关于《笨办法学C》Ex25中fgetc与stdin的困惑及代码疑问

Hey there! Let's break down stdin and fgets (I notice you mentioned fgetc but your code uses fgets—I'll focus on the function in your snippet since that's what's relevant here) using your Ex25 code as context.

stdin is a predefined file pointer from the C standard library (declared in <stdio.h>) that represents the standard input stream. By default, this stream is connected to your computer's keyboard—so when you use stdin as a source for input functions like fgets, you're telling the program to read input that you type in the terminal/console.

You don't need to open or close stdin manually; the C runtime handles that for you automatically when your program starts and exits. In your code, passing stdin to fgets means "read input from the user's keyboard".

Let's look at the line from your function:

char *result = fgets(*out_string, max_buffer, stdin);

The fgets function has three key parameters:

1. *out_string: This is the buffer (memory you allocated with calloc) where the input string will be stored.
2. max_buffer: This tells fgets the maximum number of characters it can read. Critically, fgets will only read up to max_buffer - 1 characters—it saves the last spot for the null terminator ('\0') that every C string needs to be valid.
3. stdin: As we covered, this is the input source (user's keyboard).

How it works step-by-step in your code: #

• When the user types a line of text and presses Enter, fgets reads that input until either:
  • It hits the newline character ('\n'), or
  • It has read max_buffer - 1 characters (whichever comes first)
• It automatically appends a null terminator ('\0') to the end of the input it stores in *out_string, so you get a properly formatted C string.
• The return value (result) is the same as the buffer pointer (*out_string) if the read was successful. If it fails (for example, if there's an input error or the user hits EOF like Ctrl+D on Unix/Ctrl+Z on Windows), fgets returns NULL—which is exactly what your check macro is checking for with check(result != NULL, "Input error.").

Why this is better than riskier functions like gets #

Unlike the old, unsafe gets function (which doesn't check buffer size and can cause buffer overflows), fgets enforces a maximum read length—this makes your code much more secure and avoids crashes or memory corruption from too-long input.

Quick note on your memory handling #

You used calloc to allocate *out_string—this is smart because calloc initializes all the memory to 0. Even if fgets didn't fill the entire buffer, the pre-zeroed memory ensures there's still a valid null terminator somewhere (though fgets adds its own anyway, so this is just an extra layer of safety). If the read fails, your error block frees the allocated memory to avoid memory leaks—great practice!

内容的提问来源于stack exchange，提问作者Leo Huang