How can I use GDB to debug code with signal handling? - gdb

Question

I am running below code in debug. As soon as parent process receives a signal the debugger show that I am in parent handler code on stepping in. But from there on , I have no control over GDB. It enters some file when I hit continue and stops with following message:
Program received signal SIGTRAP, Trace/breakpoint trap.
_dl_runtime_resolve () at ../sysdeps/x86_64/dl-trampoline.S:34
34 ../sysdeps/x86_64/dl-trampoline.S: No such file or directory.
(gdb) continue
And when I try to continue , it repeats with new files.
Output of show follow-fork-mode and show detach-on-fork
(gdb) b 78
Breakpoint 1 at 0x400bba: file main.c, line 78.
(gdb) run
Starting program: /home/samkit/Posix_Programming/unix_Simple_prodcon_con_prob/main
Child sending signal
Breakpoint 1, main (argc=1, argv=0x7fffffffdf48) at main.c:79
79 if(cpid==0){
(gdb) show follow-fork-mode
Debugger response to a program call of fork or vfork is "parent".
(gdb) show detach-on-fork
Whether gdb will detach the child of a fork is on.
(gdb)
Also tried switching to child process in GDB , setting breakpoint at line 78 and then RUN. Child process sends signal to parent and receives SIGUSR2 from parent. And hence enters its own handler. And after that the GDB is not able to return(similar to above) Below is the output:
(gdb) set follow-fork-mode child
(gdb) set detach-on-fork on
(gdb) b 78
Breakpoint 1 at 0x400bba: file main.c, line 78.
(gdb) run
Starting program: /home/samkit/Posix_Programming/unix_Simple_prodcon_con_prob/main
[New process 31109]
[Switching to process 31109]
Breakpoint 1, main (argc=1, argv=0x7fffffffdf48) at main.c:79
79 if(cpid==0){
(gdb) next
83 if((child_sm_id = shmget(key,size*sizeof(char),0))==-1){ // Allocate shared memory
(gdb) next
87 if((child_sm_ptr = shmat(child_sm_id,NULL,0)) == (void *)-1){ // Attach shared memory to your address space
(gdb) next
95 printf("Child sending signal\n");
(gdb) next
Child sending signal
96 kill(getppid(),SIGUSR1);
(gdb) next
Received Signal 10
Sending signal
Program received signal SIGUSR2, User defined signal 2.
0x00007ffff7a4b267 in kill () at ../sysdeps/unix/syscall-template.S:81
81 ../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) next
childHandler (sig=0) at main.c:25
25 void childHandler(int sig){
(gdb) next
26 printf("Child received Signal %d\n",sig);
(gdb) next
Program received signal SIGTRAP, Trace/breakpoint trap.
0x00000000004006e6 in printf#plt ()
(gdb) next
Single stepping until exit from function printf#plt,
which has no line number information.
0x00000000004006b0 in ?? ()
(gdb) next
Cannot find bounds of current function
(gdb)
I also tried continue but I think since GDB saw a TRAP , it is re-executing the TRAP again and again , if I try continue.
(gdb) continue
Continuing.
Program received signal SIGTRAP, Trace/breakpoint trap.
0x00000000004006b6 in ?? ()
(gdb) ccontinue
Undefined command: "ccontinue". Try "help".
(gdb) continue
Continuing.
Program received signal SIGTRAP, Trace/breakpoint trap.
_dl_runtime_resolve () at ../sysdeps/x86_64/dl-trampoline.S:34
34 ../sysdeps/x86_64/dl-trampoline.S: No such file or directory.
(gdb) continue
Continuing.
Now my questions:
Q1) How can I make GDB actually continue with the execution ?
Q2) I have 2 processes.If I put a breakpoint after fork and do info threads, it shows 1 process and 1 thread. Shouldn't it show 2 processes ?
Code:
# include <signal.h>
# include <stdio.h>
# include <sys/types.h>
# include <sys/fcntl.h>
# include <sys/shm.h>
# include <sys/ipc.h>
# include <unistd.h>
# include <sys/wait.h>
#include <string.h>
#include <stdlib.h>
#define BUFF_SIZE 1024
struct smbuffer{
char buffer[1024];
int count;
};
void parentHandler(int sig){
printf("Received Signal %d\n",sig);
}
void childHandler(int sig){
printf("Child received Signal %d\n",sig);
}
int main( int argc, char* argv[] ) {
/* Child related attributes */
pid_t cpid;
int status;
int retval;
/* Shared memory initialization */
int sm_id;
struct smbuffer *sm_ptr;
key_t key = 1100;
int size = BUFF_SIZE;
if((sm_id = shmget(key , size * sizeof(char) , 0666|IPC_CREAT))== -1){ // Allocate shared memory
perror("SHMGET Failed : \n");
exit(1);
}
if((sm_ptr = shmat(sm_id,NULL,0))==(void *)-1){ // Attach shared memory to your address space
perror("SHMAT Failed : \n");
exit(1);
}
// Intilitialing Signals and handler
struct sigaction childold;
struct sigaction childnew;
childnew.sa_handler = childHandler;
sigset_t childmask ;
sigemptyset(&childmask);
childnew.sa_mask = childmask;
childnew.sa_flags = 0;
sigaction(SIGUSR2,&childnew,&childold);
// Intilitialing Signals and handler
struct sigaction parentold;
struct sigaction parentnew;
parentnew.sa_handler = parentHandler;
sigset_t parentmask ;
sigemptyset(&parentmask);
parentnew.sa_mask = parentmask;
parentnew.sa_flags = 0;
sigaction(SIGUSR1,&parentnew,&parentold);
if((cpid = fork())== -1){
perror("Fork Failed :\n");
}
if(cpid==0){
/* Child code */
struct smbuffer *child_sm_ptr;
int child_sm_id ;
if((child_sm_id = shmget(key,size*sizeof(char),0))==-1){ // Allocate shared memory
perror("Child shmget failed :\n");
exit(2);
}
if((child_sm_ptr = shmat(child_sm_id,NULL,0)) == (void *)-1){ // Attach shared memory to your address space
perror("Child shmat failed :\n");
exit(2);
}
// Signal parent and wait for signal from parent. Loop
while(1){
printf("Child sending signal\n");
kill(getppid(),SIGUSR1);
sigsuspend(&childmask);
sleep(1);
}
if(shmdt(child_sm_ptr)==-1){ // De-attach shared memory
perror("Child shmdt failed :\n");
exit(2);
}
printf("Child exit\n");
return 0;
}
else{
/* Parent code */
// Wait for child signal and signal child. Loop
while(1){
sigsuspend(&parentmask);
printf("Sending signal\n");
kill(cpid,SIGUSR2);
sleep(1);
}
}
if(shmdt(sm_ptr)==-1){ // De-attach shared memory
perror("SHMDT failed :\n");
exit(1);
}
// Free shared memory
if((shmctl(sm_id, IPC_RMID, NULL)) == -1){
perror("Shmctl failed \n");
exit(1);
}
printf("End of code\n");
return 0;
} // End of main