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Re: CVS, Python's subprocess.Ppen() and STDOUT redirection


From: Petr Pisar
Subject: Re: CVS, Python's subprocess.Ppen() and STDOUT redirection
Date: Wed, 25 Aug 2010 10:44:40 +0200
User-agent: Mutt/1.5.20 (2009-12-10)

On Tue, Aug 24, 2010 at 10:54:52PM +0200, Michał Górny wrote:
> #include <stdio.h>
> 
> int main(void) {
>     fprintf(stdout, "foo\n");
>     fclose(stdout);
>     fprintf(stderr, "bar\n");
> }
> 
> As I've expected, calling it with Popen(..., stdout=sys.stderr) causes
> only 'foo' to be output.
> 
> I'm not sure if the problem is rather in CVS or Python, but I
> personally think it's not a good idea to close STDOUT anyway.
> 
I'm not sure what Python's Popen() does, but simple dup2(2) does not exhibit
the issue and prints both strings:


#include <stdio.h>
#include <stdlib.h>

int main(void) {
    close(1);
    if (-1 == dup2(2, 1)) {
        perror("Stdout to stderr dupplication failed");
        exit(EXIT_FAILURE);
    }

    fprintf(stdout, "foo\n");
    fclose(stdout);
    fprintf(stderr, "bar\n");
}


And this is strace output:

close(1)                                = 0
dup2(2, 1)                              = 1
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 8), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 
0x7f3b05fd8000
write(1, "foo\n", 4foo
)                    = 4
close(1)                                = 0
munmap(0x7f3b05fd8000, 4096)            = 0
write(2, "bar\n", 4bar
)                    = 4
exit_group(4)                           = ?

As you can see, it closes stdout, duplicates it to stderr (the
stdout=sys.stderr construct, I guess) and write to stderr succeeds even if
stdout has been closed before.

-- Petr

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