linux/unix use c to read file

read.c

// #include <error.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <time.h>

// forked process can use the file descriptor from parent's process without open again. Then two process can synchronize the file operations.
// ref:
// https://stackoverflow.com/questions/5284062/two-file-descriptors-to-same-file
// paragraph1:
//  It depends on where you got the two file descriptors. If they come from a dup(2) call,
// then they share file offset and status, so doing a write(2) on one will affect the position on the other.
// If, on the other hand, they come from two separate open(2) calls, each will have their own file offset and status.
// paragraph2:
//  This is guaranteed if they both refer to the same file description, aka you got them from "dup" or "dup2" (or inherited via fork()).
// paragraph3:
//  when you use dup() or dup2() or fork() , the file table is shared by both of the file descriptors. so if you write something from one file descriptor , and again write something through other file descriptor , then it is appended not overwritten.

// ref2:
//  https://stackoverflow.com/questions/42319310/is-a-file-descriptor-local-to-its-process-or-global-on-unix

// steps:
// build: cc test.c
// run: ./a.out
int main(int argc, const char *argv[])
{
    //3
    int fd = open("test.txt", O_RDWR | O_CREAT, 0666);
    if (fd == -1) {
        printf("open fail");
        // perror("open file mytest");
    }
    printf("open ok1:%d", fd);
    printf("\n");

    //result: 4. different value than fd
    // int fd2 = open("test.txt", O_RDWR | O_CREAT, 0666);
    // if (fd2 == -1) {
    //     printf("open 2 fail");
    //     // perror("open file mytest");
    // }
    // printf("open ok2:%d", fd2);
    // printf("\n");

    // if use another terminal to execute the compiled program, file descriptor is the same, 3
    // it seems normal and the oepration is independent although they have the same number.
    while (1) {
        sleep(1);
    }

    close(fd);
    return 0;
}