C, UNIX: How to redirect 'stdout' to a file from a C code?
I am looking for a way to redirect standard output to a file from a C-code;
so, any 'cout<<..' or 'printf(...)' will be written into a file.
I have a server source that I need to debug.
That program called by RPC (remote procedure call) and has no any session to print out anything.
I have some my source with set of macro and functions that are useful for debugging any source; but, it is writing into the standard output.
Therefore I would like to redirect the stdout to a file to use those debug-tools into the server source.
Thank you, jim mcnamara, it is perfectly helps!
Sure here is more than I need in my case, but everything is clear and strait forward!
Additionaly, your code keep a way to switch back that not needed right now, but could be a task later on such approach!
Also I've found useful (by another reply) the 'freopen()' C-function and did it in very simple way, too.
(... for anybody else with the same task and for myself later, here is how I did it with freopen() ) :
But, again, thanks for your solution!!
freopen DOES NOT redirect standard output! You are only redirecting the C file pointer 'stdout', not the file descriptor it uses.
Basically, you're ripping the floor out from under your program without guaranteeing that the Right Thing has happened to put a new floor back. The 'stdout' file pointer will continue to work, but anything not informed of the change might not -- like cout, and any subprocesses you happen to run. Their output may not go where you expected, or go nowhere at all, or crash. It might work right, if the next file opened happens to land at file descriptor 1 -- or it might not. The behavior is undefined and at the mercy of what libraries and compiler you're using. All that's guaranteed after you do that is that stdio routines like printf() will go where you redirected.
Whatever file descriptor 1 went to before, might not be properly closed after freopen, either. If stdout was an open file descriptor to a terminal preventing your ssh window from closing, such is life.
The "right thing" to do if you really want to redirect "standard output", not just the stdio external variable "stdout", is to ensure that the file you want is opened specifically as file descriptor one, which is what dup2() does in the example you were given. Which is much simpler than it looks once you realize it's redirecting twice: Once for stdout, once for stderr. Re-opening onto file descriptor one with dup2() also guarantees that whatever was there before, is forced to close. This could be especially important if that happened to be a terminal or device file.
Also, your example is concerning. Never mix printf and cout, for starters, especially if you're going to play funny games with redirection.
Last edited by Corona688; 10-13-2016 at 08:04 PM..
Dear all,
redirecting STDOUT & STDERR to file is quite simple, I'm currently using:
Code:
exec 1>>/tmp/tmp.log; exec 2>>/tmp/tmp.log
But during script execution I would like the output come back again to screen, how to do that?
Thanks
Luc
edit by bakunin: please use CODE-tags like the... (6 Replies)
Dear all,
redirecting STDOUT & STDERR to file is quite simple, I'm currently using:
exec 1>>/tmp/tmp.log; exec 2>>/tmp/tmp.logBut during script execution I would like the output come back again to screen, how to do that?
Thanks
Lucas (4 Replies)
Hi All,
I am using centOS.
When I try to redirect STDOUT to a file, it ends up in getting some funny characters. For example ...
STDOUT of the command as follows.
$ ls
H3k27me3
H3k36me3
H3k4me1
H3k4me2
H3k4me3
H3k9ac
H4k20me1
$ ls >test
$ cat test
^ (1 Reply)
Hello
I read a lot of post related to this topic, but nothing helped me. :mad:
I'm running a ksh script with subshell what processing some ldap command. I need to check output for possible errors.
#!/bin/ksh
...
readinput < $QCHAT_INPUT |&
while read -p line
do
echo $line
... (3 Replies)
Hi,
I would like to avoid re-directing line by line to a file.
What is the best way to re-direct STDOUT to a file in a subshell?
Thanks in advance.
Cheers
Vj (1 Reply)
Hi
I am not if this is possible: is it possible in bach (or another shell) to redirect GLOBALLY the stdout/stderr channels to a file.
So, if I have a script
script.sh
cmd1
cmd2
cmd3
I want all stdout/stderr goes to a file. I know I can do:
./script.sh 1>file 2>&1
OR
... (2 Replies)
Hi friends
I am facing one problem while redirecting the out of the stderr and stdout to a file
let example my problem with a simple example
I have a file (say test.sh)in which i run 2 command in the background
ps -ef &
ls &
and now i am run this file and redirect the output to a file... (8 Replies)
Hi all,
I have the following script:
-------------------------------------------------
#SCRIPT TO CHECK WHO HAS ACCESSED THE LOG/FILE IN PAST 'N' MINUTES, AND MAIL ACCORDINGLY.
MYPATH="/clocal/mqbrkrs/user/mqsiadm/sanjay/"
MAIL_RECIPIENTS="vg517@dcx.com"
Subject="File accessed in last... (6 Replies)
Hi all hope you can help as I am going MAD!!! :eek:
The below is in a shell script but the redirection in the sed line does not work and outputs to the screen and the $fname_2 does note get created ?????
Can any one help ??
#!/bin/ksh
cd /app/
for fname in `ls -1 X*`
do
sed 1d $fname... (3 Replies)