[BUG] Multithreaded benchmark results in invalid JSON #2039
Description
Describe the bug
In some cases (but strangely, not all cases), a multi-threaded benchmark results in invalid JSON output when using --benchmark_format=json.
System
I was able to reproduce this on macOS and on Compiler Explorer, so I think this clearly is independent of the platform.
To reproduce
A benchmark like this reproduces the issue (unreliably):
#include "benchmark/benchmark.h"
static void BM_Ostream_number(benchmark::State& state) {
while (state.KeepRunning()) {
}
}
BENCHMARK(BM_Ostream_number)->Arg(3)->Threads(2);
BENCHMARK_MAIN();To really reproduce the issue reliably, it's best to perform something in the loop. For example: https://godbolt.org/z/Y97e6veaE
That example results in the following JSON:
{
"context": {
"date": "2025-09-12T13:38:31+00:00",
"host_name": "ce",
"executable": "./output.s",
"num_cpus": 2,
"mhz_per_cpu": 3598,
"cpu_scaling_enabled": false,
"aslr_enabled": false,
"caches": [
],
"load_avg": [0.679688,0.585449,0.533691],
"library_version": "v1.9.4-39-g16144154",
"library_build_type": "debug",
"json_schema_version": 1
},
"benchmarks": [
{
"name": "BM_Ostream_number/3/threads:2",
"family_index": 0,
"per_family_instance_index": 0,
"run_name": "BM_Ostream_number/3/threads:2",
"run_type": "iteration",
"repetitions": 1,
"repetition_index": 0,
"threads": 2,
"iterations": 1,172,451,248,
"real_time": 5.9873469468256479e-01,
"cpu_time": 5.9655409228580569e-01,
"time_unit": "ns"
}
]
}
Notice that the iterations key of BM_Ostream_number/3/threads:2 contains multiple comma-separated values, which is invalid JSON.
Expected behavior
The JSON should be valid, otherwise tooling can't do anything with the benchmark results.