Printf is not working in C signal handler

Question

Code in question first (minimized case):

#include <stdio.h> #include <signal.h> int counter = 0; void react_to_signal(int n) { fprintf(stderr, "Caught!\n"); counter++; } int main(int argc, char** argv) { signal(SIGINFO, react_to_signal); while (1) { printf("%d\n", counter); } return 0; }

I run the code, it loops as it should, printing out 0. Then in another shell..

kill -s SIGINFO <pid_of_my_process>

Signal is delivered, c is incremented .. but the fprintf doesn't happen.

Why is this so? In what environment/context does handler code run? Where can I read up on this?

Don't use printf() et.al. inside signal handlers. They are not signal safe (eg: can call malloc(), which is not signal-safe) — wildplasser
– wildplasser, Commented Mar 3, 2012 at 17:05
Amusingly, I could've sworn I'd tried a variant with fflush before I posted. Either never recompiled and ran, or shot myself in the foot with the infinite loop moving the message off screen. So unromantic and dumb. — ntl0ve
– ntl0ve, Commented Mar 3, 2012 at 18:43

Pavan Manjunath · Accepted Answer · 2023-10-09 21:56:49Z

21

In short: you cannot safely use printf within a signal handler.

There's a list of authorized functions in signal handler's man page. There is not fprintf in it.

That's because this function is not reentrant, mainly because it can use malloc and free. See this post for a detailed explanation.

edited Oct 9, 2023 at 21:56

Pavan Manjunath

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answered Mar 3, 2012 at 17:05

Coren

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1 Comment

Chris Dodd Over a year ago

In addition to potentially calling malloc/free (for buffer management), its internal buffer management is not reentrant, so if a signal occurs in the middle of a printf call, the fprintf call may end up corrupting the buffer in odd ways, leading to garbage or missed output.

James McLeod · Accepted Answer · 2012-03-03 17:03:08Z

2

You may need to fflush stderr to get the message to write before the program exits.

answered Mar 3, 2012 at 17:03

James McLeod

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2 Comments

adentinger Over a year ago

Doesn't printf automatically flush when it reaches a newline?

James McLeod Over a year ago

No. See stackoverflow.com/questions/5229096/…

Chris Dodd · Accepted Answer · 2023-10-09 22:29:11Z

In addition to the reentrancy problems, since counter is not volatile, its read might get hoisted out of the while(1) loop, leading to the printf in the loop always printing 0 even when the signal handler increments the global variable.

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