base/debug/stack_trace_posix.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "base/debug/stack_trace.h"
   6
   7 #include <errno.h>
   8 #include <execinfo.h>
   9 #include <fcntl.h>
  10 #include <signal.h>
  11 #include <stdio.h>
  12 #include <stdlib.h>
  13 #include <sys/param.h>
  14 #include <sys/stat.h>
  15 #include <sys/types.h>
  16 #include <unistd.h>
  17
  18 #include <ostream>
  19
  20 #if defined(__GLIBCXX__)
  21 #include <cxxabi.h>
  22 #endif
  23
  24 #if defined(OS_MACOSX)
  25 #include <AvailabilityMacros.h>
  26 #endif
  27
  28 #include "base/basictypes.h"
  29 #include "base/debug/debugger.h"
  30 #include "base/logging.h"
  31 #include "base/memory/scoped_ptr.h"
  32 #include "base/posix/eintr_wrapper.h"
  33 #include "base/strings/string_number_conversions.h"
  34
  35 #if defined(USE_SYMBOLIZE)
  36 #include "base/third_party/symbolize/symbolize.h"
  37 #endif
  38
  39 namespace base {
  40 namespace debug {
  41
  42 namespace {
  43
  44 volatile sig_atomic_t in_signal_handler = 0;
  45
  46 // The prefix used for mangled symbols, per the Itanium C++ ABI:
  47 // http://www.codesourcery.com/cxx-abi/abi.html#mangling
  48 const char kMangledSymbolPrefix[] = "_Z";
  49
  50 // Characters that can be used for symbols, generated by Ruby:
  51 // (('a'..'z').to_a+('A'..'Z').to_a+('0'..'9').to_a + ['_']).join
  52 const char kSymbolCharacters[] =
  53     "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_";
  54
  55 #if !defined(USE_SYMBOLIZE)
  56 // Demangles C++ symbols in the given text. Example:
  57 //
  58 // "out/Debug/base_unittests(_ZN10StackTraceC1Ev+0x20) [0x817778c]"
  59 // =>
  60 // "out/Debug/base_unittests(StackTrace::StackTrace()+0x20) [0x817778c]"
  61 void DemangleSymbols(std::string* text) {
  62   // Note: code in this function is NOT async-signal safe (std::string uses
  63   // malloc internally).
  64
  65 #if defined(__GLIBCXX__)
  66
  67   std::string::size_type search_from = 0;
  68   while (search_from < text->size()) {
  69     // Look for the start of a mangled symbol, from search_from.
  70     std::string::size_type mangled_start =
  71         text->find(kMangledSymbolPrefix, search_from);
  72     if (mangled_start == std::string::npos) {
  73       break;  // Mangled symbol not found.
  74     }
  75
  76     // Look for the end of the mangled symbol.
  77     std::string::size_type mangled_end =
  78         text->find_first_not_of(kSymbolCharacters, mangled_start);
  79     if (mangled_end == std::string::npos) {
  80       mangled_end = text->size();
  81     }
  82     std::string mangled_symbol =
  83         text->substr(mangled_start, mangled_end - mangled_start);
  84
  85     // Try to demangle the mangled symbol candidate.
  86     int status = 0;
  87     scoped_ptr_malloc<char> demangled_symbol(
  88         abi::__cxa_demangle(mangled_symbol.c_str(), NULL, 0, &status));
  89     if (status == 0) {  // Demangling is successful.
  90       // Remove the mangled symbol.
  91       text->erase(mangled_start, mangled_end - mangled_start);
  92       // Insert the demangled symbol.
  93       text->insert(mangled_start, demangled_symbol.get());
  94       // Next time, we'll start right after the demangled symbol we inserted.
  95       search_from = mangled_start + strlen(demangled_symbol.get());
  96     } else {
  97       // Failed to demangle.  Retry after the "_Z" we just found.
