#include <stdio.h> #include <stdlib.h> #include <ctype.h> int main() { FILE *fp; char str[81]; int i; if ((fp = fopen("D:\\TEST\\ex10_5.txt", "w")) == NULL) { printf("Open file error!\n"); exit(1); } while (fgets(str, MAX_LINE_LEN, stdin) != NULL) { for (i = 0; str[i] != '\0'; i++) { str[i] = toupper(str[i]); } fputs(str, fp); } fclose(fp); if ((fp = fopen("D:\\TEST\\ex10_5.txt", "r")) == NULL) { printf("Open file error!\n"); exit(1); } while (fgets(str, MAX_LINE_LEN, fp) != NULL) { printf("%s", str); } fclose(fp); return 0; }

这段代码中没有定义`MAX_LINE_LEN`的值，也没有包含头文件，应该将头文件和常量定义加入代码中。以下是修改后的代码：

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

#define MAX_LINE_LEN 100

int main() {
    FILE *fp;
    char str[MAX_LINE_LEN];
    int i;
    if ((fp = fopen("D:\\TEST\\ex10_5.txt", "w")) == NULL) {
        printf("Open file error!\n");
        exit(1);
    }
    while (fgets(str, MAX_LINE_LEN, stdin) != NULL) {
        for (i = 0; str[i] != '\0'; i++) {
            str[i] = toupper(str[i]);
        }
        fputs(str, fp);
    }
    fclose(fp);
    if ((fp = fopen("D:\\TEST\\ex10_5.txt", "r")) == NULL) {
        printf("Open file error!\n");
        exit(1);
    }
    while (fgets(str, MAX_LINE_LEN, fp) != NULL) {
        printf("%s", str);
    }
    fclose(fp);
    return 0;
}

这样，代码中的`MAX_LINE_LEN`常量就被定义为100，并且头文件也被正确包含了。