The Purpose of STDIN_FILENO and its Difference from stdin

1. Purpose of STDIN_FILENO

STDIN_FILENO belongs to the system API library, declared as an int type. It is an open file descriptor, and its corresponding functions primarily include system calls like open, read, write, close.

File APIs provided at the operating system level all represent files using file descriptors. STDIN_FILENO is the file descriptor for the standard input device (usually the keyboard).

2. Differences

1. Inconsistent Data Types: stdin is of type FILE* STDIN_FILENO is of type int Functions that use stdin primarily include fread, fwrite, fclose, etc., most of which start with 'f'. Functions that use STDIN_FILENO include read, write, close, etc.

2. stdin and similar are FILE* types, belonging to standard I/O, which are high-level input/output functions. They are defined in <stdio.h>. STDIN_FILENO and similar are file descriptors, which are non-negative integers, typically defined as 0, 1, 2. They belong to unbuffered I/O, directly invoking system calls, and are defined in <unistd.h>.

3. STDIN_FILENO is the file descriptor for standard input. See /usr/include/unistd.h for details. Code: /* Standard file descriptors. / #define STDIN_FILENO 0 / Standard input. / #define STDOUT_FILENO 1 / Standard output. / #define STDERR_FILENO 2 / Standard error output. */

4. Different levels of abstraction. stdin belongs to the input stream handled by the standard library, declared as a FILE type, and its corresponding functions all start with 'f', such as standard library calls like fopen, fread, fwrite, fclose. STDIN_FILENO belongs to the system API library, declared as an int type. It is an open file descriptor, and its corresponding functions primarily include system calls like open, read, write, close.

The standard library encapsulates system API calls; for example, fread internally calls read.

5. File APIs provided at the operating system level all represent files using file descriptors. STDIN_FILENO is the file descriptor for the standard input device (usually the keyboard).

6. I once wondered why an integer file descriptor (fd), like STDIN_FILENO=0, could represent an open file. Later I understood it's similar to our phone numbers: a phone number is just a 9-digit integer, but within the mobile communication network, it can be used to distinguish different mobile terminals. File operation function libraries provided at the standard C level all use FILE* to represent files, and stdin is a FILE* pointing to the standard input device file.