Some special-purpose macros

1. Block unused parameters warning

Such a macro was used in Ob_define.h in oceanbase to force the incoming data into void type.

   #ifndef UNUSED    #define UNUSED (v) ((void) (v))    #endif

Such a macro is primarily intended to mask the "unused parameters" warning, as the following function uses two parameters. However, some compilers will report a warning when none is used: they feel that P and mod_id are unused.

All of us use UNUSE macros so that two of the parameters are used, but we don't actually do anything effective to block the "unused parameters" warning.

virtual void Mod_free (void* p, const int32_t mod_id = 0) {     UNUSED (p);     UNUSED (mod_id);  }

2. Specify the byte alignment

In the default case. The compiler allocates space for each variable or data unit according to its natural boundary conditions. In general. The default boundary conditions can be changed by the following methods:
1) Use pseudo #pragma pack (n) -directives. The compiler will align to n bytes.

2) use pseudo #pragma pack () -directives to cancel your own definition of byte alignment.

In addition, there is one way to do this:
3) __attribute__((aligned (n))) . Aligns the members of the structure to the N-byte natural boundary. Assume that there are members in the structure that are longer than N. Is aligned according to the length of the maximum member.

4) __attribute__ ((packed)) , the optimization of the structure in the compilation process alignment, according to the actual number of bytes to align.

Above n = 1, 2, 4, 8, 16 ... The first approach is more common.

#define Cache_aligned __attribute__ ((ALIGNED (cache_align_size))

3. variable parameter Macros

We specify that some functions have variable parameters, such as the printf function. Specifies that the first is a formatted string after. The following can be the same as the variable number of parameters. A macro can also specify more than one number of parameters. There are two ways to do this.
One is args... to use and ##args , two is to use ... and __VA_ARGS__ . The following are two macros defined in two ways. Both macros can be in front of the printf string. Output a string first. Achieve the same effect.

#define P_DEBUG (Format,args ...) {    printf ("[DeBug]");    printf (format,# #args);} while (false) #define P_RELEASE (format,...) \do{    printf ("[Release]");    printf (format,__va_args__);} while (false)

When used as:

P_debug ("%s:%d\n", "good", 100); P_release ("%s:%d\n", "good", 100);

4. #， ##, #@The difference

If you define a 3 macro,

#define A (x) t_# #x # define B (x) #@x#define C (x) #x

The following expands to
A (1)------>t_1
B (1)------> ' 1 '
C (1)------> "1"

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Some special-purpose macros