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Include header file if it exists to redefine macros in C

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1

1

I have two header files, one that is a library with a number of default macros defined
i.e:

libraryheader.h:

#define ANAME 1
#define BNAME 2
.
.
.

and I want to be able to have another optional header that the user can supply to redefine these macros in the project

optional.h:

#define ANAME 5
#define BNAME 15
.
.
.

The header optional.h doesn't always exist and I only want to include it if it does exist. Is there a way in the makefile or library header to check if this header exists and redefine the macros.

I'm thinking to check if the file exists in the makefile and set a flag by putting the following in the makefile

ifdef $(test -f optional.h && echo "present")

and then somehow check the flag macro in the library header.
But I haven't had any success yet. Thank you

c makefile header preprocessor

share|improve this question

asked Mar 26 at 3:21

rotalcomorotalcomo

3144 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Well, you could have your makefile update the compile flags to define HAS_OPTIONAL_H macro, and then you can wrap your #include line in a #ifdef.
  
  – paddy
  Mar 26 at 3:46
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  myprog:CPPFLAGS += $(if $(wildcard optional.h),-include optional.h)
  
  – Matt
  Mar 26 at 3:49
  

  
  
  
  

add a comment | 

1

1

I have two header files, one that is a library with a number of default macros defined
i.e:

libraryheader.h:

#define ANAME 1
#define BNAME 2
.
.
.

and I want to be able to have another optional header that the user can supply to redefine these macros in the project

optional.h:

#define ANAME 5
#define BNAME 15
.
.
.

The header optional.h doesn't always exist and I only want to include it if it does exist. Is there a way in the makefile or library header to check if this header exists and redefine the macros.

I'm thinking to check if the file exists in the makefile and set a flag by putting the following in the makefile

ifdef $(test -f optional.h && echo "present")

and then somehow check the flag macro in the library header.
But I haven't had any success yet. Thank you

c makefile header preprocessor

share|improve this question

asked Mar 26 at 3:21

rotalcomorotalcomo

3144 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Well, you could have your makefile update the compile flags to define HAS_OPTIONAL_H macro, and then you can wrap your #include line in a #ifdef.
  
  – paddy
  Mar 26 at 3:46
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  myprog:CPPFLAGS += $(if $(wildcard optional.h),-include optional.h)
  
  – Matt
  Mar 26 at 3:49
  

  
  
  
  

add a comment | 

I have two header files, one that is a library with a number of default macros defined
i.e:

libraryheader.h:

#define ANAME 1
#define BNAME 2
.
.
.

and I want to be able to have another optional header that the user can supply to redefine these macros in the project

optional.h:

#define ANAME 5
#define BNAME 15
.
.
.

The header optional.h doesn't always exist and I only want to include it if it does exist. Is there a way in the makefile or library header to check if this header exists and redefine the macros.

I'm thinking to check if the file exists in the makefile and set a flag by putting the following in the makefile

ifdef $(test -f optional.h && echo "present")

and then somehow check the flag macro in the library header.
But I haven't had any success yet. Thank you

c makefile header preprocessor

share|improve this question

asked Mar 26 at 3:21

rotalcomorotalcomo

3144 bronze badges

#define ANAME 1
#define BNAME 2
.
.
.

#define ANAME 5
#define BNAME 15
.
.
.

ifdef $(test -f optional.h && echo "present")

share|improve this question

asked Mar 26 at 3:21

rotalcomorotalcomo

3144 bronze badges

share|improve this question

asked Mar 26 at 3:21

rotalcomorotalcomo

3144 bronze badges

asked Mar 26 at 3:21

rotalcomorotalcomo

3144 bronze badges

asked Mar 26 at 3:21

rotalcomorotalcomo

3144 bronze badges

1 Answer
1

active

oldest

votes

1

As @Matt mentioned in comment "wildcard" can be used.

However, when I tested it using a small program I had to do the following things.

If I include the "optional.h" in my .c file always, then it will give build error when the .h file is not present. Similarly, if I dont include, even if the file is present I will not be able to update the macros. So I did the following in my makefile.

CFLAGS += $(if $(wildcard ./optional.h),-include ./optional.h -DINCLUDE_OPTIONAL_H)

In my .c file I used INCLUDE_OPTIONAL_H for including the optional.h as below.

