Handling stdout

Posted on 2000-03-31
Last Modified: 2010-04-22
In a C program, I want to close the file stream associated with stdout [ something like fclose(stdout);] and open it again. Who can tell me how can I do this?
I use Linux and gcc.
Question by:mateif
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LVL 10

Expert Comment

ID: 2672559
man freopen

Author Comment

ID: 2672647
What I want to do is to reopen the stream and associate it with stdout again. Using freopen would require a first argument ( char *path) and I don't know which would that be. I tried NULL and it didn't work out.
LVL 10

Expert Comment

ID: 2672656
So why want you to close it. If you just want to flush buffers use fflush (stdin);
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LVL 10

Expert Comment

ID: 2672658
Sorry use
fflush (stdout);

Author Comment

ID: 2672757
What I want is to display two pages with a cgi, without stopping the cgi. The two pages are constructed by the cgi, and i don't want to save them on the hard-disk. So I saw that if i close stdout it displays the first page, so i need to reopen it to display the second. I tried to input EOF in stdout, but it didn't work.
LVL 40

Expert Comment

ID: 2675943
I think that rbr's suggestion is what you need. Try flushing the buffers and it should push the data out.

Author Comment

ID: 2679032
Flushing the buffer has no effect. I tried.
LVL 10

Expert Comment

ID: 2679301
Can you post your code pls. I'm not sure what your problem is.

Expert Comment

ID: 2679683
have you tried to write a formfeed ('\f') or an Unix EOF ('\004')? note: the constant EOF is not a valid character to write to stream.

Expert Comment

ID: 2680250

If I understand correctly what you want to do, the answer is: 'You cannot, because the CGI protocol prevents it'.

You cannot output two CGI pages during the same transaction, because the web server will just leave and close the client transaction after you're done with the first one. When your CGI starts up, your stdout is connected - through a socket - to the browser. After you're done with the transaction, the browser renders the page and *goes away* - it does not come back looking for seconds.

If persistent/multistep  connections are what you want, you need to go with different techniques (Cookies, hidden fields, php, and/or more). Fiddling with file handles won't buy you anything (regretfully).


Accepted Solution

zx9r earned 100 total points
ID: 2735128
do the following steps:

1. copy the file descriptor of the stdout :
       int fd;
       fd = dup(stdout);

2. close stdout

3. do the job
      here is important to not open any file or if you open any, make sure that       you close it before continuing with the next step

4. to restaure stdout you only have to do
      dup2(fd, stdout);

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