C小技巧

comment.c

/*
 * 注释嵌套测试
 */
#include <stdio.h>

int main(int argc, char **argv)
{
    int i;
    i = /*/*/0*/**/1;

    printf("%d: %s\n", i, i? "支持嵌套注释": "不支持嵌套注释");
    /*
     *	输出0表示不支持注释镶嵌。
     *	输出1表示支持注释镶嵌。
     */

    return 0;
}

data.c

/*
 * 函数法计算日期到天数！！
 */
#include <stdio.h>

int days(int y, int m, int d);

int main(int argc, char **argv)
{
    int i;
    for (i = 2; i < 13; i++)
        printf("%d\n", days(2015, i, 1)-days(2015, 1, 1));
    return 0;
}

/*
 * 已3-1为一年开始来推算的
 */
int days(int y, int m, int d)
{
    m = (m+9)%12;
    y = y - m/10;
    return 365*y+y/4-y/100+y/400+(m*306+5)/10+(d-1);
}

duffs-device.c

/*
 * 达夫设备
 * 一种探索循环展开的程序
 */
#include <stdio.h>
#include <string.h>

#define TO_MAX		20
#define FROM_MAX	20
#define COUNT_MAX	20

#define ARRAY_MAX	32

void send(int *to, int *from, int count);
void print(int *to, int *from, int array_max);

int main(int argc, char **argv)
{
	int to[ARRAY_MAX], from[ARRAY_MAX], count;
	int ito, ifrom, icount;

	for(ito = 1; ito < TO_MAX; ito ++)
		for(ifrom = 1; ifrom < FROM_MAX; ifrom ++)
			for(icount = 1; icount < COUNT_MAX; icount ++) {
				memset(to, 0x0, ARRAY_MAX * sizeof(int));
				memset(from, 0x0, ARRAY_MAX * sizeof(int));
				*to = ito;
				*from = ifrom;
				count = icount;
				printf("to:%d\tfrom:%d\tcount:%d\n", ito, ifrom, count);
				send(to, from, count);
				print(to, from, ARRAY_MAX);
				putchar('\n');
			}
	return 0;
}

void send(int *to, int *from, int count)
{
	int n = (count + 7) / 8;
	switch(count % 8) {
		case 0:	do { 	*to ++ = *from ++;
		case 7: 		*to ++ = *from ++;
		case 6: 		*to ++ = *from ++;
		case 5: 		*to ++ = *from ++;
		case 4: 		*to ++ = *from ++;
		case 3: 		*to ++ = *from ++;
		case 2: 		*to ++ = *from ++;
		case 1: 		*to ++ = *from ++;
			}while(-- n > 0);
	}
}

void print(int *to, int *from, int array_max)
{
	int i;
	for(i = 0; i < array_max; i ++)
		printf("%d ", to[i]);
	putchar('\n');
	for(i = 0; i < array_max; i ++)
		printf("%d ", from[i]);
	putchar('\n');
}

floor-sqrt.c

floor(sqrt(number)+0.5);
/* 避免浮点数误差 */

functions-returning-itself.c

#include <stdio.h>

typedef void fun_t(int a);
typedef fun_t* recursion_t(int a);

/* NOTE maybe it is not portable */
static fun_t *fun(int n)
{
    printf("run_fun: %d\n", n);
    return (fun_t*)fun;
}

static void run_fun(int cnt)
{
    recursion_t *f = (recursion_t*)fun;
    while (cnt--) {
        f = (recursion_t*)f(cnt+1);
    }
}

int main()
{
    printf("sizeof(void*):   %d\n", sizeof(void*));
    printf("sizeof(fun_t*):  %d\n", sizeof(fun_t*));
    printf("sizeof(fun):     %d\n", sizeof(fun));   /* it is undefine behavior */
    printf("sizeof(run_fun): %d\n", sizeof(run_fun));
    printf("sizeof(void (*)()): %d\n", sizeof(void (*)()));
    printf("org: %p ptr: %p\n", fun, fun(-1));
    int n;
    scanf("%d", &n);
    run_fun(n);
    return 0;
}

limits-float.md

limits.h和float.h

limits.h

• INT_MAX

float.h

• DBL_MAX

man-or-boy.c

/*
 * man or boy test
 */
#include <stdio.h>
#include <stdlib.h>

/* thunks */
typedef struct arg {
	int (*fn)(struct arg*);
	int *k;
	struct arg *x1, *x2, *x3, *x4, *x5;
}arg_t;

/* lambdas */
int f_1(arg_t* _) { return -1; }
int f0(arg_t* _) { return 0;  }
int f1(arg_t* _) { return 1;  }

/* helper */
/* TODO 预处理define语句的多参数?? */
int eval(arg_t *a) { return a->fn(a); }
#define MAKE_ARG(...) (&(arg_t){__VA_ARGS__})
#define FUN(...) MAKE_ARG(B, &k, __VA_ARGS__)

int A(arg_t*);

