C - change stdin and wait for user input

Question

I have the following code:

#include <stdio.h>

int main()
{
   char c = 0;
   fclose(stdin);
   stdin = fopen("newin", "r");
   if(stdin != NULLL)
   {
      scanf("%c", &c);
      printf("%d", c);
   }
   else
       printf("Error");
}

I want my program to wait for a change in the stdin file. I created it blank, but it returns 0.

If a put like a 'a' char in it it prints 97 like it should.

How can I make the scanf wait for a change in the file, like it was waiting for me to write in the terminal window?

Why are you closing stdin? You might want to check out this answer: http://stackoverflow.com/a/5925575/1212725. You might also want to check the return code from fclose. — bruceg, Jan 06 '17 at 18:38
You want to write in your file and then your program read what you have writed? Kind of like you did in terminal? — Gabriel Pellegrino, Jan 06 '17 at 18:45

score 1 · Answer 1 · answered Jan 06 '17 at 18:51

1

How can I make the scanf wait for a change in the file, like it was waiting for me to write in the terminal window?

You can't.

Input from stdin and a file from disk are handled differently. When you are reading from a file, the file must have everything in order before you open it to read from it.

answered Jan 06 '17 at 18:51

R Sahu

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Is there any way to simulate that? Like I want to have a process running and another one giving inputs to it. – user7122079 Jan 06 '17 at 18:56
Sure. You can fork a child process and use pipes between them. See http://stackoverflow.com/questions/4812891/fork-and-pipes-in-c. – R Sahu Jan 06 '17 at 18:58
If you put your program in a loop reading the file and make it acts when it founds a signal, and you're able to write in your archive. – Gabriel Pellegrino Jan 06 '17 at 19:03
I don't understand why using pipes is no a general solution. – R Sahu Jan 06 '17 at 19:04
1

Because you have to create a parent and a child process, I just want to create a process and then send and receive information from it, by another processes, that can be running or not... – user7122079 Jan 06 '17 at 19:11
I also would like to use it when a scanf is called not a read or a write... – user7122079 Jan 06 '17 at 19:14
You just experienced [the XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). I suggest creating a new post where you state the actual problem you want to solve, what you have tried to solve it, and what obstacles you ran into. – R Sahu Jan 06 '17 at 19:36

C - change stdin and wait for user input

1 Answers1