What is the purpose of a these #define within an e

问题:

I found this code in the linux headers, /usr/include/dirent.h:

enum   
  {    
    DT_UNKNOWN = 0,
# define DT_UNKNOWN DT_UNKNOWN
    DT_FIFO = 1,
# define DT_FIFO    DT_FIFO   
    DT_CHR = 2,
# define DT_CHR     DT_CHR
    DT_DIR = 4,
# define DT_DIR     DT_DIR
    DT_BLK = 6,
# define DT_BLK     DT_BLK
    DT_REG = 8,
# define DT_REG     DT_REG
    DT_LNK = 10,
# define DT_LNK     DT_LNK
    DT_SOCK = 12,
# define DT_SOCK    DT_SOCK   
    DT_WHT = 14
# define DT_WHT     DT_WHT
  };   

What is this construct for? - why define something with an identical string, which will then compile to the int value?

回答1:

My guess is that some other code can then check if one of (or more of) these enum values have been defined using #ifdef.

回答2:

Besides the other answers which are very good - I would go with them for the main reason - the compiler could generate warnings or errors if you try to redefine DT_UNKNOWN.

回答3:

My (uneducated) guess is that the #define statements allow conditional tests to see if the constant has been defined.

For example:

#ifdef DT_UNKNOWN
    // do something
#endif

回答4:

I think that Luchian Grigore's answer was correct.

Code without defines:

#include <stdio.h>

// Defined somewhere in headers
#define DT_UNKNOWN 0

enum
{
    DT_UNKNOWN = 0,
    DT_FIFO = 1,
};

int main(int argc, char **argv)
{
    printf("DT_UNKNOWN is %d\n", DT_UNKNOWN);
    return 0;
}

From the compiler's output it is unclear why some line of code inside enum doesn't want to build:

alexander@ubuntu-10:~/tmp$ gcc -Wall ./main.c 
./main.c:7: error: expected identifier before numeric constant

After we add such defines:

#include <stdio.h>

// Defined somewhere in headers
#define DT_UNKNOWN 0

enum
{
    DT_UNKNOWN = 0,
    # define DT_UNKNOWN DT_UNKNOWN
    DT_FIFO = 1,
    # define DT_FIFO    DT_FIFO
};

int main(int argc, char **argv)
{
    printf("DT_UNKNOWN is %d\n", DT_UNKNOWN);
    return 0;
}

Compiler would tell us that DT_UNKNOWN is redefined and a place where it is redefined:

alexander@ubuntu-10:~/tmp$ gcc -Wall ./main2.c 
./main2.c:7: error: expected identifier before numeric constant
./main2.c:8:1: warning: "DT_UNKNOWN" redefined
./main2.c:3:1: warning: this is the location of the previous definition

回答5:

I used -E and -dD arguments (and also -fdump-tree-all) in gcc to see the preprocessor output and found nothing useful. So I guess this code does not have any functionality other than perhaps displaying the symbolic names when debugging using a debugger like gdb.