Handler - get PID of child process

**Empair** · 11-20-2020

Code:

#include <stdio.h>#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/wait.h>


int const pipeCount = 1;
int pipes[pipeCount][2];




#undef si_pid


void handler_readPipeSIGIO(int sig, siginfo_t *info, void *secret) 
{


    printf( "info->_sifields._sigchld.si_pid: %x\n",  info->_sifields._sigchld.si_pid);


    char buf[ 333 ];
    int ret = read(pipes[0][0], buf, sizeof( buf ));
    if ( ret < 0 ) 
    {
        printf( "Read fail! %d\n", sig);
        fflush(stdout);
    }
    else
    {
        buf[ ret ] = 0;
        printf( "Read data form Process: %s", buf );
        fflush(stdout);
    }


}


int main () 
{
    // Create Pipes
    for(int i = 0; i<pipeCount; i++)
    {
        pipe(pipes[i]);
    }


    // Create Children
    for(int j = 0; j<pipeCount; j++)
    {
        if(fork()==0)
        {
        //CloseAllPipiesButTheOne
            for(int i=0; i<pipeCount; i++)
            {
                close(pipes[i][0]);
                //printf("Process %d Pipe %d End 0 is CLOSE\n", j, i);
                if(j==i)
                {
                    ///printf("Process %d Pipe %d End 1 is OPEN\n", j, i);
                    //printf("MyChildD %d \n", getpid());
                }
                else
                {
                    //printf("Process %d Pipe %d End 1 is CLOSE\n", j, i);
                    close(pipes[i][1]);
                }
            }
            //fflush(stdout);
            //Write data to 0 End of pipes ... 
            while ( 1 )
            {   
                //printf("Writing to Pipe from Child - %d \n", getpid());
                fflush(stdout);
                char buf[ 333 ];
                //sprintf( buf, "Pid: (%d) Random Number: %d\n", getpid(), rand() % 10000 );
                sprintf( buf, "Pid: (%d)\n", getpid());
                write( pipes[j][1], buf, strlen( buf ) );
                sleep( 2 + rand() % 5 );
            }
            return 0;
        }
    }


    //close Pipes
    for(int i=0; i<pipeCount; i++)
    {
        close(pipes[i][1]);
    }


    //SigAction Define


    struct sigaction sa;
    sa.sa_sigaction = handler_readPipeSIGIO; 
    sa.sa_flags = SA_RESTART | SA_SIGINFO; 
    sigemptyset( &sa.sa_mask ); 


    sigaction(SIGIO, &sa, NULL);


    //config read reaction for SIGIO


    for(int i=0; i<pipeCount; i++)
    {
        fcntl( pipes[i][0], F_SETFL, fcntl( pipes[i][0], F_GETFL ) | O_ASYNC );
        fcntl( pipes[i][0], F_SETSIG, SIGIO );
        fcntl( pipes[i][0], F_SETOWN, getpid() );
    }




    printf("Some out ... \n");
    fflush(stdout);
    while(1) sleep (1);
    //wait(NULL);
    return(0);
}

and my output is

Code:

info->_sifields._sigchld.si_pid: 41Read data form Process: Pid: (6327)
info->_sifields._sigchld.si_pid: 41
Read data form Process: Pid: (6327)
info->_sifields._sigchld.si_pid: 41
Read data form Process: Pid: (6327)

where is my mistake?

**Salem** · 11-20-2020

> where is my mistake?
How do we know what you were expecting?

You shouldn't be calling printf in signal handlers.
signal-safety(7) - Linux manual page

**Empair** · 11-21-2020

Originally Posted by Salem

> where is my mistake?
How do we know what you were expecting?

You shouldn't be calling printf in signal handlers.
signal-safety(7) - Linux manual page

ok I expressed myself wrong
i am using prinf in handler only for debug ...

So my questing is: How a get i PID of process witch calling a Handler?

**Salem** · 11-21-2020

Your code doesn't compile.

**flp1969** · 11-21-2020

Originally Posted by Empair

So my questing is: How a get i PID of process witch calling a Handler?

Didn't you mean how to get the PID of the process dispatching the signal? Because the PID of the process "calling" the handler will be the same as the process which the handler was registered by sigaction, isn't it?

And you already did it... But I recomend you do this:

Code:

// Initialize sa_flags and sa_sigaction and ZERO anything else!
struct sigaction sa = { .sa_flags = SA_SIGINFO, .sa_sigaction = handler_readPipeSIGIO };

// and then register the handler.
sigaction( SIGIO, &sa, NULL );

**Empair** · 11-21-2020

Originally Posted by Salem

Your code doesn't compile.

strange

my be i upload one of my attempts
sorry

try this

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/wait.h>

int const pipeCount = 1;
int pipes[pipeCount][2];

void handler_readPipeSIGIO(int sig, siginfo_t *info, void *secret) 
{

    printf( "info->si_pid: %d\n",  info->si_pid);

    char buf[ 333 ];
    int ret = read(pipes[0][0], buf, sizeof( buf ));
    if ( ret < 0 ) 
    {
        printf( "Read fail! %d\n", sig);
        fflush(stdout);
    }
    else
    {
        buf[ ret ] = 0;
        printf( "Read data form Process: %s", buf );
        fflush(stdout);
    }

}

int main () 
{
    // Create Pipes
    for(int i = 0; i<pipeCount; i++)
    {
        pipe(pipes[i]);
    }

