobs: abs(0)
What you could do to prevent the warning is the following:
#ifndef NDEBUG
#define FOO(arg) foo(arg)
#else
#define FOO(arg) abs(0)
#endif
I'm not saying this is ideal (you'd have to make sure stdlib.h is included everywhere, for example) but it does prevent the warning.
I'd do something that is dependent on the C version. In the header file:
#if __STDC_VERSION__ > 199900L
inline int foo(const char* bar) { return 0; }
#else
int foo(const char* bar);
#endif
in one compilation unit
#if __STDC_VERSION__ < 199900L
int foo(const char* bar) { return 0; }
#else
int foo(const char* bar);
#endif
or use for the oldish C version something like Job's answer, that is a function that is certain to be optimized out but that doesn't produce the warning.

C preprocessor in include header files

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.

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