Overriding macro values defined in source code

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1

Hi Team,
Consider the below C code,
#define AVAL 5
void func()
{
int a=5;
if(a==AVAL)
{
//Do something
}
else
{
//Do something else
}

}
My question is
Is it possible to override the value of AVAL through the -D option?
For eg is it possible to set the value of AVAL to 2 through -D in clang ?

2

This is more a question for the cfe-users list, but as far as I know
there's no way to prevent redefinitions in source files.

Cheers.

Tim.

3

Hi Team,
Consider the below C code,
#define AVAL 5
void func()
{
int a=5;
if(a==AVAL)
{
//Do something
}
else
{
//Do something else
}

}
My question is
Is it possible to override the value of AVAL through the -D option?
For eg is it possible to set the value of AVAL to 2 through -D in clang so that the else part executes?

Also apart from -D option we are also open to other ways of achieving this
Thanks in advance

4

From: "Tim Northover" <t.p.northover@gmail.com>
Date: May 1, 2019 3:48 PM
Subject: Re: [llvm-dev] Overriding macro values defined in source code
To: "Sudhindra kulkarni" <sudhindrakkulkarni102@gmail.com>
Cc: "LLVM Developers Mailing List" <llvm-dev@lists.llvm.org>

> Is it possible to override the value of AVAL through the -D option?
> For eg is it possible to set the value of AVAL to 2 through -D in clang ?

This is more a question for the cfe-users list, but as far as I know
there's no way to prevent redefinitions in source files.

Cheers.

Tim.

Hi Team,
                 Consider the below C code,
#define AVAL 5
void func()
{
int a=5;
if(a==AVAL)
{
//Do something
}
else
{
//Do something else
}

}
My question is
Is it possible to override the value of AVAL through the -D option?
For eg is it possible to set the value of AVAL to 2 through -D in clang so that the else part executes?
Also apart from -D option we are also open to other ways of achieving this

The normal approach is to change the source to something like

#ifndef AVAL
#define AVAL 5
#endif

5

Thanks Richard for the answer.

I have another question regarding the include(-I) option.

If we have a c file called cfile.c and a header file called header.h both in the same directory say (dir x). cfile.c includes header.h as shown below
#include “header.h”
And if in some other directory say (dir y)we have another header file with the same name(header.h) but with a different content then when I try to generate the preprocessed file using the below command,
clang.exe -E path of cfile.c -I path of dir y
The header.h present in the directory dir x is getting considered even though -I points to dir y.

Is this the intended behavior that the current directory is searched first for the header files irrespective of the directory provided against the -I option.
If not how to make clang consider only the directories provided against the -I option and not the current directory.

Thanks in advance

6

Thanks Richard for the answer.

I have another question regarding the include(-I) option.

If we have a c file called cfile.c and a header file called header.h both in the same directory say (dir x). cfile.c includes header.h as shown below
#include "header.h"
And if in some other directory say (dir y)we have another header file with the same name(header.h) but with a different content then when I try to generate the preprocessed file using the below command,
clang.exe -E path of cfile.c -I path of dir y
The header.h present in the directory dir x is getting considered even though -I points to dir y.

Is this the intended behavior that the current directory is searched first for the header files irrespective of the directory provided against the -I option.
If not how to make clang consider only the directories provided against the -I option and not the current directory.

I think the only way to suppress the search in the directory of the
file containing the #include is with the -I- (dash, capital i, dash)
option. (That option also has other effects on the #include path, and
it matters where you place it relative to other -I flags.) It'd
probably be worth adding a dedicated flag for just the "don't look in
the directory containing the file" effect.

7

Hi Richard,
Thanks for the quick response.
I tried the -I- option.
I ran the below command
clang.exe -E path of cfile.c -I path of dir y -I-
With this I get the below error
clang.exe:error :-I- not supported , please use -iquote instead.

I tried using -iquote as well
clang.exe -E path of cfile.c -I path of dir y -iquote path of dir y

Even this did not work and it is still picking up the header file from the same directory as that of the c file ie dir x.

I am using clang 6.0.1
Is the -I- option deprecated in clang 6.0.1?
If not is the usage of the flag(-I- and -iquote) not proper.

Thanks in advance