从 PL/0 编译程序源码中抽出词法分析部分，只改写main函数，构建 PL/0 独立词法分析程序。功能：输入为 PL/0 源程序， 输出为单词符号的序列；对于标识符和无符号整数，显示单词种别和单词自身的值两项内容；对于其他单词符号，仅显示其单词种别。

以下是一个简单的 PL/0 独立词法分析程序，它从标准输入中读取 PL/0 源程序，将其转换为单词符号的序列，并在标准输出中显示单词种别和单词的值（对于标识符和无符号整数），或者仅显示单词种别（对于其他单词符号）。

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

#define MAX_IDENT_LEN 32
#define MAX_NUM_LEN 10

enum {
    SYM_NULL,
    SYM_IDENTIFIER,
    SYM_NUMBER,
    SYM_PLUS,
    SYM_MINUS,
    SYM_TIMES,
    SYM_SLASH,
    SYM_ODD,
    SYM_EQU,
    SYM_NEQ,
    SYM_LES,
    SYM_LEQ,
    SYM_GTR,
    SYM_GEQ,
    SYM_LPAREN,
    SYM_RPAREN,
    SYM_COMMA,
    SYM_SEMICOLON,
    SYM_PERIOD,
    SYM_BECOMES,
    SYM_BEGIN,
    SYM_END,
    SYM_IF,
    SYM_THEN,
    SYM_WHILE,
    SYM_DO,
    SYM_CALL,
    SYM_CONST,
    SYM_VAR,
    SYM_PROCEDURE
};

static char ch = ' ';

static int sym;
static char ident[MAX_IDENT_LEN + 1];
static int num;

static void next_ch(void)
{
    ch = getchar();
}

static void error(const char *msg)
{
    fprintf(stderr, "%s\n", msg);
    exit(1);
}

static void get_sym(void)
{
    while (isspace(ch))
        next_ch();

    if (isalpha(ch)) {
        int i = 0;
        do {
            if (i < MAX_IDENT_LEN)
                ident[i++] = ch;
            next_ch();
        } while (isalnum(ch));
        ident[i] = '\0';
        if (strcmp(ident, "odd") == 0)
            sym = SYM_ODD;
        else if (strcmp(ident, "begin") == 0)
            sym = SYM_BEGIN;
        else if (strcmp(ident, "end") == 0)
            sym = SYM_END;
        else if (strcmp(ident, "if") == 0)
            sym = SYM_IF;
        else if (strcmp(ident, "then") == 0)
            sym = SYM_THEN;
        else if (strcmp(ident, "while") == 0)
            sym = SYM_WHILE;
        else if (strcmp(ident, "do") == 0)
            sym = SYM_DO;
        else if (strcmp(ident, "call") == 0)
            sym = SYM_CALL;
        else if (strcmp(ident, "const") == 0)
            sym = SYM_CONST;
        else if (strcmp(ident, "var") == 0)
            sym = SYM_VAR;
        else if (strcmp(ident, "procedure") == 0)
            sym = SYM_PROCEDURE;
        else
            sym = SYM_IDENTIFIER;
    } else if (isdigit(ch)) {
        num = 0;
        do {
            num = num * 10 + (ch - '0');
            next_ch();
        } while (isdigit(ch));
        sym = SYM_NUMBER;
    } else {
        switch (ch) {
        case '+':
            sym = SYM_PLUS;
            next_ch();
            break;
        case '-':
            sym = SYM_MINUS;
            next_ch();
            break;
        case '*':
            sym = SYM_TIMES;
            next_ch();
            break;
        case '/':
            sym = SYM_SLASH;
            next_ch();
            break;
        case '=':
            sym = SYM_EQU;
            next_ch();
            break;
        case '<':
            next_ch();
            if (ch == '=') {
                sym = SYM_LEQ;
                next_ch();
            } else if (ch == '>') {
                sym = SYM_NEQ;
                next_ch();
            } else {
                sym = SYM_LES;
            }
            break;
        case '>':
            next_ch();
            if (ch == '=') {
                sym = SYM_GEQ;
                next_ch();
            } else {
                sym = SYM_GTR;
            }
            break;
        case '(':
            sym = SYM_LPAREN;
            next_ch();
            break;
        case ')':
            sym = SYM_RPAREN;
            next_ch();
            break;
        case ',':
            sym = SYM_COMMA;
            next_ch();
            break;
        case ';':
            sym = SYM_SEMICOLON;
            next_ch();
            break;
        case '.':
            sym = SYM_PERIOD;
            next_ch();
            break;
        case ':':
            next_ch();
            if (ch == '=') {
                sym = SYM_BECOMES;
                next_ch();
            } else {
                error("invalid symbol");
            }
            break;
        default:
            error("invalid symbol");
            break;
        }
    }
}

int main(void)
{
    while (ch != EOF) {
        get_sym();
        switch (sym) {
        case SYM_IDENTIFIER:
            printf("%d %s\n", SYM_IDENTIFIER, ident);
            break;
        case SYM_NUMBER:
            printf("%d %d\n", SYM_NUMBER, num);
            break;
        default:
            printf("%d\n", sym);
            break;
        }
    }
    return 0;
}

可以将上述代码保存为 `pl0lex.c`，并使用 C 编译器进行编译，例如：

$ gcc -o pl0lex pl0lex.c

然后，可以使用以下命令运行该程序：

$ ./pl0lex < input.pl0

其中 `input.pl0` 是 PL/0 源程序的文件名。程序将输出单词符号的序列，每个单词符号占一行，其中标识符和无符号整数的值也将被显示出来。