Re: Why can't lseek the STDIN_FILENO?

[wen-yi]

OK, I see, I'm so sorry for my action.
I will ask in cpplang slack community in the future.
And really thanks for your advice.



Your,
Wen Yi





------------------ Original ------------------
From:                                                                           
                                             "John McKown"                      
                                                              
<john.archie.mck...@gmail.com&gt;;
Date:&nbsp;Sat, Jun 24, 2023 11:02 AM
To:&nbsp;
Cc:&nbsp;"pgsql-general"<pgsql-general@lists.postgresql.org&gt;;
Subject:&nbsp;Re: Why can't lseek the STDIN_FILENO?



My best advice would be to ask a C language question on a C language forum. 
This forum is really only for questions about the SQL language for the 
PostgreSQL database. I.e. no MariaDB, MySQL, MS SQL questions.


First, you didn't say what OS and she'll you're using. I an guessing BASH and 
Linux.&nbsp;


Second, you did NO error checking. I would purely guess that the lseek() is 
getting a return value of -1, probably with an error of ESPIPE.&nbsp;


This is probably a better 
explanation:&nbsp;https://unix.stackexchange.com/questions/502518/problems-when-test-whether-standard-input-is-capable-of-seeking


The bottom line from the above post is that STDIN is not seekable when it is a 
terminal.

On Fri, Jun 23, 2023, 21:17 Wen Yi <896634...@qq.com&gt; wrote:

Hi community,
I am testing the lseek &amp; write &amp; read, and I write the code like this:


/*
&nbsp;&nbsp; &nbsp;lseek_test.c
&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp; Test the lseek
&nbsp;&nbsp; &nbsp;Wen Yi
*/
#include <unistd.h&gt;
#include <fcntl.h&gt;
int main()
{
&nbsp;&nbsp; &nbsp;int fd = 0;
&nbsp;&nbsp; &nbsp;char buffer[16] = {};
&nbsp;&nbsp; &nbsp;write(STDIN_FILENO, "Hello world\n", sizeof("Hello 
world\n"));
&nbsp;&nbsp; &nbsp;lseek(STDIN_FILENO, 0, SEEK_SET);
&nbsp;&nbsp; &nbsp;read(STDIN_FILENO, buffer, sizeof(buffer));
&nbsp;&nbsp; &nbsp;write(STDIN_FILENO, buffer, sizeof(buffer));
&nbsp;&nbsp; &nbsp;return 0;
}


And I run the program ("Something Input" is my input content)



[beginnerc@bogon ???? C????]$ gcc lseek_test.c
[beginnerc@bogon ???? C????]$ ./a.out
Hello world
Something Input
Something Input
[beginnerc@bogon ???? C????]$ 



I really don't know, why the buffer's content not be "Hello world\n"? (I use 
the lseek to move the cursor to the beginning region)


Can someone give me some advice?
Thanks in advance!


Yours,
Wen Yi