CODEWITHSUNDEEP
cincludec-preprocessorkernighan-and-ritchie
CS Student
CS Student

Reputation: 1643

C pre-processor error

I expected the code to output suse.sys but it actually prints win.sys. Why is this the case?

 #define SYS SUSE

 #if SYS == WIN
   #define HDR "win.sys"
 #elif SYS == SUSE
   #define HDR "suse.sys"
 #else
   #define HDR "default.sys"
 #endif
 #include HDR
 #include <stdio.h>

 int main()
 {
   char *name = HDR;
   printf("%s\n", name);
   return 0;
 }

This is similar to the example in the C Programming language second edition. The .sys files don't contain anything, they have no real use.

Upvotes: 0

Views: 68

Answers (1)

Arkku
Arkku

Reputation: 42159

The preprocessor comparison with == works on integer values, not strings or names of macros. You should be able to fix this by first defining the macros SUSE and WIN with integer values, e.g.,

#define SUSE 1
#define WIN 2
#define SYS SUSE

After this both SYS and SUSE resolve to the integer 1, and the comparison should work.

However, I would suggest a more conventional approach of defining different macros altogether for the systems, e.g.:

#define SYS_SUSE
//#define SYS_WIN

#if defined(SYS_SUSE)
#define HDR "suse.sys"
#elif defined(SYS_WIN)
#define HDR "win.sys"
#else
#define HDR "default.sys"
#endif

This approach has the advantage of being able to more conveniently specify the system on command-line, makefiles, etc. without depending on the numeric constants being defined in every context:

cc -DSYS_WIN -c foo.c

Upvotes: 3

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