Introduction: Computer language is the basis and specification for compiler and programmer to communicate. GNU C is a special function of GCC and widely used in Linux kernel.
Help: gcc.gnu.org/onlinedocs/…
The keyword typeof is used to get the data typeof the expression.
A simple example looks like Listing 1:
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- char *chptr01;
- typeof (*chptr01) ch; // equivalent to char ch;
- typeof (ch) *chptr02; Char *chptr02;
- typeof (chptr01) chparray[5]; // equivalent to char *chparray[5];
In this example, the data type of CHpTR01 is char *, and that of *chptr01 is char.
A complex example, like Listing 2:
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- #include <stdio.h>
- #define array(type, size) typeof(type [size])
- int func(int num)
- {
- return num + 5;
- }
- int main(void)
- {
- typeof (func) *pfunc = func; Int (*pfunc)(int) = func;
- printf(“pfunc(10) = %d\n”, (*pfunc)(10));
- array(char, ) charray = “hello world!” ; Char charray[] = “Hello world!” ;
- typeof (char *) charptr = charray; Char *charptr = charray;
- printf(“%s\n”, charptr);
- return 0;
- }
Example output:
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- pfunc(10) = 15
- hello world!
If typeof’s operand is a data type, its result is that data type, as in line 16.
In the Linux kernel, as shown in Listing 3:
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- / * Linux – 2.6.38.8 / include/Linux/kernel h * /
- #define min(x, y) ({ \
- typeof(x) _min1 = (x); \
- typeof(y) _min2 = (y); \
- (void) (&_min1 == &_min2); \
- _min1 < _min2 ? _min1 : _min2; })
- #define max(x, y) ({ \
- typeof(x) _max1 = (x); \
- typeof(y) _max2 = (y); \
- (void) (&_max1 == &_max2); \
- _max1 > _max2 ? _max1 : _max2; })
Get the data types of x and y from Typeof, then define two temporary variables and assign the values of x and y to these two temporary variables, and finally compare.
In addition, the (void)(&_min1 == &_min2) statement in the macro definition is used to warn that x and y cannot belong to different data types.