C language Macro definition #define Usage summary.

1. Simple macro definition
#define Identifier substitution list (alternate list can be number, string literal, punctuation, operator, identifier, keyword, Word constants quantity.) Note: The substitution list is nullable.
Typical error:
#define N = 100
int a[n]; /* This will become an int a[= 100], which will be processed as an identification mark.


#define N 100;
int a[n]; /* The definition of a semicolon will become an int a[100;] This is a very common error.

#define PIN (int*);
Pin a,b;
int* a,b; /* The intention is both A and B are int pointers, but in fact it becomes int* a,b;a is an int pointer, and b is an int variable. This should use a typedef instead of define so that A and B are all int pointers. */

Typical ways to use:
Using a macro definition we can define or even change the grammar habits of C language according to our own habits, for example:
#define BEGIN {
#define END}
int main () BEGIN
printf ("DEFINE----\ n");
End

Define a loop
#define Loop for (;;)

Redefining data types
#define IT INT


2. Macros with parameters
#define Identifier (X1,X2,X3...XN) replaces the list (note: x1,x2,x3.) is a macro parameter with no spaces between the identifier and its left bracket
How to use:
#define MAX (X,y) (x) > (y)? ( x):(y))
I=max (J+K,M-N);
To be replaced by:
I=max (J+k) > (m-n)? ( J+K):(m-n));
#define SI_EX (N) ((n)%2==0)
if (SI_EX (i)) i++;
To be replaced by:
if (SI_EX ((i)%2==0))
We can see from the example above that the identifier takes a parameter (x1,x2,x3 ...). will be replaced with the list (Y1,y2,y3 ...). ) corresponding substitution, but not in order.


3. Special Single-line definition of macros
#define A (x) t_# #x
#define B (x) #@x
#define C (x) #x
X=1
To be replaced by:
A (1)------> t_1 (t_# #x, # #符号是记号粘黏符号, paste the characters before and after. ）
B (1)------> ' 1 ' (#@x, #@ symbols will be literal quantization of the parameters of the macro, and add ' "number)
C (1)------> "1" (#x, #符号会将宏的参数进行字符串字面量化 and Plus "")


Multi-line definition for 4.define
Define can replace multiple lines of code, adding a "\" for each line change
#define MAX (x,y) do {\
Statement 1; \
Statement 2; \
* * The writing of the annotation * *
while (0)/* (no trailing;) * *


5. In the large-scale development process, especially cross-platform and the system software, define most important function is the conditional compilation.
#ifdef WINDOWS
......
......
#endif
#ifdef LINUX
......
......
#endif
You can set up the compilation environment by #define at compile time
6. Cancel Macros
#undef identifier


7. Conditional compilation
#ifdef XXX ... (#else) ... #endif


8. Predefined macros
A number of useful macros are included in the C language that provide the current compilation information.
__line__ the number of rows of the compiled file (integer)
__FILE__ the name of the compiled file (character type)
__date__ Compile date (character type)
__time__ compile time (character type)
__stdc__ if the compiler accepts standard C, then the value is 1. (Integral type)


Usually when we compile, we can use these predefined macros for scheduling errors, such as:
In general, when the compiler is not prompted when there is zero, we can customize a workaround.
#define Checkzero (divisor) \
if (divisor==0) \
printf ("***attempt to DIVIDE by ZERO in line%d of file%s ***\n", __line__,__file__); \


Reference method
Checkzero (j);
k=i/j;