/* functions */
int B(arg_t *a)
{
	int k = *a->k -= 1;
	return A(FUN(a, a->x1, a->x2, a->x3, a->x4));
}

int A(arg_t *a)
{
	return *a->k <= 0 ? eval(a->x4)+eval(a->x5) : B(a);
}

int main(int argc, char **argv)
{
	int k = argc == 2? strtol(argv[1], 0, 0) : 10;
	printf("%d\n", A(FUN(MAKE_ARG(f1), MAKE_ARG(f_1), MAKE_ARG(f_1),
					MAKE_ARG(f1), MAKE_ARG(f0))));

	return 0;
}

setjmp.c

/* setjmp的妙用 */
jmp_buf env;
setjmp(env); // 获取机器寄存器的值

void *stack_start;
int foo = 42;
stack_start = &foo;
/* do some */
/* ... */

void *stack_end;
int foo2 = 0;
stack_end = &foo2;  /* 这样可以获取栈的使用大小 */

shuffle.c

int cmp(void const *a, void const *b)
{
    return rand()-(RAND_MAX/2);
}
qsort(nums, nele, elesize, cmp);
/* 虽然效率受到影响 */

stack-grow.c

/* 在基于数组的栈时有如下技巧 */
typedef struct {
  size_t top;
  size_t elesize;
  void *mem;
}stack_t;

stack_t stk;
stk.top = 0;  /* 初始为0表示没有元素 */
/* 核心的一句，二进制数在增长时只有达到1111型时下一步才会达到10000型的界 */
if ((stk.top+1) ^ (stk.top) > stk.top)) {
  stk.mem = realloc(stk.mem, ((stk.top<<1)|0x1) * sizeof(stk.elesize));
}
stk.meem[stk.top++] = ele;
/* 每次push元素时使用上面一句来检测 */
/* NOTE 也就是 (num+1)^num > num 时num由1111型变为10000的关键！ */

switch.c

#if 0 /* C语言里不能实现。。。在javascript里可以！ */
switch (1) {
case (strcmp(name, "name1") && age == 21):
    do_name1_21();
break;
case (strcmp(name, "name2") && age == 21):
    do_name2_21();
braek;
    /* .... */
default:
break;
}
#endif
/* 实现 */
if (strcmp(name, "name1") && age == 21))
  do_name1_21();
else if (strcmp(name, "name2") && age == 21)
  do_name1_21();
else
  do_other();

testing-drive-and-macro.c

static int main_ret = 0;
static int test_count = 0;
static int test_pass = 0;

/* NOTE 也许可以加入 #name 来 stringze 名字，以便显示表达式 */

/* 相等测试 */
#define EXPECT_EQ_BASE(equality, expect, actual, format) \
    do { \
        ++test_count; \
        if (equality) ++test_pass; \
        else { \
            fprintf(stderr, "%s: %d: expect: "format"\tactual: "format"\n", \
                    __FILE__, __LINE__, expect, actual); \
            main_ret = 1; \
        } \
    } while (0)
/* 值近似测试 */
#define EXPECT_SIM_BASE(expect, deviation, actual, format) \
    do { \
        ++test_count; \
        if ((expect)-(deviation) <= (actual) && (actual) <= (expect)+(deviation)) { \
            ++test_pass; \
        } else { \
            fprintf(stderr, "%s: %d: expect: ["format ", "format"]\tactual: "format"\n",\
                    __FILE__, __LINE__, (expect)-(deviation), (expect)+(deviation), actual); \
            main_ret = 1; \
        } \
    } while (0)
#define TEST_EQ_INT(a, b)   EXPECT_EQ_BASE((a)==(b), (a), (b), "%d")
#define TEST_NEQ_INT(e, a)  EXPECT_EQ_BASE((e)!=(a), "NOT EQ", "EQ", "%s")
#define TEST_AS(asval, act) EXPECT_SIM_BASE(asval, 1, act, "%d")
#define TEST_TRUE(e)        EXPECT_EQ_BASE((e), 1, (e), "%d")
#define TEST_FALSE(e)       EXPECT_EQ_BASE(!(e), 0, (e), "%d")
#define TEST_NOTNULL(p)     EXPECT_EQ_BASE(p, "NOTNULL", "NULL", "%s")
#define TEST_ISNULL(p)      EXPECT_EQ_BASE(!p, "NULL", "NOTNULL", "%s")

variable_declare.c

#include <stdio.h>

typedef void (declare_f)(int a);
/*上述是不同于 typedef void (*function_t)(int a); */

#define DECLARE_F(name) void name(int a)

/* 声明 */
declare_f f1;
DECLARE_F(f2);

declare_f *f3;  /* a variable of pointing to a function. */

void f1(int a)
{
    printf("%d\n", a);
}
void f2(int b)
{
    printf("f2: %d\n", b);
}

int main()
{
    f1(10);
    f2(20);
    f3 = f2;
    f3(30);
    return 0;
}