[Issue]: [Bug][Linux] flm serve crash: C++ assertion failure '!this->empty()' in std::vector<logits_t>::back() #725

Problem Description

Crashed Thread Stack Trace:

1 Thread 1 (Thread 0x7f92069fd6c0 (LWP 3577901)):
2 #0 __pthread_kill_implementation (threadid=, signo=signo@entry=6, no_tid=no_tid@entry=0) at
pthread_kill.c:44
3 #1 0x00007f934609a243 in __pthread_kill_internal (threadid=, signo=6) at pthread_kill.c:89
4 #2 0x00007f934603e5d0 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
5 #3 0x00007f9346025685 in __GI_abort () at abort.c:77
6 #4 0x00007f934649d5bd in std::__glibcxx_assert_fail (file=, line=, function=, condition=) at ../../../../../gcc/libstdc++-v3/src/c++11/assert_fail.cc:41
7 #5 0x0000562ef5ae03ae in ?? ()
8 #6 0x0000562ef5d3bd67 in ?? ()

Root Cause & Technical Analysis (GDB Core Extraction):
By examining the assembly and register values inside flm right before the abort call (std::__glibcxx_assert_fail), we
extracted the RIP-relative string parameters:

• Assertion File: /usr/include/c++/16/bits/stl_vector.h (Line 1360)
• Assertion Function:

1 constexpr std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::back() [with _Tp = logits_t; _Alloc =
std::allocator<logits_t>; reference = logits_t&]

• Failed Condition: !this->empty()

The program counter at 0x0000562ef5ae03ae is preceded by the assertion instruction block:

1 0x0000562ef5ae038e: lea 0x6a5b31(%rip),%rcx # 0x562ef6185ec6 -> "!this->empty()"
2 0x0000562ef5ae0395: lea 0x6beb34(%rip),%rdx # 0x562ef619eed0 -> "std::vector<logits_t>::back()"
3 0x0000562ef5ae039c: mov $0x550,%esi # 0x550 = Line 1360
4 0x0000562ef5ae03a1: lea 0x6b3708(%rip),%rdi # 0x562ef6193ab0 -> "/usr/include/c++/16/bits/stl_vector.h"
5 0x0000562ef5ae03a8: call *0x94e75a(%rip) # Calls __glibcxx_assert_fail

This proves that flm called logits.back() when logits had a size of 0.

Environment & Package Details:

• OS: Arch Linux (Kernel 6.x / AMD Ryzen AI NPU configuration)
• Compiler/Libc: GCC 16 / glibc 2.40 / libstdc++ 16.x (compiled with standard -D_GLIBCXX_ASSERTIONS)
• FastFlowLM Version: fastflowlm 1.0.4-1
• Model in use: qwen3-it:4b (Qwen3-4B-Instruct-2507-NPU2)
• Hardware: AMD Ryzen NPU (managed via /usr/lib/libxrt_driver_xdna.so.2)

Proposed Fix:
Add a safety check to ensure that the logits vector is populated before fetching the trailing element:

1 if (!logits.empty()) {
2 auto last_logit = logits.back();
3 // ... process logit
4 } else {
5 // Handle empty logits list gracefully or return error code
6 }

Filed by Gemini via Gemini CLI.

Operating System

Omarchy 4.0.3-1

CPU

AMD Ryzen AI 5 340 w/ Radeon 840M

GPU

iGPU: AMD Radeon 840M (Krackan, 1002:1114) NPU: NPU Krackan 1 [0000:c2:00.1] (used by FastFlowLM)

ROCm Version

ROCm not installed. FastFlowLM uses XRT 2.21.75 + NPU firmware 1.1.2.64 (not the ROCm GPU stack).

Installation Method

Other (describe in Additional Information)

Installed ROCm Packages / Versions

ROCm Component

No response

Steps to Reproduce

No response

(Optional for Linux users) Output of rocminfo --support

Paste output here

Additional Information

No response