  98       search_from = mangled_start + 2;
  99     }
 100   }
 101
 102 #endif  // defined(__GLIBCXX__)
 103 }
 104 #endif  // !defined(USE_SYMBOLIZE)
 105
 106 class BacktraceOutputHandler {
 107  public:
 108   virtual void HandleOutput(const char* output) = 0;
 109
 110  protected:
 111   virtual ~BacktraceOutputHandler() {}
 112 };
 113
 114 void OutputPointer(void* pointer, BacktraceOutputHandler* handler) {
 115   char buf[1024] = { '\0' };
 116   handler->HandleOutput(" [0x");
 117   internal::itoa_r(reinterpret_cast<intptr_t>(pointer),
 118                    buf, sizeof(buf), 16, 12);
 119   handler->HandleOutput(buf);
 120   handler->HandleOutput("]");
 121 }
 122
 123 void ProcessBacktrace(void *const *trace,
 124                       int size,
 125                       BacktraceOutputHandler* handler) {
 126   // NOTE: This code MUST be async-signal safe (it's used by in-process
 127   // stack dumping signal handler). NO malloc or stdio is allowed here.
 128
 129 #if defined(USE_SYMBOLIZE)
 130   for (int i = 0; i < size; ++i) {
 131     OutputPointer(trace[i], handler);
 132     handler->HandleOutput(" ");
 133
 134     char buf[1024] = { '\0' };
 135
 136     // Subtract by one as return address of function may be in the next
 137     // function when a function is annotated as noreturn.
 138     void* address = static_cast<char*>(trace[i]) - 1;
 139     if (google::Symbolize(address, buf, sizeof(buf)))
 140       handler->HandleOutput(buf);
 141     else
 142       handler->HandleOutput("<unknown>");
 143
 144     handler->HandleOutput("\n");
 145   }
 146 #else
 147   bool printed = false;
 148
 149   // Below part is async-signal unsafe (uses malloc), so execute it only
 150   // when we are not executing the signal handler.
 151   if (in_signal_handler == 0) {
 152     scoped_ptr_malloc<char*> trace_symbols(backtrace_symbols(trace, size));
 153     if (trace_symbols.get()) {
 154       for (int i = 0; i < size; ++i) {
 155         std::string trace_symbol = trace_symbols.get()[i];
 156         DemangleSymbols(&trace_symbol);
 157         handler->HandleOutput(trace_symbol.c_str());
 158         handler->HandleOutput("\n");
 159       }
 160
 161       printed = true;
 162     }
 163   }
 164
 165   if (!printed) {
 166     for (int i = 0; i < size; ++i) {
 167       OutputPointer(trace[i], handler);
 168       handler->HandleOutput("\n");
 169     }
 170   }
 171 #endif  // defined(USE_SYMBOLIZE)
 172 }
 173
 174 void PrintToStderr(const char* output) {
 175   // NOTE: This code MUST be async-signal safe (it's used by in-process
 176   // stack dumping signal handler). NO malloc or stdio is allowed here.
 177   ignore_result(HANDLE_EINTR(write(STDERR_FILENO, output, strlen(output))));
 178 }
 179
 180 void StackDumpSignalHandler(int signal, siginfo_t* info, void* void_context) {
 181   // NOTE: This code MUST be async-signal safe.
 182   // NO malloc or stdio is allowed here.
 183
 184   // Record the fact that we are in the signal handler now, so that the rest
 185   // of StackTrace can behave in an async-signal-safe manner.
