What is a good reference documenting patterns of use of X-Macros in C (or possibly C++)?

Question

A basic definition and example and a few references for "X-Macros" is given in this wikipedia entry on the C pre-processor:

An X-Macro is a header file (commonly using a ".def" extension instead of the traditional ".h") that contains a list of similar macro calls (which can be referred to as "component macros").

What are some good sources of information on how to use this powerful technique? Are there well-known open source libraries using this method?

Answer 1

I use X Macros() in code a lot. The value comes from only adding new data only to the "X list" and not modifying any other code.

The most common use of X Macros() is for associating error text with error codes. When new error codes are added, programmers must remember to add the code and the text, typically in separate places. The X Macro allows the new error data to be added in a single place and get automatically populated anywhere it is needed.

Unfortunately, the mechanisms use a lot of pre-compiler magic that can make the code somewhat hard to read (e.g. string joining with token1##token2, string creation with #token). Because of this I typically explain what the X Macro is doing in the comments.

Here is an example using the error/return values. All new data gets added to the "X_ERROR" list. None of the other code hast to be modified.

/* 
 * X Macro() data list
 * Format: Enum, Value, Text
 */
#define X_ERROR \
  X(ERROR_NONE,   1, "Success") \
  X(ERROR_SYNTAX, 5, "Invalid syntax") \
  X(ERROR_RANGE,  8, "Out of range")

/* 
 * Build an array of error return values
 *   e.g. {0,5,8}
 */
static int ErrorVal[] =
{
  #define X(Enum,Val,Text)     Val,
   X_ERROR
  #undef X
};

/* 
 * Build an array of error enum names
 *   e.g. {"ERROR_NONE","ERROR_SYNTAX","ERROR_RANGE"}
 */

static char * ErrorEnum[] = {
  #define X(Enum,Val,Text)     #Enum,
   X_ERROR
  #undef X
};

/* 
 * Build an array of error strings
 *   e.g. {"Success","Invalid syntax","Out of range"}
 */
static char * ErrorText[] = {
  #define X(Enum,Val,Text)     Text,
   X_ERROR
  #undef X
};

/* 
 * Create an enumerated list of error indexes
 *   e.g. 0,1,2
 */
enum {
  #define X(Enum,Val,Text)     IDX_##Enum,
   X_ERROR
  #undef X
  IDX_MAX   /* Array size */
};

void showErrorInfo(void)
{
    int i;

    /* 
     * Access the values
     */
    for (i=0; i<IDX_MAX; i++)
        printf(" %s == %d [%s]\n", ErrorEnum[i], ErrorVal[i], ErrorText[i]);

}

You can also use X Macros() to generate code. For example to test if an error value is "known", the X Macro can generate cases in a switch statement:

 /*
  * Test validity of an error value
  *      case ERROR_SUCCESS:
  *      case ERROR_SYNTAX:
  *      case ERROR_RANGE:
  */

  switch(value)
  {

  #define X(Enum,Val,Text)     case Val:
   X_ERROR
  #undef X
         printf("Error %d is ok\n",value);
         break;
      default:
         printf("Invalid error: %d\n",value);
         break;
  }

Answer 2

I discovered X-macros a couple of years ago when I started making use of function pointers in my code. I am an embedded programmer and I use state machines frequently. Often I would write code like this:

/* declare an enumeration of state codes */
enum{ STATE0, STATE1, STATE2, ... , STATEX, NUM_STATES};

/* declare a table of function pointers */
p_func_t jumptable[NUM_STATES] = {func0, func1, func2, ... , funcX};

The problem was that I considered it very error prone to have to maintain the ordering of my function pointer table such that it matched the ordering of my enumeration of states.

A friend of mine introduced me to X-macros and it was like a light-bulb went off in my head. Seriously, where have you been all my life x-macros!

