简易C语言声明解析器(cdecl)——C语言实现

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1 什么是cdecl

cdecl是C语言声明的简称，通俗来讲就是把C语言的声明翻译成通俗语言。

2 代码

本程序暂时没有考虑错误处理，而且在处理结构、枚举和联合时只简单地用“struct”，“enum”，“union”来代表它们的具体内容。最后，这个程序假定函数的括号内没有参数列表。

# include <stdio.h>
# include <string.h>
# include <ctype.h>
# include <stdlib.h>
# define MAXTOKENS 100
# define MAXTOKENLEN 64enum type_tag {IDENTIFIER, QUALIFIER, TYPE};struct token {char    type;char    string[MAXTOKENLEN];
};int top = -1;
struct token stack[MAXTOKENS];
struct token this;#define pop stack[top--]
#define push(s) stack[++top] = senum type_tag classify_string(void){/* 推断标识符的类型 */char *s = this.string;if(!strcmp(s, "const")){strcpy(s, "read-only");return QUALIFIER;}if(!strcmp(s, "volatile")) return QUALIFIER;if(!strcmp(s, "void"))  return TYPE; if(!strcmp(s, "char"))  return TYPE; if(!strcmp(s, "signed"))  return TYPE;if(!strcmp(s, "unsigned")) return TYPE;if(!strcmp(s, "short"))  return TYPE;if(!strcmp(s, "int"))  return TYPE;if(!strcmp(s, "long"))  return TYPE;if(!strcmp(s, "float")) return TYPE;if(!strcmp(s, "double"))  return TYPE;if(!strcmp(s, "struct"))  return TYPE; if(!strcmp(s, "union"))  return TYPE; if(!strcmp(s, "enum")) return TYPE; return  IDENTIFIER;
}void gettoken(void){/* 读取下一个标记到this */char *p = this.string;/* 略过空白字符 */while((*p = getchar()) == ' ');if(isalnum(*p)){/* 读入的标识符以A-Z, 0-9开头 */while(isalnum(*++p = getchar()));ungetc(*p, stdin);*p = '\0';this.type = classify_string();return;}if(*p == '*'){strcpy(this.string, "pointer to");this.type = '*';return;}this.string[1] = '\0';this.type = *p;
}/* 理解所有分析过程的代码段 */
void read_to_first_identifer(){gettoken();while(this.type != IDENTIFIER){push(this);gettoken();}printf("%s is ", this.string); //打印标识符gettoken();
}void deal_with_array() {while(this.type == '['){printf("array ");gettoken(); //数字或者‘]’if(isdigit(this.string[0])){printf("0..%d ", (atoi(this.string)-1));gettoken();  //读取‘]’}gettoken();  //读取‘]’之后的再一个标记printf("of ");}
}void deal_with_function_args() {while (this.type != ')'){gettoken();}gettoken();printf("function returning ");
}void deal_with_pointers() {while(stack[top].type == '*'){printf("%s ", pop.string);}
}void deal_with_declarator() {/* 处理标识符之后可能存在的数组/函数 */switch (this.type){case '[': deal_with_array();break;case '(': deal_with_function_args();}deal_with_pointers();//处理在读入到标识符之前压入到堆栈中的符号while(top >= 0){if(stack[top].type == '('){pop;gettoken(); //读取')'之后的符号deal_with_declarator();}else{printf("%s ", pop.string);}}
}int main() {//将标记压入堆栈中，直到遇见标识符printf("Please enter a C declaration: ");read_to_first_identifer();deal_with_declarator();printf("\n");return 0;
}/*
输出示例：
Please enter a C declaration: int (*func)()
func is pointer to function returning intPlease enter a C declaration: int *(*a[3])();
a is array 0..2 of pointer to function returning pointer to intPlease enter a C declaration: struct pm i;
pm is struct
*/

3 基本思路

设计方案是弄一个堆栈，从输入流中依次向右读取，把各个标记依次压入堆栈，直到读到标识符为止。然后继续向右读入一个标记，也就是标识符右边的那个标记。接着，观察标识符左边的那个标记（需要从堆栈中弹出）。

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写在最后

本文是博主阅读《C专家编程》时的摘录，为了日后方便理解复习C声明的知识，特意作此文章，同时也希望可以帮到各位，Thank you very much!

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