c - 什么是二进制文件的 EOF？

Question

我的程序需要从一个二进制文件读取到一个链表，这个函数做得很好，并为它分配了正确的内存，但由于某种原因，它在中断之前做了另一个循环。尝试为它寻找一个好的解决方案但没有运气，链表中的最后一个结构变得垃圾。

结构:

typedef struct
{
    char id[10];
    char * first_name;
    char * last_name;
    int age;
    char gender;
    char * username;
    char * password;
    char * description;
    int hobbies[4];
    struct Person * next_client;
}Person;

这里是一些代码:

Person * input_from_file(Person * member)
{
int str_size;
Person * clients_start = NULL;
FILE * filePointerRead;

filePointerRead = fopen("input.bin", "rb");

if (filePointerRead != NULL){
    while (1){

        member = NULL;
        member = (Person*)malloc(sizeof(Person));

        fread(&member->id, sizeof(char), ID_DIGITS + 1, filePointerRead);

        fread(&str_size, sizeof(int), 1, filePointerRead);
        member->first_name = (char*)malloc(str_size*sizeof(char));
        fread(member->first_name, sizeof(char), str_size, filePointerRead);

        //more reading from file

        member->next_client = NULL;
        clients_start = receive_clients_info(clients_start, member);  //function to put the received struct from file to end of the linked list

        if (feof(filePointerRead))
            break;
    }
    fclose(filePointerRead);
}
return clients_start;
}

Answer 1

调用 feof 的问题在于，除非您在 EOF 处尝试读取，否则它不会返回“true”。换句话说，如果您的文件恰好有 100 个字节，并且您已经成功地尝试读取恰好 100 个字节，feof 将返回“false”，直到您尝试至少再读取一个字节。

这就是为什么你应该避免使用 feof 来检查 fread 的返回值的原因，它会告诉你从文件中读取了多少字节:

if (fread(&member->id, sizeof(char), ID_DIGITS + 1, filePointerRead) != ID_DIGITS + 1) {
    // The code above knows that sizeof(char) is always 1.
    // For other data types you need to compute the actual size
    // by multiplying sizeof(T) by the number of items that you read.
    break;
}

在你调用 fread 的所有地方做同样的事情。

与 != 比较是有效的，因为 fread 在完成请求时总是返回您传递的确切大小:

Upon successful completion, fread() shall return the number of elements successfully read which is less than nitems only if a read error or end-of-file is encountered.