Input and output · Programming in C

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Input and output

Predict what a program writes depending on whether its output goes to the terminal or to a file, write wc, tee and tail with read, write and lseek, follow shared file offsets, send integers through a pipe. Chapter 5.

All programs are compiled with gcc -std=c11 -Wall -Wextra -Werror -D_POSIX_C_SOURCE=200809L. We do not use <stdio.h> to read or write the data, only for error messages and for displaying results.

Exercise · Terminal or file?

#include <stdio.h>
#include <sys/wait.h>
#include <unistd.h>

int main(void)
{
    printf("A\n");
    printf("B");
    write(STDOUT_FILENO, "C\n", 2);
    if (fork() == 0)
    {
        printf("D\n");
        return 0;
    }
    wait(NULL);
    printf("E\n");
    return 0;
}

What does this program print when run in a terminal?
What is in f.txt after ./q1 > f.txt?
Where should you add a single call so that both outputs are identical and no letter is repeated?

Exercise · A tee

tee file copies its standard input both to its standard output and into file. Write montee. Each block read must be written in full to both places.

$ printf 'un\ndeux\n' | ./montee copie.txt | wc -l
2
$ cat copie.txt
un
deux

Exercise · Who writes where?

Without running it, give the contents of the three files after this program. od -c p1.txt shows a file byte by byte, so you can check.

#include <fcntl.h>
#include <unistd.h>

int main(void)
{
    int a = open("p1.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666);
    int b = dup(a);
    write(a, "bonjour", 7);
    write(b, "!!", 2);
    close(a);
    close(b);

    int c = open("p2.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666);
    int d = open("p2.txt", O_WRONLY);
    write(c, "bonjour", 7);
    write(d, "!!", 2);
    close(c);
    close(d);

    int e = open("p3.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666);
    write(e, "bonjour", 7);
    lseek(e, 0, SEEK_SET);
    write(e, "B", 1);
    lseek(e, 2, SEEK_END);
    write(e, "!", 1);
    close(e);
    return 0;
}