From Newsgroup: comp.unix.programmer

Hi

I've been playing around with signalfd() on linux and it seems to work but there also seems to be zero information other than the signal number in the signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler
set using sigaction with the siginfo_t passed to the handler, the below code returns 0 for everything in the signalfd_siginfo structure.

The relevant parts of the code are below. Have I done something wrong?

struct signalfd_siginfo siginfo;
:
:
sigemptyset(&sigset);
sigaddset(&sigset,SIGINT); /* Control-C */
sigaddset(&sigset,SIGTSTP); /* Control-Z */

if (sigprocmask(SIG_BLOCK,&sigset,NULL) == -1)
{
perror("ERROR: sigprocmask()");
return 1;
}

if ((fd = signalfd(-1,&sigset,0)) == -1)
{
perror("ERROR: signalfd()");
return 1;
}

:
:

while(1)
{
FD_ZERO(&mask);
FD_SET(fd,&mask);

switch(select(FD_SETSIZE,&mask,0,0,NULL))
:
:


if (FD_ISSET(fd,&mask))
{
len = read(fd,&siginfo,sizeof(siginfo));
if (len == -1)
{
perror("ERROR: read(fd)");
return 1;
}
if (len != sizeof(siginfo))
{
printf("WARNING: Expected %lu bytes, got %d\n",
sizeof(siginfo),len);
}
printf("RX signal %d:\n",siginfo.ssi_signo);
printf(" Error: %d\n",siginfo.ssi_errno);
printf(" PID : %d\n",siginfo.ssi_pid);
printf(" UID : %d\n",siginfo.ssi_uid);
printf(" UTime: %lu\n",siginfo.ssi_utime);
printf(" STime: %lu\n",siginfo.ssi_stime);
}
}


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From Newsgroup: comp.unix.programmer

Muttley@DastardlyHQ.org writes:

Hi

I've been playing around with signalfd() on linux and it seems to work but >there also seems to be zero information other than the signal number in the >signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler >set using sigaction with the siginfo_t passed to the handler, the below code >returns 0 for everything in the signalfd_siginfo structure.

I wonder if you still need to call sigaction with SA_SIGINFO before
using signalfd?
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From Newsgroup: comp.unix.programmer

Muttley@DastardlyHQ.org writes:

Hi

I've been playing around with signalfd() on linux and it seems to work but there also seems to be zero information other than the signal number in the signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler set using sigaction with the siginfo_t passed to the handler, the below code returns 0 for everything in the signalfd_siginfo structure.

The relevant parts of the code are below. Have I done something wrong?

struct signalfd_siginfo siginfo;
:
:
sigemptyset(&sigset);
sigaddset(&sigset,SIGINT); /* Control-C */
sigaddset(&sigset,SIGTSTP); /* Control-Z */

[...]

sigaction(2) documents the details regarding which fields in a siginfo structure will be set under which conditions and if were using C-c and
C-z to generate signals, the pid and uid values shouldn't be set. They
should be set when using kill(2) or sigqueue(2) to send signals.
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From Newsgroup: comp.unix.programmer

On Mon, 24 Feb 2025 14:18:44 GMT
scott@slp53.sl.home (Scott Lurndal) wibbled:

Muttley@DastardlyHQ.org writes:

Hi

I've been playing around with signalfd() on linux and it seems to work but >>there also seems to be zero information other than the signal number in the >>signalfd_siginfo structure returned from a read().

Where I'd expect PID and UID to be set as in reception of a signal handler >>set using sigaction with the siginfo_t passed to the handler, the below code >>returns 0 for everything in the signalfd_siginfo structure.

I wonder if you still need to call sigaction with SA_SIGINFO before
using signalfd?

Actually my mistake - the pid and uid are only zerop when it receives a signal from the terminal but when another process sends it it works ok. Thanks for
the idea though.

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From Newsgroup: comp.unix.programmer

On Mon, 24 Feb 2025 15:51:29 -0000 (UTC), Muttley wrote:

... the pid and uid are only zerop when it receives a
signal from the terminal but when another process sends it it works ok.

Just confirmed that with the following test program:

import asyncio
import linuxproc as lxp
from linuxproc import \
SIG, \
SigProcMask, \
SignalFile

async def main() :
to_block = SigProcMask({SIG.TERM, SIG.INT})
notif = SignalFile.create(to_block)
to_block.apply_replace()
while True :
evt = await notif.get_next_async(timeout = 1)
print(evt)
#end while
#end main

asyncio.run(main())

using my Python wrappers at <https://gitlab.com/ldo/python_linuxfs>.

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