C语言编译错误求助：语法及fread参数不足等问题排查

Hey there! Making the jump from Pascal to C comes with some syntax adjustments—let's walk through each of your errors and break down how to fix them, since these are super common for folks coming from Pascal's more verbose (but different) type system.

Let's Tackle Each Error One by One #

1. Error: expected expression before struct #

This usually stems from a key difference between Pascal and C's struct handling:

• In Pascal, once you define a Book type, you can use it directly. In C, if you define a struct like:

  struct Book {
      char title[50];
      int book_id;
  };
  

  You need to either prefix variable declarations with struct (e.g., struct Book my_book;) or use typedef to create a shorthand for the struct type:

  typedef struct {
      char title[50];
      int book_id;
  } Book;
  // Now you can declare variables like this (no struct prefix needed)
  Book my_book;
  

• Another possible cause: You tried to use struct in an expression where it doesn't belong, like struct.title (this is invalid—you need to use a struct variable, not the type itself).

2. [Error] expected identifier or '(' before '.' token #

This error pops up when you use the . member access operator on something that isn't a struct variable. For example:

• If you wrote struct Book.title (using the struct type instead of a variable), that's wrong. You need to access members on an instance of the struct:

  Book my_book;
  strcpy(my_book.title, "The C Programming Language"); // Correct
  

• Double-check your function definitions too—this error can also happen if you have a typo like void .display_book() instead of void display_book().

3. [Error] expected expression before 'books' #

Assuming books is a struct array or variable, this usually means one of two things:

• You're using books as a type instead of a variable. For example, if you declared struct Book books[]; but then tried to use it in an expression like struct Book books = ... (redeclaring it incorrectly).
• You forgot to declare books before using it. In C, variables must be declared before they're referenced (unlike some Pascal dialects where you can declare them later in the block).

4. [Error] too few arguments to function 'fread' #

C's fread requires 4 mandatory arguments (Pascal's file-reading functions have different signatures):

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

Let's break down what each means:

• ptr: Pointer to the buffer where you'll store the read data (e.g., your books array)
• size: Size of a single element (e.g., sizeof(Book))
• nmemb: Number of elements you want to read
• stream: Pointer to the open file (from fopen())

Example of correct usage:

Book books[10];
FILE *file_ptr = fopen("library.dat", "rb");
if (file_ptr != NULL) {
    // Read 10 Book structs into the books array
    fread(books, sizeof(Book), 10, file_ptr);
    fclose(file_ptr);
}

You probably missed one or more of these arguments (like omitting the file pointer or number of elements).

Quick Pascal-to-C Tips to Avoid These Issues #

• Structs: Always remember to either use struct prefixes or typedef for shorthand.
• Variable Scope: Declare variables at the top of blocks (or before use) until you're comfortable with C99's block-scoped declarations.
• Function Calls: Double-check standard library function signatures (like fread)—C is stricter about argument counts and types than Pascal.
• Passing Structs: In Pascal, var parameters pass by reference; in C, you'll need to use pointers if you want to modify a struct in a function (e.g., void update_book(Book *book_ptr) instead of void update_book(Book book)).

内容的提问来源于stack exchange，提问作者Ingrid Ryes