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redirect to dev null (2)
If you indeed used command /bin/null on CentOS, you have created a common file file named /bin/null on your system. You may say 'It worked!' , if you want.This file now contains the stdout and stderr output of your command.Usually, in /bin/ there are only executable files. And usually, only the root user is allowed to create files there. So if that file is there, you did run your command as. If you indeed used command /bin/null on CentOS, you have created a common file file named /bin/null on your system. You may say 'It worked!' , if you want.This file now contains the stdout and stderr output of your command.
If you are sure that thing does not redirect output (e.g. to
/dev/tty/
, which would be standard-out again) (which I don't think), you could redirect before calling them.I am using a library that is printing a warning message to
cout
or cerr
. I don't want this warning message to reach the output of my program. How can I catch this output and put it into /dev/null
or similar?MWE:
The output should be:
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How should I modify
main
to get the desired output? (foo
must not be changed.)Sears auto center tune up. I tried using
freopen()
and fclose(stdout)
as suggested in this question How can I redirect stdout to some visible display in a Windows Application?. The result is that nothing is printed. May I suggest a hack? Set a bad/fail bit on the relevant stream before use of a library function.
ikeaboy> Hello.
ikeaboy> I wonder if this is the right way to redirect stdout, stderr
ikeaboy> and stdin to /dev/null:
ikeaboy> I wonder if this is the right way to redirect stdout, stderr
ikeaboy> and stdin to /dev/null:
ikeaboy> freopen('/dev/null', 'w', stdout);
ikeaboy> freopen('/dev/null', 'w', stderr);
ikeaboy> freopen('/dev/null', 'r', stdin);
ikeaboy> freopen('/dev/null', 'w', stderr);
ikeaboy> freopen('/dev/null', 'r', stdin);
ikeaboy> Is the mode correct ? Would 'w+' be better ?
ikeaboy> Also, in lot of open source code is this:
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ikeaboy> close(0); close(1); close(2);
ikeaboy> open('/dev/null',O_RDWR); dup(0); dup(0);
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ikeaboy> open('/dev/null',O_RDWR); dup(0); dup(0);
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ikeaboy> Which approach is better and more portable ?
The first approach isn't guaranteed to work, because freopen() isn't
guaranteed to reuse the same underlying file descriptors for the new
stream. You may find it works anyway as a result of pure coincidence.
If the same descriptors _aren't_ reused, then stdio functions used to
write to 'stderr' will write to /dev/null, but POSIX writes to
STDERR_FILENO will not (and will possibly go to some completely
unexpected place if you're really unlucky). (Note that some library
functions can write to standard error, and in some cases this means
STDERR_FILENO rather than 'stderr'.)
guaranteed to reuse the same underlying file descriptors for the new
stream. You may find it works anyway as a result of pure coincidence.
If the same descriptors _aren't_ reused, then stdio functions used to
write to 'stderr' will write to /dev/null, but POSIX writes to
STDERR_FILENO will not (and will possibly go to some completely
unexpected place if you're really unlucky). (Note that some library
functions can write to standard error, and in some cases this means
STDERR_FILENO rather than 'stderr'.)
Unix Dev Null
The second approach avoids this problem. However, a slightly safer
way of doing it is:
way of doing it is:
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int fd = open('/dev/null',O_RDWR);
/* handle failure of open() somehow */
dup2(fd,0);
dup2(fd,1);
dup2(fd,2);
if (fd > 2)
close(fd);
/* handle failure of open() somehow */
dup2(fd,0);
dup2(fd,1);
dup2(fd,2);
if (fd > 2)
close(fd);
This has the same effect as the code you posted, but has the advantage
of being thread-safe. (relying on open()/dup() to pick the right
descriptor numbers is only safe if there are no other threads running)
of being thread-safe. (relying on open()/dup() to pick the right
descriptor numbers is only safe if there are no other threads running)
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--
Andrew.
Andrew.
Redirect Stdout To Dev Null C
comp.unix.programmer FAQ: see <URL: http://www.erlenstar.demon.co.uk/unix/>