c/c++函数接口

/* populator.h */
#ifndef POPULATOR_H
#define POPULATOR_H

typedef struct {
     int number;
     char msg[20];
} unit_t;

extern void set_unit(unit_t *);

#endif

/* populator.c */
#include <string.h>
#include "populator.h"

void set_unit(unit_t *p)
{
     if (p == NULL)
          return; /* ignore NULL parameter */
     p->number = 3;
     strcpy(p->msg, "Hello World!");
}

/* main.c */
#include <stdio.h>
#include "populator.h"

int main(void)
{
     unit_t u;

     set_unit(&u);
     printf("number: %d\nmsg: %s\n", u.number, u.msg);
     return 0;
}

/* redirect_ptr.h */
#ifndef REDIRECT_PTR_H
#define REDIRECT_PTR_H

extern void get_a_day(const char **);

#endif

想一想，这里的参数指针是const char **，有const限定符，却不是传入参数而是传出参数，为什么？如果是传入参数应该怎么表示？

/* redirect_ptr.c */
#include "redirect_ptr.h"

static const char *msg[] = {"Sunday", "Monday", "Tuesday", "Wednesday",
			"Thursday", "Friday", "Saturday"};
void get_a_day(const char **pp)
{
     static int i = 0;
     *pp = msg[i%7];
     i++;
}

/* main.c */
#include <stdio.h>
#include "redirect_ptr.h"

int main(void)
{
     const char *firstday = NULL;
     const char *secondday = NULL;
     get_a_day(&firstday);
     get_a_day(&secondday);
     printf("%s\t%s\n", firstday, secondday);
     return 0;
}

/* para_allocator.h */
#ifndef PARA_ALLOCATOR_H
#define PARA_ALLOCATOR_H

typedef struct {
     int number;
     char *msg;
} unit_t;

extern void alloc_unit(unit_t **);
extern void free_unit(unit_t *);

#endif

/* para_allocator.c */
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "para_allocator.h"

void alloc_unit(unit_t **pp)
{
     unit_t *p = malloc(sizeof(unit_t));
     if(p == NULL) {
	  printf("out of memory\n");
	  exit(1);
     }
     p->number = 3;
     p->msg = malloc(20);
     strcpy(p->msg, "Hello World!");
     *pp = p;
}

void free_unit(unit_t *p)
{
     free(p->msg);
     free(p);
}

/* main.c */
#include <stdio.h>
#include "para_allocator.h"

int main(void)
{
     unit_t *p = NULL;

     alloc_unit(&p);
     printf("number: %d\nmsg: %s\n", p->number, p->msg);
     free_unit(p);
     p = NULL;
     return 0;
}

/* ret_ptr.h */
#ifndef RET_PTR_H
#define RET_PTR_H

extern char *get_a_day(int idx);

#endif

/* ret_ptr.c */
#include <string.h>
#include "ret_ptr.h"

static const char *msg[] = {"Sunday", "Monday", "Tuesday", "Wednesday",
			"Thursday", "Friday", "Saturday"};

char *get_a_day(int idx)
{
     static char buf[20];
     strcpy(buf, msg[idx]);
     return buf;
}

/* main.c */
#include <stdio.h>
#include "ret_ptr.h"

int main(void)
{
     printf("%s %s\n", get_a_day(0), get_a_day(1));
     return 0;
}

/* ret_allocator.h */
#ifndef RET_ALLOCATOR_H
#define RET_ALLOCATOR_H

typedef struct {
     int number;
     char *msg;
} unit_t;

extern unit_t *alloc_unit(void);
extern void free_unit(unit_t *);

#endif

/* ret_allocator.c */
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "ret_allocator.h"

unit_t *alloc_unit(void)
{
     unit_t *p = malloc(sizeof(unit_t));
     if(p == NULL) {
	  printf("out of memory\n");
	  exit(1);
     }
     p->number = 3;
     p->msg = malloc(20);
     strcpy(p->msg, "Hello world!");
     return p;
}

void free_unit(unit_t *p)
{
     free(p->msg);
     free(p);
}

/* main.c */
#include <stdio.h>
#include "ret_allocator.h"

int main(void)
{
     unit_t *p = alloc_unit();

     printf("number: %d\nmsg: %s\n", p->number, p->msg);
     free_unit(p);
     p = NULL;
     return 0;
}

