Redirecting stderr of a running process May 4, 2018 on Drew DeVault's blog

During the KDE sprint in Berlin, Roman Gilg leaned over to me and asked if I knew how to redirect the stderr of an already-running process to a file. I Googled it and found underwhelming answers using strace and trying to decipher the output by reading the write syscalls. Instead, I thought a gdb based approach would work better, and after putting the pieces together Roman insisted I wrote a blog post on the topic.

gdb, the GNU debugger, has two important features that make this possible:

With this it’s actually quite straightforward. The process is the following:

  1. Attach gdb to the running process
  2. Run compile code -- dup2(open("/tmp/log", 65), 2)

The magic 65 here is the value of O_CREAT | O_WRONLY on Linux, which is easily found with a little program like this:

#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char **argv) {
    printf("%d\n", O_CREAT | O_WRONLY);
    return 0;
}

2 is always the file descriptor assigned to stderr. What happens here is:

  1. Via open, the file you want to redirect to is created.
  2. Via dup2, stderr is overwritten with this new file.

The compile code gdb command will compile some arbitrary C code and run the result in the target process, presumably by mapping some executable RAM and loading it in, then jumping to the blob. Closing gdb (control+d) will continue the process, and it should start writing out to the file you created.

There are lots of other cool (and hacky) things you can do with gdb. I once disconnected someone from an internet radio by attaching gdb to nginx and closing their file descriptor, for example. Thanks to Roman for giving me the chance to write an interesting blog post on the subject!

Articles from blogs I read Generated by openring

The Future of Forums is Lies, I Guess

In my free time, I help run a small Mastodon server for roughly six hundred queer leatherfolk. When a new member signs up, we require them to write a short application—just a sentence or two. There’s a small text box in the signup form which says: Please t…

via Aphyr: Posts July 7, 2025

Raised Catholic

My introduction to CatholicismIn my young life, I was quite involved in Catholicism. I was raised this way by my mother, who wholeheartedly believes in it. In contrast, my father is not religious, and only goes to church for special occasions like the Chr…

via Cadence's Weblog July 6, 2025

Hare 0.25.2 released

I am pleased to announce the release of Hare 0.25.2 today. 🎉 It has been almost one year since the previous release, Hare 0.24.2, and the new release is packed with the numerous language features, standard libarary features, and countless bugfixes and small…

via Blogs on The Hare programming language June 21, 2025