GDB debugging practice (7) signal processing

GDB Sending signal

stay GDB Debugging status , You can enter... At the command number signal Signal name To send a signal to the program .

Input signal Press down tab The signal name is displayed ：

(gdb) signal 
Display all 152 possibilities? (y or n)
32                   SIG108               SIG125               SIG46                SIG63                SIG80                SIG97                SIGIO                SIGSTOP
33                   SIG109               SIG126               SIG47                SIG64                SIG81                SIG98                SIGKILL              SIGSYS
34                   SIG110               SIG127               SIG48                SIG65                SIG82                SIG99                SIGLIBRT             SIGTERM
EXC_ARITHMETIC       SIG111               SIG32                SIG49                SIG66                SIG83                SIGABRT              SIGLOST              SIGTRAP
EXC_BAD_ACCESS       SIG112               SIG33                SIG50                SIG67                SIG84                SIGALRM              SIGLWP               SIGTSTP
EXC_BAD_INSTRUCTION  SIG113               SIG34                SIG51                SIG68                SIG85                SIGBUS               SIGMSG               SIGTTIN
EXC_BREAKPOINT       SIG114               SIG35                SIG52                SIG69                SIG86                SIGCANCEL            SIGPHONE             SIGTTOU
EXC_EMULATION        SIG115               SIG36                SIG53                SIG70                SIG87                SIGCHLD              SIGPIPE              SIGURG
EXC_SOFTWARE         SIG116               SIG37                SIG54                SIG71                SIG88                SIGCONT              SIGPOLL              SIGUSR1
SIG100               SIG117               SIG38                SIG55                SIG72                SIG89                SIGDANGER            SIGPRIO              SIGUSR2
SIG101               SIG118               SIG39                SIG56                SIG73                SIG90                SIGEMT               SIGPROF              SIGVTALRM
SIG102               SIG119               SIG40                SIG57                SIG74                SIG91                SIGFPE               SIGPWR               SIGWAITING
SIG103               SIG120               SIG41                SIG58                SIG75                SIG92                SIGGRANT             SIGQUIT              SIGWINCH
SIG104               SIG121               SIG42                SIG59                SIG76                SIG93                SIGHUP               SIGRETRACT           SIGWIND
SIG105               SIG122               SIG43                SIG60                SIG77                SIG94                SIGILL               SIGSAK               SIGXCPU
SIG106               SIG123               SIG44                SIG61                SIG78                SIG95                SIGINFO              SIGSEGV              SIGXFSZ
SIG107               SIG124               SIG45                SIG62                SIG79                SIG96                SIGINT               SIGSOUND        

GDB Processing signals

background ：gdb The default behavior of is to capture the signal , Instead of passing it to the debugger , We can change this behavior through command configuration

stay GDB in handle The command is used to set GDB For signal processing , You can enter help handle Check it out. .

(gdb) help handle
Specify how to handle signals.
Usage: handle SIGNAL [ACTIONS]
Args are signals and actions to apply to those signals.
If no actions are specified, the current settings for the specified signals
will be displayed instead.

Symbolic signals (e.g. SIGSEGV) are recommended but numeric signals
from 1-15 are allowed for compatibility with old versions of GDB.
Numeric ranges may be specified with the form LOW-HIGH (e.g. 1-5).
The special arg "all" is recognized to mean all signals except those
used by the debugger, typically SIGTRAP and SIGINT.

Recognized actions include "stop", "nostop", "print", "noprint",
"pass", "nopass", "ignore", or "noignore".
Stop means reenter debugger if this signal happens (implies print).
Print means print a message if this signal happens.
Pass means let program see this signal; otherwise program doesn't know.
Ignore is a synonym for nopass and noignore is a synonym for pass.
Pass and Stop may be combined.

Multiple signals may be specified.  Signal numbers and signal names
may be interspersed with actions, with the actions being performed for
all signals cumulatively specified.

Summarize the general meaning ：

• nostop
  When the debugged program receives a signal ,GDB It won't stop the program , But a message will be sent to tell you that you receive this signal .
• stop
  When the debugged program receives a signal ,GDB Will stop your program .
• print
  When the debugged program receives a signal ,GDB A message will be displayed .
• noprint
  When the debugged program receives a signal ,GDB Will not tell you the message of the signal .
• pass 、noignore
  When the debugged program receives a signal ,GDB Do not process signals . This means ,GDB Will give this signal to the debugged program to process .
• nopass、ignore
  When the debugged program receives a signal ,GDB The debugged program will not be allowed to process this signal .

above 6 Item Configuration , Actually, it should be divided into 3 A state in which groups can be superimposed on each other . namely ：
stop/nostop

print/noprint

pass/nopass

stay GDB Can be used in info signals and info handle To see what signals are being GDB Detection neutralization GDB Treatment of it :

(gdb) info signals
Signal        Stop	Print	Pass to program	Description

SIGHUP        Yes	Yes	Yes		Hangup
SIGINT        Yes	Yes	No		Interrupt
SIGQUIT       Yes	Yes	Yes		Quit
SIGILL        Yes	Yes	Yes		Illegal instruction
SIGTRAP       Yes	Yes	No		Trace/breakpoint trap
SIGABRT       Yes	Yes	Yes		Aborted
SIGEMT        Yes	Yes	Yes		Emulation trap
SIGFPE        Yes	Yes	Yes		Arithmetic exception
SIGKILL       Yes	Yes	Yes		Killed
SIGBUS        Yes	Yes	Yes		Bus error
SIGSEGV       Yes	Yes	Yes		Segmentation fault
SIGSYS        Yes	Yes	Yes		Bad system call
SIGPIPE       Yes	Yes	Yes		Broken pipe
SIGALRM       No	No	Yes		Alarm clock
...

actual combat

#include <signal.h>
#include <stdio.h>

void signalHandle(int para)
{
    
    printf("in signal handle,signal num:%d\n", para);
}

void signalHandle2(int para)
{
    
    printf("in signal handle,signal num:%d\n", para);
    exit(0);
}


int main(void)
{
    
    signal(SIGUSR1, signalHandle);
    signal(SIGUSR2, signalHandle2);

    while(1)
    {
    
        printf("press any key to send a signal\n");
        getchar();
        raise(SIGUSR1);
     }

    return 0;
}

For better results , Run the program first , then gdb attach To the process , Get into gdb Debug mode

(gdb) c
Continuing.

Program received signal SIGUSR1, User defined signal 1.
0x00007fd1f1ec9337 in raise () from /lib64/libc.so.6
(gdb) c
Continuing.

You can find that when you press enter in the program , The process sent itself SIGUSR1 The signal , also GDB When the signal is captured, stop , We continue to run ,
take GDB Yes SIGUSR1 The signal processing is set to nostop

(gdb) handle SIGUSR1 nostop
Signal        Stop	Print	Pass to program	Description
SIGUSR1       No	Yes	Yes		User defined signal 1
(gdb) c
Continuing.

Program received signal SIGUSR1, User defined signal 1

You can see that only , and gdb Won't stop , Next
take GDB Yes SIGUSR1 The signal processing is set to noprint

(gdb) handle SIGUSR1 noprint
Signal        Stop	Print	Pass to program	Description
SIGUSR1       No	No	Yes		User defined signal 1
(gdb) c
Continuing

here GDB Neither print nor stop .