C Essense (7) - Function interface

1. malloc and free

malloc is used to dynamiclly allocate memory by calling system call brk. After allocation, we should use free to release the memory, return the memory to malloc.

#include <stdlib.h>
void *malloc(size_t size);
void free  (void *ptr)


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct {
    int number;
    char *msg;
} unit_t;

int main(void)
    unit_t *p = malloc(sizeof(unit_t));
    if (p == NULL) {
        printf("out of memory\n");
    p->number = 3;
    p->msg = malloc(20);
    strcpy(p->msg, "Hello world!"); printf("number: %d\nmsg: %s\n", p->number, p->msg);
    p = NULL;
    return 0;

Several things to notice:

  • unit_t *p = malloc(sizeof(unit_t));, void * can be implicit converted to any other pointer type.
  • Although normally memory will not be exhausted, but after malloc, we should check whether it is successful, by checking whether the return value is NULL
  • After free( p ), the memory space of p is returned, but the value of p is not changed. Now p is a wild pointer(pointing to somewhere not exist). We should set p = NULL after free( p )
  • When calling free( p ), the program knows how large space has been allocated to p by malloc. It will free the part allocated to p.

2. pass-in parameter & pass-out parameter

void func(unit_t *p);

If *p points to somewhere exist, and func() will read data from the space, then *p is called pass-in parameter.

If *p points to nowhere, and func() will assign *p the address, then *p is called pass-out parameter.

If *p points to somewhere exist, and func() will read data and change data in that space, then *p is called value-result parameter.

3. double pointer


/* redirect_ptr.h */
extern void get_a_day(const char **); #endif

/* 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++;
/* main1.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;

Compile and Run

gcc main1.c redirect_ptr.c -o main

4. Callback function

void func(void (*f)(void *), void *p);

The first parameter of func is a function pointer f. f should take one parameter of type void *, and return a void * value. The second parameter is just a void * value.

Example: comparison function

/* generics.h */
#ifndef GENERICS_H
#define GENERICS_H
typedef int (*cmp_t)(void *, void *);
extern void *max(void *data[], int num, cmp_t cmp);

/* 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;

/* main2.c */
#include <stdio.h>
#include "generics.h"
typedef struct {
    const char *name;
    int score;
} student_t;

// Comparison function of two student
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;
        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;

5. Variable length arguments

int printf(const char *format, )

Example: an implementation of printf

#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);
            case 's': {
                char *p = va_arg(ap, char *); fputs(p, stdout);
int main(void)
    myprintf("c\ts\n", '1', "hello");
    return 0;

Variable length arguments requires stdarg.h, and va_list type and va_start, va_atg, va_end macro.

  • va_list ap: ap is a pointer
  • va_start(ap, format): let ap points to the next memory position of format
  • va_arg(ap, int): get next value in ap as int, and move ap to the next position
  • va_end(ap): is a callback. Will be called at here.
#define va_start(v,l)   __builtin_va_start(v,l)
#define va_end(v)   __builtin_va_end(v)
#define va_arg(v,l) __builtin_va_arg(v,l)