/* para_callback.h */
#ifndef PARA_CALLBACK_H
#define PARA_CALLBACK_H

typedef void (*callback_t)(void *);
extern void repeat_three_times(callback_t, void *);

#endif

/* para_callback.c */
#include "para_callback.h"

void repeat_three_times(callback_t f, void *para)
{
     f(para);
     f(para);
     f(para);
}

/* main.c */
#include <stdio.h>
#include "para_callback.h"

void say_hello(void *str)
{
     printf("Hello %s\n", (const char *)str);
}

void count_numbers(void *num)
{
     int i;
     for(i=1; i<=(int)num; i++)
	  printf("%d ", i);
     putchar('\n');
}

int main(void)
{
     repeat_three_times(say_hello, "Guys");
     repeat_three_times(count_numbers, (void *)4);
     return 0;
}

/* generics.h */
#ifndef GENERICS_H
#define GENERICS_H

typedef int (*cmp_t)(void *, void *);
extern void *max(void *data[], int num, cmp_t cmp);

#endif

/* generics.c */
#include "generics.h"

void *max(void *data[], int num, cmp_t cmp)
{
     int i;
     void *temp = data[0];
     for(i=1; i<num; i++) {
	  if(cmp(temp, data[i])<0)
	       temp = data[i];
     }
     return temp;
}

/* main.c */
#include <stdio.h>
#include "generics.h"

typedef struct {
     const char *name;
     int score;
} student_t;

int cmp_student(void *a, void *b)
{
     if(((student_t *)a)->score > ((student_t *)b)->score)
	  return 1;
     else if(((student_t *)a)->score == ((student_t *)b)->score)
	  return 0;
     else
	  return -1;
}

int main(void)
{
     student_t list[4] = {{"Tom", 68}, {"Jerry", 72},
		       {"Moby", 60}, {"Kirby", 89}};
     student_t *plist[4] = {&list[0], &list[1], &list[2], &list[3]};
     student_t *pmax = max((void **)plist, 4, cmp_student);
     printf("%s gets the highest score %d\n", pmax->name, pmax->score);

     return 0;
}

#include <stdio.h>
#include <stdarg.h>

void myprintf(const char *format, ...)
{
     va_list ap;
     char c;

     va_start(ap, format);
     while (c = *format++) {
	  switch(c) {
	  case 'c': {
	       /* char is promoted to int when passed through '...' */
	       char ch = va_arg(ap, int);
	       putchar(ch);
	       break;
	  }
	  case 's': {
	       char *p = va_arg(ap, char *);
	       fputs(p, stdout);
	       break;
	  }
	  default:
	       putchar(c);
	  }
     }
     va_end(ap);
}

int main(void)
{
     myprintf("c\ts\n", '1', "hello");
     return 0;
}

/* stdarg.h standard header */
#ifndef _STDARG
#define _STDARG

/* type definitions */
typedef char *va_list;
/* macros */
#define va_arg(ap, T) \
	(* (T *)(((ap) += _Bnd(T, 3U)) - _Bnd(T, 3U)))
#define va_end(ap) (void)0
#define va_start(ap, A) \
	(void)((ap) = (char *)&(A) + _Bnd(A, 3U))
#define _Bnd(X, bnd) (sizeof (X) + (bnd) & ~(bnd))
#endif

#include <stdio.h>
#include <stdarg.h>

void printlist(int begin, ...)
{
     va_list ap;
     char *p;

     va_start(ap, begin);
     p = va_arg(ap, char *);

     while (p != NULL) {
	  fputs(p, stdout);
	  putchar('\n');
	  p = va_arg(ap, char*);
     }
     va_end(ap);
}

int main(void)
{
     printlist(0, "hello", "world", "foo", "bar", NULL);
     return 0;
}