c - 理解 strspn(const char *s, const char *accept) 函数

Question

我正在编写一个程序，逐行读取文件以将单词和翻译分开。下面的代码有效。但是，我无法理解 /* separate word and translation */ 部分的 load_dictionary 函数实际上是如何工作的。 .

不清楚的地方:

• p 行的输出
• 在 word = line + strspn(line, DELIMS) 之后输出 p word。 strspn 不应该读取到 DELIMS\t 并打印 - ants\t。

文件:字典.txt

WORD    TRANSLATION

ants    anttt
anti    eti
ante    soggy
anda    eggs

函数:主要

 /* maximum number of characters for word to search */
 #define WORD_MAX 256

 /* maximum number of characters in line */
 #ifndef LINE_MAX
 #define LINE_MAX 2048
 #endif

 int main(int argc, char * argv[]) {
        char word[WORD_MAX], * translation;
        int len;

        if (argc <= 1)
            return 0; /* no dictionary specified */

        /* load dictionary */
        load_dictionary(argv[1]);
        return 0;
  }

功能:load_dictionary:- 读取字典文件

/* delimiter for dictionary */
#define DELIMS "\t"

unsigned void load_dictionary(const char * filename) {
        FILE * pfile;
        char line[LINE_MAX], * word, * translation;

        /* ensure file can be opened */
        if ( !(pfile = fopen(filename,"r")) )
            return icount;

        /* read lines */
        while ( (fgets(line, LINE_MAX, pfile)) ) {
            /* strip trailing newline */
            int len = strlen(line);
            if (len > 0 && line[len-1] == '\n') {
              line[len-1] = '\0';
              --len;
            }

            /* separate word and translation */
            word = line + strspn(line, DELIMS);

            if ( !word[0] )
                continue; /* no word in line */
            translation = word + strcspn(word, DELIMS);
            *translation++ = '\0';
            translation += strspn(translation, DELIMS);
        }
 }

Answer 1

strspn 将给出 DELIM

中存在的初始字符数

strcspn 将给出 不 DELIM

中的初始字符数

(参见 http://man7.org/linux/man-pages/man3/strspn.3.html)

所以代码的想法是使用简单的指针算法使 word 和 translation 指针指向输入中的第一个单词和输入中的第二个单词.此外，代码在第一个单词后添加了一个 NUL 终止符，使其看起来像两个字符串。

例子:

line: \t\t\t\tC++\0\t\t\tA programming language
              ^   ^      ^
              |   |      |
              |   |      translation points here
              |   |      
              |   NUL added here
              |  
              word points here

所以打印 word 和 translation 将给出:

C++
A programming language

带有附加注释的代码:

        word = line + strspn(line, DELIMS);  // Skip tabs, i.e. 
                                             // make word point to the
                                             // first character which is
                                             // not a tab (aka \t)

        if ( !word[0] )
            continue; /* no word in line */

        translation = word + strcspn(word, DELIMS); // Make translation point to the
                                                    // first character after word 
                                                    // which is a tab (aka \t), i.e. it 
                                                    // points to the character just after
                                                    // the first word in line



        *translation++ = '\0';               // Add the NUL termination and
                                             // increment translation

        translation += strspn(translation, DELIMS);   // Skip tabs, i.e. 
                                                      // make translation point to the
                                                      // second word in line which is