The Linux Programming Interface Exercise 5-6

5-6

After each of the calls to write() in the following code, explain what the content of
the output file would be, and why:
fd1 = open(file, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
fd2 = dup(fd1);
fd3 = open(file, O_RDWR);
write(fd1, "Hello,", 6);
write(fd2, "world", 6);
lseek(fd2, 0, SEEK_SET);
write(fd1, "HELLO,", 6);
write(fd3, "Gidday", 6);


Content is:
Gidday\00world\00
fd1 and fd2 share the same file offset, fd3 opens with offset 0 from SEEK_SET

5-6.c

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char const *argv[])
{
    int fd1, fd2, fd3;
    fd1 = open("test.txt", O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
    fd2 = dup(fd1);
    fd3 = open("test.txt", O_RDWR);
    write(fd1, "Hello,", sizeof("Hello,"));
    write(fd2, "world", sizeof("world"));
    lseek(fd2, 0, SEEK_SET);
    write(fd1, "HELLO,", 6);
    write(fd3, "Gidday", 6);
    return 0;
}