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: stackoverflow.com/a/5925575/1212725. You might also want to check the return code from fclose. — bruceg
– bruceg, Commented Jan 6, 2017 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
– Gabriel Pellegrino, Commented Jan 6, 2017 at 18:45

R Sahu · Accepted Answer · 2017-01-06 18:51:31Z

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 6, 2017 at 18:51

R Sahu

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7 Comments

user7122079 Over a year ago

Is there any way to simulate that? Like I want to have a process running and another one giving inputs to it.

R Sahu Over a year ago

Sure. You can fork a child process and use pipes between them. See stackoverflow.com/questions/4812891/fork-and-pipes-in-c.

Gabriel Pellegrino Over a year ago

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.

R Sahu Over a year ago

I don't understand why using pipes is no a general solution.

user7122079 Over a year ago

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...

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