    // Create Children
    for(int j = 0; j<pipeCount; j++)
    {
        if(fork()==0)
        {
        //CloseAllPipiesButTheOne
            for(int i=0; i<pipeCount; i++)
            {
                close(pipes[i][0]);
                //printf("Process %d Pipe %d End 0 is CLOSE\n", j, i);
                if(j==i)
                {
                    ///printf("Process %d Pipe %d End 1 is OPEN\n", j, i);
                    //printf("MyChildD %d \n", getpid());
                }
                else
                {
                    //printf("Process %d Pipe %d End 1 is CLOSE\n", j, i);
                    close(pipes[i][1]);
                }
            }
            //fflush(stdout);
            //Write data to 0 End of pipes ... 
            while ( 1 )
            {   
                //printf("Writing to Pipe from Child - %d \n", getpid());
                fflush(stdout);
                char buf[ 333 ];
                //sprintf( buf, "Pid: (%d) Random Number: %d\n", getpid(), rand() % 10000 );
                sprintf( buf, "Pid: (%d)\n", getpid());
                write( pipes[j][1], buf, strlen( buf ) );
                sleep( 2 + rand() % 5 );
            }
            return 0;
        }
    }

    //close Pipes
    for(int i=0; i<pipeCount; i++)
    {
        close(pipes[i][1]);
    }

    //SigAction Define

    struct sigaction sa;
    sa.sa_sigaction = handler_readPipeSIGIO; 
    sa.sa_flags = SA_RESTART | SA_SIGINFO; 
    sigemptyset( &sa.sa_mask ); 

    sigaction(SIGIO, &sa, NULL);

    //config read reaction for SIGIO

    for(int i=0; i<pipeCount; i++)
    {
        fcntl( pipes[i][0], F_SETFL, fcntl( pipes[i][0], F_GETFL ) | O_ASYNC );
        fcntl( pipes[i][0], F_SETSIG, SIGIO );
        fcntl( pipes[i][0], F_SETOWN, getpid() );
    }


    printf("Some out ... \n");
    fflush(stdout);
    while(1) sleep (1);
    //wait(NULL);
    return(0);
}

**flp1969** · 11-21-2020

Originally Posted by Empair

So my questing is: How a get i PID of process witch calling a Handler?

Didn't you mean how to get the PID of the process dispatching the signal? Because the PID of the process "calling" the handler will be the same as the process which the handler was registered by sigaction, isn't it?

And you already did it... But I recomend you do this:

Code:

// Initialize sa_flags and sa_sigaction and ZERO anything else!
struct sigaction sa = { .sa_flags = SA_SIGINFO, .sa_sigaction = handler_readPipeSIGIO };

// and then register the handler.
sigaction( SIGIO, &sa, NULL );

And, as always, Salem is right. printf() family of functions aren't AS-Safe. You should use write() syscall and your own itoa() function (since it's not present anymore).

**Salem** · 11-21-2020

Well it might compile, but you need to compile this 'C' code using a C++ compiler.
So I made your 'const int' a #define instead.

Code:

$ g++ -D_GNU_SOURCE  -Wall main.c
$ gcc -D_GNU_SOURCE  -Wall main.c
main.c:11:5: error: variably modified ‘pipes’ at file scope
 int pipes[pipeCount][2];
     ^
$ gcc -D_GNU_SOURCE  -Wall main.c

The next problem is that si_pid isn't a unique data member, at least not on my system.

Code:

$ gcc -D_GNU_SOURCE -g -Wall main.c
$ gdb -q ./a.out
Reading symbols from ./a.out...done.
(gdb) b handler_readPipeSIGIO 
Breakpoint 1 at 0x400a25: file main.c, line 14.
(gdb) run
Starting program: ./a.out 

Breakpoint 1, handler_readPipeSIGIO (sig=29, info=0x7fffffffd5b0, secret=0x7fffffffd480) at main.c:14
14	{
(gdb) p *info
$1 = {si_signo = 29, si_errno = 0, si_code = 1, _sifields = {_pad = {65, 0, 3, 0, 0, 0, -136507304, 32767, -9600, 32767, -134229952, 32767, 0, 0, 0, 0, 1024, 0, -136418554, 32767, 1, 0, 
      0, 0, 404, 0, -136481568, 32767}, _kill = {si_pid = 65, si_uid = 0}, _timer = {si_tid = 65, si_overrun = 0, si_sigval = {sival_int = 3, sival_ptr = 0x3}}, _rt = {si_pid = 65, 
      si_uid = 0, si_sigval = {sival_int = 3, sival_ptr = 0x3}}, _sigchld = {si_pid = 65, si_uid = 0, si_status = 3, si_utime = 0, si_stime = 140737351848024}, _sigfault = {
      si_addr = 0x41, si_addr_lsb = 3, si_addr_bnd = {_lower = 0x0, _upper = 0x7ffff7dd1058}}, _sigpoll = {si_band = 65, si_fd = 3}, _sigsys = {_call_addr = 0x41, _syscall = 3, 
      _arch = 0}}}
(gdb) ptype info
type = struct {
    int si_signo;
    int si_errno;
    int si_code;
    union {
        int _pad[28];
        struct {...} _kill;
        struct {...} _timer;
        struct {...} _rt;
        struct {...} _sigchld;
        struct {...} _sigfault;
        struct {...} _sigpoll;
        struct {...} _sigsys;
    } _sifields;
} *

You need to use si_code to figure out which union member of _sifields to go looking in.

Not that it would help much in your case, but it doesn't look like there is anything resembling the pid of the signalling process in the struct in any of the union variants.

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