Tencent/matrix · error

Failed to create file regs.txt

Error message

Failed to create file regs.txt

What it means

In libunwindstack's unwind_for_offline tool, SaveRegs() opens 'regs.txt' in the current working directory with fopen("regs.txt","w+") to dump the captured register values. If the file cannot be created (fopen returns nullptr), the tool reports 'Failed to create file regs.txt' via perror and aborts the save. perror appends the errno reason (e.g. Permission denied, Read-only file system) to the message.

Solutions

1. cd to a writable directory (e.g. /data/local/tmp) before running unwind_for_offline
2. Check and fix errno details printed by perror: fix permissions (chmod/chown) or free disk space
3. Remove or rename an existing regs.txt that is a directory or otherwise unopenable
4. Run with a user/SELinux context permitted to write in the target directory

Example fix

// before
fp(fopen("regs.txt", "w+"), &fclose);
// after (run from a writable dir, or pass an output path)
chdir("/data/local/tmp");
fp(fopen("regs.txt", "w+"), &fclose);

if (access(".", W_OK) != 0 || access("regs.txt", F_OK) == 0 && access("regs.txt", W_OK) != 0) {
  fprintf(stderr, "cwd not writable for regs.txt\n"); return 1;
}

FILE *fp = fopen("regs.txt", "w+");
if (!fp) { perror("Failed to create file regs.txt"); /* fallback dir */ fp = fopen("/data/local/tmp/regs.txt", "w+"); }
if (!fp) return false;

When it happens

Trigger: Running the unwind_for_offline tool with a current working directory that is not writable, a read-only filesystem, disk full, or regs.txt existing as a directory/locked file; SaveData calls SaveRegs right after collecting registers from a core dump or live process.

Common situations: Running the tool from a root-owned or read-only directory on a device/emulator shell, SELinux blocking writes in /data or /system paths, or a full tmpfs when capturing offline unwind data for backtrace debugging.

Understand the failure class

Background: "open() failed", "failed to open file", "cannot create file" — what a file open error means and how to fix it — this error's family across 42 libraries.

Related errors

• Failed to create stack.data
• Failed to create maps.txt
• Matrix init, Matrix should not be null.
• you must init Matrix sdk first
• matrix init, application is null

AI-assisted analysis of Tencent/matrix@3b8293bd65 (2026-09-08). Data as JSON: /api/errors/387dd568e0d5be21. Report an issue: GitHub.

Appendix: source

Thrown at matrix/matrix-android/matrix-backtrace/src/main/cpp/external/libunwindstack/tools/unwind_for_offline.cpp:79

    return false;
  }

  // Allow at least 1 second to attach properly.
  for (size_t i = 0; i < 1000; i++) {
    siginfo_t si;
    if (ptrace(PTRACE_GETSIGINFO, pid, 0, &si) == 0) {
      return true;
    }
    usleep(1000);
  }
  printf("%d: Failed to stop.\n", pid);
  return false;
}

bool SaveRegs(unwindstack::Regs* regs) {
  std::unique_ptr<FILE, decltype(&fclose)> fp(fopen("regs.txt", "w+"), &fclose);
  if (fp == nullptr) {
    perror("Failed to create file regs.txt");
    return false;
  }
  regs->IterateRegisters([&fp](const char* name, uint64_t value) {
    fprintf(fp.get(), "%s: %" PRIx64 "\n", name, value);
  });

  return true;
}

bool SaveStack(pid_t pid, const std::vector<std::pair<uint64_t, uint64_t>>& stacks) {
  for (size_t i = 0; i < stacks.size(); i++) {
    std::string file_name;
    if (stacks.size() != 1) {
      file_name = "stack" + std::to_string(i) + ".data";
    } else {
      file_name = "stack.data";
    }

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