 186   in_signal_handler = 1;
 187
 188   if (BeingDebugged())
 189     BreakDebugger();
 190
 191   PrintToStderr("Received signal ");
 192   char buf[1024] = { 0 };
 193   internal::itoa_r(signal, buf, sizeof(buf), 10, 0);
 194   PrintToStderr(buf);
 195   if (signal == SIGBUS) {
 196     if (info->si_code == BUS_ADRALN)
 197       PrintToStderr(" BUS_ADRALN ");
 198     else if (info->si_code == BUS_ADRERR)
 199       PrintToStderr(" BUS_ADRERR ");
 200     else if (info->si_code == BUS_OBJERR)
 201       PrintToStderr(" BUS_OBJERR ");
 202     else
 203       PrintToStderr(" <unknown> ");
 204   } else if (signal == SIGFPE) {
 205     if (info->si_code == FPE_FLTDIV)
 206       PrintToStderr(" FPE_FLTDIV ");
 207     else if (info->si_code == FPE_FLTINV)
 208       PrintToStderr(" FPE_FLTINV ");
 209     else if (info->si_code == FPE_FLTOVF)
 210       PrintToStderr(" FPE_FLTOVF ");
 211     else if (info->si_code == FPE_FLTRES)
 212       PrintToStderr(" FPE_FLTRES ");
 213     else if (info->si_code == FPE_FLTSUB)
 214       PrintToStderr(" FPE_FLTSUB ");
 215     else if (info->si_code == FPE_FLTUND)
 216       PrintToStderr(" FPE_FLTUND ");
 217     else if (info->si_code == FPE_INTDIV)
 218       PrintToStderr(" FPE_INTDIV ");
 219     else if (info->si_code == FPE_INTOVF)
 220       PrintToStderr(" FPE_INTOVF ");
 221     else
 222       PrintToStderr(" <unknown> ");
 223   } else if (signal == SIGILL) {
 224     if (info->si_code == ILL_BADSTK)
 225       PrintToStderr(" ILL_BADSTK ");
 226     else if (info->si_code == ILL_COPROC)
 227       PrintToStderr(" ILL_COPROC ");
 228     else if (info->si_code == ILL_ILLOPN)
 229       PrintToStderr(" ILL_ILLOPN ");
 230     else if (info->si_code == ILL_ILLADR)
 231       PrintToStderr(" ILL_ILLADR ");
 232     else if (info->si_code == ILL_ILLTRP)
 233       PrintToStderr(" ILL_ILLTRP ");
 234     else if (info->si_code == ILL_PRVOPC)
 235       PrintToStderr(" ILL_PRVOPC ");
 236     else if (info->si_code == ILL_PRVREG)
 237       PrintToStderr(" ILL_PRVREG ");
 238     else
 239       PrintToStderr(" <unknown> ");
 240   } else if (signal == SIGSEGV) {
 241     if (info->si_code == SEGV_MAPERR)
 242       PrintToStderr(" SEGV_MAPERR ");
 243     else if (info->si_code == SEGV_ACCERR)
 244       PrintToStderr(" SEGV_ACCERR ");
 245     else
 246       PrintToStderr(" <unknown> ");
 247   }
 248   if (signal == SIGBUS || signal == SIGFPE ||
 249       signal == SIGILL || signal == SIGSEGV) {
 250     internal::itoa_r(reinterpret_cast<intptr_t>(info->si_addr),
 251                      buf, sizeof(buf), 16, 12);
 252     PrintToStderr(buf);
 253   }
 254   PrintToStderr("\n");
 255
 256   debug::StackTrace().PrintBacktrace();
 257
 258 #if defined(OS_LINUX)
 259 #if ARCH_CPU_X86_FAMILY
 260   ucontext_t* context = reinterpret_cast<ucontext_t*>(void_context);
 261   const struct {
 262     const char* label;
 263     greg_t value;
 264   } registers[] = {
 265 #if ARCH_CPU_32_BITS
 266     { "  gs: ", context->uc_mcontext.gregs[REG_GS] },
 267     { "  fs: ", context->uc_mcontext.gregs[REG_FS] },
 268     { "  es: ", context->uc_mcontext.gregs[REG_ES] },
 269     { "  ds: ", context->uc_mcontext.gregs[REG_DS] },
 270     { " edi: ", context->uc_mcontext.gregs[REG_EDI] },
 271     { " esi: ", context->uc_mcontext.gregs[REG_ESI] },
 272     { " ebp: ", context->uc_mcontext.gregs[REG_EBP] },
 273     { " esp: ", context->uc_mcontext.gregs[REG_ESP] },
 274     { " ebx: ", context->uc_mcontext.gregs[REG_EBX] },
 275     { " edx: ", context->uc_mcontext.gregs[REG_EDX] },
 276     { " ecx: ", context->uc_mcontext.gregs[REG_ECX] },
