[C] 切割字串函數 strtok

今天寫了 strtok 的範例：『如何分離網路 mac address』程式碼如下，大家一定會有疑問 strtok 第一次呼叫，第一參數輸入愈分離的字串，在 while 迴圈，則是輸入 NULL 呢？底下就來解析 strtok.c 的程式碼。

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/* * * Author      : appleboy * Date        : 2010.04.01 * Filename    : strtok.c * */   #include "string.h" #include "stdlib.h" #include "stdio.h"   int main() {   char str[]="00:22:33:4B:55:5A";   char *delim = ":";   char * pch;   printf ("Splitting string \"%s\" into tokens:\n",str);   pch = strtok(str,delim);   while (pch != NULL)   {     printf ("%s\n",pch);     pch = strtok (NULL, delim);   }        system("pause");   return 0; }

執行結果如下圖： strtok.c 在 FreeBSD 7.1 Release 裡面路徑是 /usr/src/lib/libc/string/strtok.c，可以看到底下函式 __strtok_r

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__strtok_r(char *s, const char *delim, char **last) {     char *spanp, *tok;     int c, sc;       if (s == NULL && (s = *last) == NULL)         return (NULL);       /*      * Skip (span) leading delimiters (s += strspn(s, delim), sort of).      */ cont:     c = *s++;     for (spanp = (char *)delim; (sc = *spanp++) != 0;) {         if (c == sc)             goto cont;     }       if (c == 0) {       /* no non-delimiter characters */         *last = NULL;         return (NULL);     }     tok = s - 1;       /*      * Scan token (scan for delimiters: s += strcspn(s, delim), sort of).      * Note that delim must have one NUL; we stop if we see that, too.      */     for (;;) {         c = *s++;         spanp = (char *)delim;         do {             if ((sc = *spanp++) == c) {                 if (c == 0)                     s = NULL;                 else                     s[-1] = '\0';                 *last = s;                 return (tok);             }         } while (sc != 0);     }     /* NOTREACHED */ }

大家可以看到，在第一次執行 strtok 時候，會針對傳入s字串每一個字進行比對，c = *s++; 意思就是 c 先設定成 *s，這行執行結束之後，會將 *s 指標加1，也就是字母 T -> h 的意思，這地方必須注意，如果第一個字母符合 delim 分隔符號，就會執行 goto cont;，如果不是，則會將 tok 指標指向 s 字串第一個位址，再來跑 for 迴圈找出下一個分隔字串，將其字串設定成 \0 中斷點，回傳 tok 指標，並且將s字串初始值指向分隔字串的下一個位址。 接下來程式只要繼續執行 strtok(NULL, delim)，程式就會依照上次所執行的 s 字串繼續比對下去，等到 *last 被指向 NULL 的時候就不會在執行 strtok 了，我相信這非常好懂，微軟 Visual Studio 有不同的寫法： https://research.microsoft.com/en-us/um/redmond/projects/invisible/src/crt/strtok.c.htm

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/* 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; }

微軟用了 strpbrk 來取代 for 迴圈的字串比對，但是整個流程是差不多的，大家可以參考看看，果然看 Code 長知識。


文章出處 ： http://blog.wu-boy.com/2010/04/cc-%E5%88%87%E5%89%B2%E5%AD%97%E4%B8%B2%E5%87%BD%E6%95%B8%EF%BC%9Astrtok-network-mac-address-%E5%88%86%E5%89%B2/