On 2017-11-01 07:29, C Smith wrote:
> I finally caught all the variables in a corefile in gdb:
> (gdb) bt
> #0 0xb76d70db in pthread_cond_wait@@GLIBC_2.3.2 () from
> /lib/libpthread.so.0
> #1 0xb76f97f4 in printer_loop (arg=0x0) at rt_print.c:685
> #2 0xb76d3adf in start_thread () from /lib/libpthread.so.0
> #3 0xb746444e in clone () from /lib/libc.so.6
> (gdb) print printer_wakeup
> $1 = {__data = {__lock = 0, __futex = 0, __total_seq = 0, __wakeup_seq = 0,
> __woken_seq = 0, __mutex = 0x0, __nwaiters = 0, __broadcast_seq = 0},
> __size = '\000' <repeats 47 times>, __align = 0}
> (gdb) print buffer_lock
> $2 = {__data = {__lock = 0, __count = 0, __owner = 0, __kind = 0, __nusers
> = 0, {__spins = 0, __list = {__next = 0x0}}}, __size = '\000' <repeats 23
> times>, __align = 0}
> (gdb) print buffers
> $3 = 4
> (gdb) print arg
> No symbol "arg" in current context.
> (gdb) print mask
> No symbol "mask" in current context.
> (gdb) print unlock
> $4 = {void (void *)} 0xb76f96f9 <unlock>
> (gdb) info threads
> Id Target Id Frame
> 2 Thread 0xb73686c0 (LWP 20462) 0xffffe424 in ?? ()
> * 1 Thread 0xb76f5b40 (LWP 20464) 0xb76d70db in
> pthread_cond_wait@@GLIBC_2.3.2
> () from /lib/libpthread.so.0
> (gdb) print &printer_wakeup
> $5 = (pthread_cond_t *) 0xb76fda20
> (gdb) print &buffer_lock
> $6 = (pthread_mutex_t *) 0xb76fd9fc
>
> I see now that all variables used in this function are static on the heap,
> and thus they are not null pointers, so what could cause a segfault?
The crash is inside pthread_cond_wait, likely while it tries to access
some pointer that the condvar or the mutex structure contains. You could
check if a) both have been properly initialized prior to the crash and
b) none of them have been corrupted by some faulty code. I suspect the
latter is problem here.
Once you identified a corrupted field, you could set a memory watchpoint
on its modification, hopefully tracking down the causing code.
Jan
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