#ifdef INCLUDE_OPTIONAL_H
#include "optional.h"
#endif

In optional.h, I checked if a macro is already defined and undefined it and then updated it.

#ifdef ANAME
#undef ANAME
#endif
#define ANAME <new_value>

share|improve this answer

answered Mar 26 at 4:23

MayurKMayurK

1,43188 silver badges1919 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks! I wasn't checking the INCLUDE_OPTIONAL_H flag in the right spot.
  
  – rotalcomo
  Mar 26 at 4:42
  

  
  
  
  

add a comment | 

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1

As @Matt mentioned in comment "wildcard" can be used.

However, when I tested it using a small program I had to do the following things.

If I include the "optional.h" in my .c file always, then it will give build error when the .h file is not present. Similarly, if I dont include, even if the file is present I will not be able to update the macros. So I did the following in my makefile.

CFLAGS += $(if $(wildcard ./optional.h),-include ./optional.h -DINCLUDE_OPTIONAL_H)

In my .c file I used INCLUDE_OPTIONAL_H for including the optional.h as below.

#ifdef INCLUDE_OPTIONAL_H
#include "optional.h"
#endif

In optional.h, I checked if a macro is already defined and undefined it and then updated it.

#ifdef ANAME
#undef ANAME
#endif
#define ANAME <new_value>

share|improve this answer

answered Mar 26 at 4:23

MayurKMayurK

1,43188 silver badges1919 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks! I wasn't checking the INCLUDE_OPTIONAL_H flag in the right spot.
  
  – rotalcomo
  Mar 26 at 4:42
  

  
  
  
  

add a comment | 

1

As @Matt mentioned in comment "wildcard" can be used.

However, when I tested it using a small program I had to do the following things.

If I include the "optional.h" in my .c file always, then it will give build error when the .h file is not present. Similarly, if I dont include, even if the file is present I will not be able to update the macros. So I did the following in my makefile.

CFLAGS += $(if $(wildcard ./optional.h),-include ./optional.h -DINCLUDE_OPTIONAL_H)

In my .c file I used INCLUDE_OPTIONAL_H for including the optional.h as below.

#ifdef INCLUDE_OPTIONAL_H
#include "optional.h"
#endif

In optional.h, I checked if a macro is already defined and undefined it and then updated it.

#ifdef ANAME
#undef ANAME
#endif
#define ANAME <new_value>

share|improve this answer

answered Mar 26 at 4:23

MayurKMayurK

1,43188 silver badges1919 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks! I wasn't checking the INCLUDE_OPTIONAL_H flag in the right spot.
  
  – rotalcomo
  Mar 26 at 4:42
  

  
  
  
  

add a comment | 

As @Matt mentioned in comment "wildcard" can be used.

However, when I tested it using a small program I had to do the following things.

If I include the "optional.h" in my .c file always, then it will give build error when the .h file is not present. Similarly, if I dont include, even if the file is present I will not be able to update the macros. So I did the following in my makefile.

CFLAGS += $(if $(wildcard ./optional.h),-include ./optional.h -DINCLUDE_OPTIONAL_H)

In my .c file I used INCLUDE_OPTIONAL_H for including the optional.h as below.

#ifdef INCLUDE_OPTIONAL_H
#include "optional.h"
#endif

In optional.h, I checked if a macro is already defined and undefined it and then updated it.

#ifdef ANAME
#undef ANAME
#endif
#define ANAME <new_value>

share|improve this answer

answered Mar 26 at 4:23

MayurKMayurK

1,43188 silver badges1919 bronze badges

CFLAGS += $(if $(wildcard ./optional.h),-include ./optional.h -DINCLUDE_OPTIONAL_H)

#ifdef INCLUDE_OPTIONAL_H
#include "optional.h"
#endif

#ifdef ANAME
#undef ANAME
#endif
#define ANAME <new_value>

share|improve this answer

answered Mar 26 at 4:23

MayurKMayurK

1,43188 silver badges1919 bronze badges

answered Mar 26 at 4:23

MayurKMayurK

1,43188 silver badges1919 bronze badges

answered Mar 26 at 4:23

MayurKMayurK

1,43188 silver badges1919 bronze badges