这是对strtok()的解释.
#include char strtok( char* s1,
const char* s2 );*The first call to strtok() returns a pointer to the first token in the
string pointed to by s1. Subsequent calls to strtok() must pass a NULL
pointer as the first argument,in order to get the next token in the
string.
但是我不知道,为什么你必须传递NULL指针才能获得字符串中的下一个标记.我搜索了大约15分钟,但没有在互联网上找到解释.
解决方法
strtok()将静态变量中的一些数据保存在函数本身内,以便它可以继续从之前的调用离开它的点进行搜索.要发送要保持搜索同一个字符串的strtok(),您将传递一个NULL指针作为其第一个参数. strtok()检查它是否为NULL,如果是,则使用其当前存储的数据.如果第一个参数不为空,则将其视为新搜索,并重置所有内部数据.
也许最好的事情是搜索strtok()函数的实际实现.我已经发现一个小到这里发布,所以你了解如何处理这个NULL参数：
/* Copyright (c) Microsoft Corporation. All rights reserved. */
#include <string.h>
/* ISO/IEC 9899 7.11.5.8 strtok. DEPRECATED.
* Split string into tokens,and return one at a time while retaining state
* internally.
*
* WARNING: Only one set of state is held and this means that the
* WARNING: function is not thread-safe nor safe for multiple uses within
* WARNING: one thread.
*
* NOTE: No library may call this function.
*/
char * __cdecl strtok(char *s1,const char *delimit)
{
static char *lastToken = NULL; /* UNSAFE SHARED STATE! */
char *tmp;
/* Skip leading delimiters if new string. */
if ( s1 == NULL ) {
s1 = lastToken;
if (s1 == NULL) /* End of story? */
return NULL;
} else {
s1 += strspn(s1,delimit);
}
/* Find end of segment */
tmp = strpbrk(s1,delimit);
if (tmp) {
/* Found another delimiter,split string and save state. */
*tmp = '\0';
lastToken = tmp + 1;
} else {
/* Last segment,remember that. */
lastToken = NULL;
}
return s1;
}
