This means there was an error (some function returned -1), but the
error message was not set. Could you capture the backtrace when this
happens? It's hard to find the exact point, but going function and
checking what returns -1 should do.
Assuming by "backtrace" you mean the strace output, I went to the end of the file, and then did a reverse search for -1; here were the first hits, with some context:
25533 clock_gettime(0x7 /* CLOCK_??? */, {17970, 938005692}) = 0 25533 clock_gettime(0x7 /* CLOCK_??? */, {17970, 938038620}) = 0 25533 select(7, [3 4], [], NULL, NULL) = 1 (in [3]) 25533 clock_gettime(0x7 /* CLOCK_??? */, {17970, 938107743}) = 0 25533 read(3, "[\1\220\376xM\365a\206\350^8\236\315T\265#\353\270r\201\301|f\30<2 \347\276\37T:>|\241\2403\256\203GN\316=\233N\221\313\32\3630{\255\374O\360\263\374\353l\324\206\272o['I\205+\30\351\304LG\374\202\216\260\340H\342eL\214\377\3131(@:C\202\300\354x\235D\256\355\204G\261\rT5\247lK\364\342\7\232\231\210\25o\23\275\260\325\337\17E\255\300\211\254\277k\375g\347\347\5\273\324\255\324\220A\311\305\365{,J\351DE\245\337\272JBr\243\361\r\2226\330S\354\212$b\1J\37&C\200\225U\351\335", 8192) = 176 25533 close(4) = 0 25533 close(5) = 0 25533 clock_gettime(0x7 /* CLOCK_??? */, {17970, 938396231}) = 0 25533 clock_gettime(0x7 /* CLOCK_??? */, {17970, 938427891}) = 0 25533 close(6) = 0 25533 select(7, [3], [3], NULL, NULL) = 1 (out [3]) 25533 clock_gettime(0x7 /* CLOCK_??? */, {17970, 938544157}) = 0 25533 write(3, "e\303:\261h\272\30\335\373\316\26\323\251\16R\255\310b\226\300\227M*\0\245\326f\217\365J\177*\336\333\3427", 36) = 36 25533 rt_sigaction(SIGWINCH, NULL, {0x557af9c49660, [], SA_RESTORER, 0x7f76f92162f0}, 8) = 0 25533 rt_sigaction(SIGWINCH, {SIG_DFL, [], SA_RESTORER, 0x7f76f92162f0}, NULL, 8) = 0 25533 write(3, "\343(\317\376\241\r\36\306\6\344T\302\217mdTt\23\326\225\330'\375$\7\215\307\0301\211\226\210\\\261x\277\245\342\3149\206R\223@\26\233M\345|\320\337 x\336\325\370'\17w~", 60) = 60 25533 ioctl(0, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7ffe60de40e0) = -1 ENOTTY (Inappropriate ioctl for device) 25533 fcntl(0, F_GETFL) = 0x802 (flags O_RDWR|O_NONBLOCK) 25533 fcntl(0, F_SETFL, O_RDWR) = 0 25533 ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7ffe60de40e0) = -1 ENOTTY (Inappropriate ioctl for device) 25533 fcntl(1, F_GETFL) = 0x2 (flags O_RDWR) 25533 ioctl(2, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, 0x7ffe60de40e0) = -1 ENOTTY (Inappropriate ioctl for device) 25533 fcntl(2, F_GETFL) = 0x801 (flags O_WRONLY|O_NONBLOCK) 25533 fcntl(2, F_SETFL, O_WRONLY) = 0 25533 shutdown(3, SHUT_RDWR) = 0 25533 close(3) = 0 25533 exit_group(0) = ? 25533 +++ exited with 0 +++
Thanks for the quick reply (and apologies for the Thanksgiving-induced delay in my response),