So now I define the following table:

#define STATE_TABLE \
        ENTRY(STATE0, func0) \
        ENTRY(STATE1, func1) \
        ENTRY(STATE2, func2) \
        ...
        ENTRY(STATEX, funcX) \

And I can use it as follows:

enum
{
#define ENTRY(a,b) a,
    STATE_TABLE
#undef ENTRY
    NUM_STATES
};

and

p_func_t jumptable[NUM_STATES] =
{
#define ENTRY(a,b) b,
    STATE_TABLE
#undef ENTRY
};

as a bonus, I can also have the pre-processor build my function prototypes as follows:

#define ENTRY(a,b) static void b(void);
    STATE_TABLE
#undef ENTRY

Another usage is to declare and initialize registers

#define IO_ADDRESS_OFFSET (0x8000)
#define REGISTER_TABLE\
    ENTRY(reg0, IO_ADDRESS_OFFSET + 0, 0x11)\
    ENTRY(reg1, IO_ADDRESS_OFFSET + 1, 0x55)\
    ENTRY(reg2, IO_ADDRESS_OFFSET + 2, 0x1b)\
    ...
    ENTRY(regX, IO_ADDRESS_OFFSET + X, 0x33)\

/* declare the registers (where _at_ is a compiler specific directive) */
#define ENTRY(a, b, c) volatile uint8_t a _at_ b:
    REGISTER_TABLE
#undef ENTRY

/* initialize registers */
#def ENTRY(a, b, c) a = c;
    REGISTER_TABLE
#undef ENTRY

My favourite usage however is when it comes to communication handlers

First I create a comms table, containing each command name and code:

#define COMMAND_TABLE \
    ENTRY(RESERVED,    reserved,    0x00) \
    ENTRY(COMMAND1,    command1,    0x01) \
    ENTRY(COMMAND2,    command2,    0x02) \
    ...
    ENTRY(COMMANDX,    commandX,    0x0X) \

I have both the uppercase and lowercase names in the table, because the upper case will be used for enums and the lowercase for function names.

Then I also define structs for each command to define what each command looks like:

typedef struct {...}command1_cmd_t;
typedef struct {...}command2_cmd_t;

etc.

Likewise I define structs for each command response:

typedef struct {...}response1_resp_t;
typedef struct {...}response2_resp_t;

etc.

Then I can define my command code enumeration:

enum
{
#define ENTRY(a,b,c) a##_CMD = c,
    COMMAND_TABLE
#undef ENTRY
};

I can define my command length enumeration:

enum
{
#define ENTRY(a,b,c) a##_CMD_LENGTH = sizeof(b##_cmd_t);
    COMMAND_TABLE
#undef ENTRY
};

I can define my response length enumeration:

enum
{
#define ENTRY(a,b,c) a##_RESP_LENGTH = sizeof(b##_resp_t);
    COMMAND_TABLE
#undef ENTRY
};

I can determine how many commands there are as follows:

typedef struct
{
#define ENTRY(a,b,c) uint8_t b;
    COMMAND_TABLE
#undef ENTRY
} offset_struct_t;

#define NUMBER_OF_COMMANDS sizeof(offset_struct_t)

NOTE: I never actually instantiate the offset_struct_t, I just use it as a way for the compiler to generator for me my number of commands.

Note then I can generate my table of function pointers as follows:

p_func_t jump_table[NUMBER_OF_COMMANDS] = 
{
#define ENTRY(a,b,c) process_##b,
    COMMAND_TABLE
#undef ENTRY
}

And my function prototypes:

#define ENTRY(a,b,c) void process_##b(void);
    COMMAND_TABLE
#undef ENTRY

Now lastly for the coolest use ever, I can have the compiler calculate how big my transmit buffer should be.

/* reminder the sizeof a union is the size of its largest member */
typedef union
{
#define ENTRY(a,b,c) uint8_t b##_buf[sizeof(b##_cmd_t)];
    COMMAND_TABLE
#undef ENTRY
}tx_buf_t

Again this union is like my offset struct, it is not instantiated, instead I can use the sizeof operator to declare my transmit buffer size.

uint8_t tx_buf[sizeof(tx_buf_t)];

Now my transmit buffer tx_buf is the optimal size and as I add commands to this comms handler, my buffer will always be the optimal size. Cool!