Question

So this is the code I am trying to run:

#include<fcntl.h>
#include<stdio.h>
#include<errno.h>
#include<string.h>
#include<unistd.h>

int main(){

  int ret;

  ret = read(STDIN_FILENO,(int*)2000,3);
  printf("%d--%s\n",ret,strerror(errno));

  return 0;
}

and this is the output I get at the terminal

anirudh@anirudh-Aspire-5920:~/Desktop/testing$ gcc test.c 
anirudh@anirudh-Aspire-5920:~/Desktop/testing$ ./a.out 
lls
-1--Bad address
anirudh@anirudh-Aspire-5920:~/Desktop/testing$ ls
a.out  htmlget_ori.c  mysocket.cpp  Packet Sniffer.c  resolutionfinder.c  test.c
anirudh@anirudh-Aspire-5920:~/Desktop/testing$ 

Question 1: When I type the address 2000 in th read call read(STDIN_FILENO,(int*)2000,3);
then where does the address lies. I think this is the absolute address of the RAM that I am trying to access. am I right or is it offset and is added to the Stack Segment Base Address. I do not know. The program is not giving me a SEGFAULT for memory violation rather gives me Bad address

Question 2: Okay so the code crashes when I give the input as lls and bash executes the “ls” part of that “lls”. The reason is that the code crashes after reading the first “l” and the rest “ls” part is executed by bash. but why bash is executing the left “ls” part. Why is bash doing so because my code is crashed and even if bash was its parent process it should not read from the file-descriptor (STDIN_FILNO) opened by the code I wrote. ( I think so)…

Thanks for your time.

Answer 1

You are running on a CPU with paging. Your OS maintains page tables which translate from virtual to physical addresses. The page table for your process doesn’t contain anything for virtual address 2000, so read() notices, and returns -EFAULT.

stdin is connected to your terminal device (/dev/tty). Your process inherits that terminal from your shell, and the shell gets it back on process exit.