仿写c字符串库函数

c的库函数写的很nice，实现下简单版本：

strcmp strncmp:

#include <stddef.h>
#include <string.h>

int strcmp_my(const char *s1, const char *s2)
{
    register signed char __res;

    while(1) {
        if ((__res = *s1 - *s2++) != 0 || !*s1++) 
            break;
    }
    return __res;
}

int strncmp_my(const char *s1, const char *s2, size_t n)
{
    register signed char __res = 0;

    while (n) { 
        if ((__res = *s1 - *s2++) != 0 || !*s1++)
            break;
        n--;
    }
}

strcpy strncpy:

#include <stddef.h>
char * ___strtok = NULL;

char * strcpy(char *dest, const char *src)
{
    while (*src != '\0')
        *dest++ = *src++;
    *dest = '\0';

    return dest;
}

char * strcpy_my(char *dest, const char *src)
{
    char *tmp = dest;

    while ((*tmp++ = *src++) != '\0')
        /* do nothing */;
        
    return tmp;
}

char * strncpy(char *dest, const char *src, size_t n)
{
    size_t i;

    for (i = 0; i < n && *src != '\0'; ++i)
        *dest++ = *src++; 
    for (; i < n; ++i)
        *dest++ = '\0';
    
    return dest;
}

char * strncpy_my(char *dest, const char *src, size_t n)
{
    char *tmp = dest;

    while (n-- && (*tmp++ = *src++) != '\0') 
        /* do nothing */;

    return tmp;
}

strcat strncat:

#include <stddef.h>
#include <string.h>

char * ___strtok = NULL;

char * strcat(char *dest, const char *src)
{
    size_t destLength = strlen(dest);
    size_t i;

    for (i = 0; src[i] != '\0'; ++i)
        dest[destLength + i] = src[i];

    return dest;
}

char * strncat(char *dest, const char *src, size_t n)
{
    size_t destLength = strlen(dest);
    size_t i;

    for (i = 0 ; i < n && src[i] != '\0' ; ++i)
        dest[destLength + i] = src[i];
    dest[destLength + i] = '\0';

    return dest;
}

char * strcat_my(char *dest, const char *src)
{
    char *tmp = dest;

    while (*dest)
        dest++;
    while ((*tmp++ = *src++) != '\0')
        ;

    return tmp;
}

char * strncat_my(char *dest, const char *src, size_t n)
{
    char *tmp = dest;
    
    if (n) {
        while (*dest)
            dest++;
        while ((*tmp++ = *src++) != '\0') {
            if (--n)        
                break;
        } 
    }
    return tmp;
}

strchr strrchrstrlen:

#include <stddef.h>
#include <string.h>

char * strchr(const char *s, int c)
{
    for (; *s != c ; ++s)
        if (*s == '\0')
            return NULL;
    return (char *)s;
}

char * strrchr(const char *s, int c)
{
    int length = 0;
    while (*s++ != '\0')
        length++;
    s--;

    for (; length >= 0 && *s!= c ; length--, --s)
        if (length == 0)
            return NULL;
    return (char *)s;
}

size_t strlen(const char *s)
{
    const char *sc;
        
    for (sc = s; *sc != '\0'; sc++)
       /* do nothing */; 
    return sc - s;
}

strspn strbrk:

#include <stddef.h>
#include <string.h>

size_t strspn(const char *s, const char *accept)
{
    const char *p;
    const char *q;
    size_t count = 0;

    for (p = s; *p != '\0'; ++p) {
        for (q = accept; *q != '\0'; ++q) {
            if (*p == *q) 
                break;
        } 
        if (*q == '\0')
            return count;
        ++count;
    }
    return count;
}

/*
 * search the first existence letter of accept in s
 */
char * strpbrk(const char *s, const char *accept)
{
    const char *p, *q;

    for (p = s; *p != '\0'; ++p) {
        for(q = accept; *q != '\0'; ++q) {
            if (*p == *q) 
                return (char *)p;
        } 
    }
    return NULL;
}

strdup

#include<stdio.h>
#include<stdlib.h>

char * strdup( char const *string )
{
   char *new_string;

   new_string = malloc( strlen( string ) * sizeof( char ) + 1 );

   if( new_string != NULL )
       strcpy( new_string, string );
   
   return new_string;
}

strstr