What causes "%s " in dotnet/runtime?

Read the full error line printed — it names the specific CoreCLR failure.

%s

Error message

%s

What it means

log_error_info() at src/coreclr/hosts/corerun/corerun.cpp:253 is the error-writer callback registered with CoreCLR via coreclr_set_error_writer (line 596). CoreCLR invokes it with diagnostic strings (assembly-load failures, binding errors, runtime errors) and corerun writes them verbatim to stderr. Any CoreCLR-reported error surfaces through this line.

Source

Thrown at src/coreclr/hosts/corerun/corerun.cpp:253

        }

        std::fprintf(fd, "Managed assembly: %s\n", _managedAssembly);
        std::fprintf(fd, "Arguments (%d): ", _argc);
        for (int i = 0; i < _argc; ++i)
        {
            std::fprintf(fd, "%s ", _argv[i]);
        }
        std::fprintf(fd, "\n");
    }
};

// The current CoreCLR instance details.
static void* CurrentClrInstance;
static unsigned int CurrentAppDomainId;

static void log_error_info(const char* line)
{
    std::fprintf(stderr, "%s\n", line);
}

size_t HOST_CONTRACT_CALLTYPE get_runtime_property(
    const char* key,
    char* value_buffer,
    size_t value_buffer_size,
    void* contract_context)
{
    configuration* config = static_cast<configuration *>(contract_context);

    if (::strcmp(key, HOST_PROPERTY_ENTRY_ASSEMBLY_NAME) == 0)
    {
        // Compute the entry assembly name based on the entry assembly full path
        pal::string_t dir;
        pal::string_t file;
        pal::split_path_to_dir_filename(config->entry_assembly_fullpath, dir, file);
        file = file.substr(0, file.rfind(W('.')));

View on GitHub (pinned to 60108ba66e)

Solutions

Read the full error line printed — it names the specific CoreCLR failure.
Verify the TPA list and APP_PATHS properties point at existing assemblies.
Ensure System.Private.CoreLib.dll and the BCL assemblies are present in the coreclr directory.
Rebuild/restore the target framework so all dependencies exist.

Defensive patterns

Strategy: try-catch

Validate before calling

for (auto& p : required_assembly_paths) {
    if (!pal::file_exists(p.c_str())) {
        pal::fprintf(stderr, W("Missing required assembly: %s\n"), p.c_str());
        return EXIT_FAILURE;
    }
}

Try / catch

int hr = coreclr_initialize_func(exePath, ...);
if (FAILED(hr)) {
    fprintf(stderr, "coreclr_initialize failed: hr=0x%08x\n", hr);
    return EXIT_FAILURE;
}

Prevention

Validate all assembly paths exist before calling coreclr_initialize.
Inspect coreclr_initialize/coreclr_execute_assembly return codes, not just the error-writer output.

When it happens

Trigger: CoreCLR emits an error during initialize/execute/shutdown — e.g. a missing dependency assembly, TPA list misconfiguration, trust/casablanca failure, or a surfaced unhandled managed exception.

Common situations: Missing System.Private.CoreLib.dll or other TPA assemblies; wrong TRUSTED_PLATFORM_ASSEMBLIES property; APP_PATHS pointing at nonexistent assemblies; framework version mismatch.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/ab9b06bdfbfc75b4. Report an issue: GitHub.