Thread: C guru help needed URGENT

Do you see in the new trace that the poll timeout is being affected?

Code:

poll([{fd=4, events=POLLIN}, {fd=0, events=POLLIN}], 2, 5) = 0 (Timeout)
rt_sigprocmask(SIG_BLOCK, [ALRM], NULL, 8) = 0
gettimeofday({1348050630, xxxxx}, NULL) = 0
rt_sigprocmask(SIG_UNBLOCK, [ALRM], NULL, 8) = 0
poll([{fd=4, events=POLLIN}, {fd=0, events=POLLIN}], 2, 5) = 0 (Timeout)
rt_sigprocmask(SIG_BLOCK, [ALRM], NULL, 8) = 0
gettimeofday({1348050630, yyyyy}, NULL) = 0
rt_sigprocmask(SIG_UNBLOCK, [ALRM], NULL, 8) = 0

1. Read post #36 again.
If you have a statically linked executable, then PRELOAD will NOT work.
Yes, you may have the syntax sorted out, and the loader recognises it as a valid option, and it may even be loaded into the address space of the running program.
But a statically linked program will NOT call it.

Here's my test program.
The thing to note is that typing in a word beginning with c will cause it to crash.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <poll.h>

void doit ( void ) {
    while ( 1 ) {
        struct pollfd   p = { 0, POLLIN, 0 };
        int result = poll(&p,1,0);
        if ( result > 0 ) {
            char buff[100];
            int n = read(0,buff,sizeof(buff));
            int ch = buff[0];
            write(1,buff,n);
            if ( ch == 'q' ) break;
            if ( ch == 'c' ) {
                int *dummy = 0;
                *dummy = 0;
            }
        } else {
            struct timeval  now;
            int r = gettimeofday(&now,NULL);
        }
    }
}
           
int main ( ) {
    doit();
    return 0;
}

First, a bit of information for core files.
The core file can be inspected using a debugger.
How effective that is depends on the extent of the symbol table in the executable.

Here are three examples, with less information each time.

Code:

# Good - gdb will point at a line of code
$ gcc -g -static foo.c
$ ./a.out 
hello
hello
crash
crash
Segmentation fault (core dumped)
$ gdb a.out core
GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /home/sc/Documents/a.out...done.
[New LWP 4757]
Core was generated by `./a.out'.
Program terminated with signal 11, Segmentation fault.
#0  0x0000000000401102 in doit () at foo.c:86
86	                *dummy = 0;
(gdb) bt
#0  0x0000000000401102 in doit () at foo.c:86
#1  0x0000000000401149 in main () at foo.c:96
(gdb) q
$ 


# Bad - gdb will just print global function names
$ gcc -static foo.c
$ ./a.out 
hello
hello
crash
crash
Segmentation fault (core dumped)
$ gdb a.out core
GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /home/sc/Documents/a.out...(no debugging symbols found)...done.
[New LWP 4766]
Core was generated by `./a.out'.
Program terminated with signal 11, Segmentation fault.
#0  0x0000000000401102 in doit ()
(gdb) bt
#0  0x0000000000401102 in doit ()
#1  0x0000000000401149 in main ()
(gdb) q
$ 


# Ugly - gdb will just print hex addresses
$ strip a.out
$ ./a.out 
hello
hello
crash
crash
Segmentation fault (core dumped)
$ gdb a.out core
GNU gdb (Ubuntu/Linaro 7.3-0ubuntu2) 7.3-2011.08
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /home/sc/Documents/a.out...(no debugging symbols found)...done.
[New LWP 4771]
Core was generated by `./a.out'.
Program terminated with signal 11, Segmentation fault.
#0  0x0000000000401102 in ?? ()
(gdb) bt
#0  0x0000000000401102 in ?? ()
#1  0x0000000000401149 in ?? ()
#2  0x000000000040130a in ?? ()
#3  0x0000000000400f61 in ?? ()
#4  0x00007fffc78fa318 in ?? ()
#5  0x0000000000000000 in ?? ()
(gdb) q

If you've got a core file, can you try with gdb to see what other information you can obtain.

The "resource unavailable" just means that there was no data for recv() at that moment in time.
After each one, there is another call to poll() to wait for a while until more data arrives.
Do you also get these when you run just a single instance of your code?


> --- SIGSEGV (Segmentation fault) @ 0 (0) ---
> +++ killed by SIGSEGV (core dumped) +++
This is just the code trying to dereference a NULL pointer.
I get the same when I trace my test program.

Code:

poll([{fd=0, events=POLLIN}], 1, 0)     = 0 (Timeout)
poll([{fd=0, events=POLLIN}], 1, 0)     = 0 (Timeout)
poll([{fd=0, events=POLLIN}], 1, 0)     = 0 (Timeout)

poll([{fd=0, events=POLLIN}], 1, 0)     = 1 ([{fd=0, revents=POLLIN}])
read(0, "crash\n", 100)                 = 6
write(1, "crash\n", 6crash
)                  = 6
--- SIGSEGV (Segmentation fault) @ 0 (0) ---
+++ killed by SIGSEGV +++
Segmentation fault

I don't think you can infer anything from the trace about this crash.
Without source code, this is going to be tough to solve.