Question

我尝试删除一个文件，然后从一个不起作用的函数中重命名已删除文件名称中的临时文件。请帮帮我

boolean delete_user(char user_name[256])   //only for Admin
{
    boolean status = FALSE;//what does the function return
    User_Data *ud = NULL;
    boolean found_user = FALSE;
    FILE *new_file = NULL;
    FILE *fp = NULL;
    char user_name_to_copy[256];
    char password_to_copy[256];
    char old_file_name[256];

    ud = find_user(user_name);
    if (ud == NULL) {
        printf("The username wasn't found!\n");
        return FALSE;
    }
    if (!strcmp(ud->permission_type, "Admin")) {
        printf("Cant delete an admin.");
        return FALSE;
    } else {
        // the user to delete was found 
        new_file = fopen("duplicate.txt", "wt");
        strcpy(old_file_name, ud->permission_type);
        strcat(old_file_name, "s.txt"); //the name of the file is in plural and ends with .txt
        fp = fopen(old_file_name, "rt");
        while (!feof(fp)) {
            //copy all the users except the user to delete the new file 
            fscanf(fp, "%s %s\n", user_name_to_copy, password_to_copy);
            if (strcmp(user_name_to_copy, user_name)) {
                fprintf(new_file, "%s %s\n", user_name_to_copy, password_to_copy);
            }
        }
        fclose(fp);
        fclose(new_file);
        printf(" %d ", remove(old_file_name));
        rename("duplicate.txt", old_file_name);
        remove("duplicate.txt");
        return TRUE;
    }
}

当我从另一个函数调用它时，此函数不起作用，但从主函数可以正常工作。

Answer 1

您的代码中存在多个问题：

您不会检查fopen()是否已成功打开文件。
连接字符串以计算权限文件时，不会阻止潜在的缓冲区溢出。
您应该将数组大小传递给fscanf以防止潜在的缓冲区溢出。
您应使用strerror(errno)输出有意义且信息丰富的错误消息，以及失败原因。
复制权限文件的循环不正确：Why is “while ( !feof (file) )” always wrong?
即使您无法重命名但仍设法删除原始文件，您也会删除重复的文件：这两个文件可能都会丢失。相反，如果重命名操作成功，则删除重复文件是多余的。

以下是改进代码的方法：

#include <errno.h>
#include <string.h>

boolean delete_user(char user_name[256]) {  //only for Admin
    boolean status = FALSE; // the function return value
    User_Data *ud = NULL;
    boolean found_user = FALSE;
    FILE *new_file = NULL;
    FILE *fp = NULL;
    char user_name_to_copy[256];
    char password_to_copy[256];
    char old_file_name[256];

    ud = find_user(user_name);
    if (ud == NULL) {
        printf("User '%s' was not found!\n", user_name);
        return FALSE;
    }
    if (!strcmp(ud->permission_type, "Admin")) {
        printf("Cannot delete user '%s', user has admin status.\n", user_name);
        return FALSE;
    }
    // the user to delete was found 
    new_file = fopen("duplicate.txt", "wt");
    if (new_file == NULL) {
        printf("Cannot open file 'duplicate.txt': %s\n"
               strerror(errno));
        return FALSE;
    }
    // the name of the file is in plural and ends with .txt
    snprintf(old_file_name, sizeof old_file_name, "%ss.txt", ud->permission_type);
    fp = fopen(old_file_name, "rt");
    if (fp == NULL) {
        printf("Cannot open user file '%s': %s\n"
               old_file_name, strerror(errno));
        return FALSE;
    }
    // copy all the users except the user to delete the new file 
    while (fscanf(fp, "%255s %255s\n", user_name_to_copy, password_to_copy) == 2) {
        if (strcmp(user_name_to_copy, user_name)) {
            fprintf(new_file, "%s %s\n", user_name_to_copy, password_to_copy);
        }
    }
    fclose(fp);
    fclose(new_file);
    if (remove(old_file_name)) {
        printf("Error removing file '%s': %s\n",
               old_file_name, strerror(errno));
        remove("duplicate.txt");
        return FALSE;
    }
    if (rename("duplicate.txt", old_file_name)) {
        printf("Error renaming file 'duplicate.txt' to '%s': %s\n",
                old_file_name, strerror(errno));
        // keep duplicate.txt
        return FALSE;
    }
    // duplicates.txt was successfully renamed, no need to remove it.
    return TRUE;
}

注意：

您在当前目录中创建临时文件，该文件可能位于与权限文件不同的驱动器上。 rename()可能无法将重复文件从当前目录移动到该目录。如果这是失败的原因，则应使用适当的诊断运行代码。

Answer 2

    printf(" %d ", remove(old_file_name));
    rename("duplicate.txt", old_file_name);
    remove("duplicate.txt");

此代码多数是冗余的。 ~~没有必要删除old_file_name，rename会将其击败。~~（原来这是POSIX的事情，C标准不保证）。没有必要删除duplicate.txt，它已被重命名。

    if( remove(old_file_name) != 0 ) {
        fprintf( stderr, "Could not remove %s: %s", old_file_name, strerror(errno) );
    }

    if( rename("duplicate.txt", old_file_name) != 0 ) {
        fprintf( stderr, "Could not rename %s to %s: %s", "duplicate.txt", old_file_name, strerror(errno) );
    }

您需要检查 每个文件操作 。其中包括fopen和fscanf。

此外，您可能会将文件向后移动。这是rename( old, new )但看起来你写的是rename( new, old )。如果您没有将它们颠倒过来，请考虑使代码更容易理解的变量名称。

    fp = fopen(old_file_name, "rt");
    while (!feof(fp)) {
        //copy all the users except the user to delete the new file 
        fscanf(fp, "%s %s\n", user_name_to_copy, password_to_copy);
        if (strcmp(user_name_to_copy, user_name)) {
            fprintf(new_file, "%s %s\n", user_name_to_copy, password_to_copy);
        }
    }

此代码存在问题。如上所述，它无法检查fopen和fscanf是否成功。使用fscanf是一个问题，如果它不匹配，它不会推进文件指针。你可以一遍又一遍地阅读同一行，每次都失败。一般来说，avoid fscanf and scanf。

相反，请使用fgets阅读整行，并使用sscanf进行解析。一定要检查sscanf是否成功，否则你会打印出胡言乱语。请注意，sscanf限制了它将读取的字符串大小以防止缓冲区溢出。

    FILE *fp = open_file(old_file_name, "rt");
    char line[1024];
    while (fgets(line, 1024, fp)) {
        //copy all the users except the user to delete the new file 
        if( sscanf(fp, "%255s %255s\n", user_name_to_copy, password_to_copy) != 2 ) {
            fprintf(stderr, "Couldn't understand line '%s'\n", line);
            continue;
        }
        if (strcmp(user_name_to_copy, user_name)) {
            fprintf(new_file, "%s %s\n", user_name_to_copy, password_to_copy);
        }
    }
    fclose(fp);

由于检查文件是否打开是多余的，我建议写一个小功能来做到这一点。

FILE *open_file(char *filename, char *mode) {
    FILE *fp = fopen(filename, mode);
    if( fp == NULL ) {
        fprintf(
            stderr, "Could not open '%s' for '%s': %s\n",
            filename, mode, strerror(errno)
        );
        exit(1);
    }

    return fp;
}

c中的remove（）不起作用

2 个答案: