[SOLVED] STDIO redirection and syscalls

prushik · 04-21-2014, 12:36 PM

Hi everybody,
I wrote a small "cat"-like program for linux using Linux system calls for reading files and outputting to STDOUT. It also reads from STDIN if no arguments are given.
This all works fine, but I also want to be able to redirect text streams to and from the program. This does not work. For instance,

Code:

echo "test" | ./a.out

does nothing. The program does not receive the input through stdin and outputs nothing to the terminal. Also, when the program is in this state, it will no longer receive interactive input from the command line.

I am using SYS_read with stdin (1) and SYS_write with stdout (0). In normal C programming with the fread family of functions, this gets handled automatically, so I don't understand why it doesn't work with system calls.
Any ideas?

NevemTeve · 04-21-2014, 12:46 PM

How about strace and/or gdb?
Maybe I could say something about your program if I saw the source...

prushik · 04-21-2014, 11:07 PM

Quote:

Originally Posted by NevemTeve

How about strace and/or gdb?
Maybe I could say something about your program if I saw the source...

Source code looks like this:

Code:

#include <fcntl.h>
#include <sys/syscall.h>

void cat(int in, int out)
{
	int ret;
	char byte;

	do
	{
		ret = syscall(SYS_read,in,&byte,1);
		syscall(SYS_write,out,&byte,1);
	} while (ret!=0);

	return;
}

int main(int argc, char **argv)
{
	int i;

	if (argc==1)
	{
		cat(1,0);
	}
	else
	{
		for (i=1;i<argc;i++)
		{
			int fd = syscall(SYS_open,argv[i],O_RDONLY,0);
			cat(fd,0);
			syscall(SYS_close,fd);
		}
	}

//	syscall(SYS_exit);
	return 0;
}

NevemTeve · 04-21-2014, 11:18 PM

For a start:

Code:

while ((ret= syscall(SYS_read,in,&byte,1))>0)
    syscall(SYS_write,out,&byte,ret);
}

prushik · 04-22-2014, 12:04 AM

Quote:

Originally Posted by NevemTeve

For a start:

Code:

while ((ret= syscall(SYS_read,in,&byte,1))>0)
    syscall(SYS_write,out,&byte,ret);
}

Ok, yes. That would be more correct, however, it doesn't address the issue of redirection.

prushik · 04-22-2014, 12:14 AM

Quote:

Originally Posted by NevemTeve

How about strace and/or gdb?
Maybe I could say something about your program if I saw the source...

Also, according to strace, there is nothing coming in from the redirected text stream, but when something is redirected to the program it fails to write to stdout (0) with "EBADF (Bad file descriptor)". It still seems to read interactive user input fine, just not any text piped into it.

NevemTeve · 04-22-2014, 01:30 AM

Another option would be using 0 for stdin, 1 for stdout, 2 for stderr.
Or symbolic constants from unistd.h:

Code:

#define STDIN_FILENO  0
#define STDOUT_FILENO 1
#define STDERR_FILENO 2

prushik · 04-22-2014, 01:35 AM

Quote:

Originally Posted by NevemTeve

Another option would be using 0 for stdin, 1 for stdout, 2 for stderr.
Or symbolic constants from unistd.h:

Code:

#define STDIN_FILENO  0
#define STDOUT_FILENO 1
#define STDERR_FILENO 2

Ahha! Silly me... always some stupid mistake. Well that's one mystery solved.
Now why did it work at all before when I was reading from stdout and writing to stdin?

NevemTeve · 04-22-2014, 01:43 AM

Because both were special file /dev/tty
Of course, it is not guaranteed that you can read from stdout in any context, but also it is not guaranteed that you cannot.