 277     { " eax: ", context->uc_mcontext.gregs[REG_EAX] },
 278     { " trp: ", context->uc_mcontext.gregs[REG_TRAPNO] },
 279     { " err: ", context->uc_mcontext.gregs[REG_ERR] },
 280     { "  ip: ", context->uc_mcontext.gregs[REG_EIP] },
 281     { "  cs: ", context->uc_mcontext.gregs[REG_CS] },
 282     { " efl: ", context->uc_mcontext.gregs[REG_EFL] },
 283     { " usp: ", context->uc_mcontext.gregs[REG_UESP] },
 284     { "  ss: ", context->uc_mcontext.gregs[REG_SS] },
 285 #elif ARCH_CPU_64_BITS
 286     { "  r8: ", context->uc_mcontext.gregs[REG_R8] },
 287     { "  r9: ", context->uc_mcontext.gregs[REG_R9] },
 288     { " r10: ", context->uc_mcontext.gregs[REG_R10] },
 289     { " r11: ", context->uc_mcontext.gregs[REG_R11] },
 290     { " r12: ", context->uc_mcontext.gregs[REG_R12] },
 291     { " r13: ", context->uc_mcontext.gregs[REG_R13] },
 292     { " r14: ", context->uc_mcontext.gregs[REG_R14] },
 293     { " r15: ", context->uc_mcontext.gregs[REG_R15] },
 294     { "  di: ", context->uc_mcontext.gregs[REG_RDI] },
 295     { "  si: ", context->uc_mcontext.gregs[REG_RSI] },
 296     { "  bp: ", context->uc_mcontext.gregs[REG_RBP] },
 297     { "  bx: ", context->uc_mcontext.gregs[REG_RBX] },
 298     { "  dx: ", context->uc_mcontext.gregs[REG_RDX] },
 299     { "  ax: ", context->uc_mcontext.gregs[REG_RAX] },
 300     { "  cx: ", context->uc_mcontext.gregs[REG_RCX] },
 301     { "  sp: ", context->uc_mcontext.gregs[REG_RSP] },
 302     { "  ip: ", context->uc_mcontext.gregs[REG_RIP] },
 303     { " efl: ", context->uc_mcontext.gregs[REG_EFL] },
 304     { " cgf: ", context->uc_mcontext.gregs[REG_CSGSFS] },
 305     { " erf: ", context->uc_mcontext.gregs[REG_ERR] },
 306     { " trp: ", context->uc_mcontext.gregs[REG_TRAPNO] },
 307     { " msk: ", context->uc_mcontext.gregs[REG_OLDMASK] },
 308     { " cr2: ", context->uc_mcontext.gregs[REG_CR2] },
 309 #endif
 310   };
 311
 312 #if ARCH_CPU_32_BITS
 313   const int kRegisterPadding = 8;
 314 #elif ARCH_CPU_64_BITS
 315   const int kRegisterPadding = 16;
 316 #endif
 317
 318   for (size_t i = 0; i < ARRAYSIZE_UNSAFE(registers); i++) {
 319     PrintToStderr(registers[i].label);
 320     internal::itoa_r(registers[i].value, buf, sizeof(buf),
 321                      16, kRegisterPadding);
 322     PrintToStderr(buf);
 323
 324     if ((i + 1) % 4 == 0)
 325       PrintToStderr("\n");
 326   }
 327   PrintToStderr("\n");
 328 #endif
 329 #elif defined(OS_MACOSX)
 330   // TODO(shess): Port to 64-bit.
 331 #if ARCH_CPU_X86_FAMILY && ARCH_CPU_32_BITS
 332   ucontext_t* context = reinterpret_cast<ucontext_t*>(void_context);
 333   size_t len;
 334
 335   // NOTE: Even |snprintf()| is not on the approved list for signal
 336   // handlers, but buffered I/O is definitely not on the list due to
 337   // potential for |malloc()|.
 338   len = static_cast<size_t>(
 339       snprintf(buf, sizeof(buf),
 340                "ax: %x, bx: %x, cx: %x, dx: %x\n",
 341                context->uc_mcontext->__ss.__eax,
 342                context->uc_mcontext->__ss.__ebx,
 343                context->uc_mcontext->__ss.__ecx,
 344                context->uc_mcontext->__ss.__edx));
 345   write(STDERR_FILENO, buf, std::min(len, sizeof(buf) - 1));
 346
 347   len = static_cast<size_t>(
 348       snprintf(buf, sizeof(buf),
 349                "di: %x, si: %x, bp: %x, sp: %x, ss: %x, flags: %x\n",
 350                context->uc_mcontext->__ss.__edi,
 351                context->uc_mcontext->__ss.__esi,
 352                context->uc_mcontext->__ss.__ebp,
 353                context->uc_mcontext->__ss.__esp,
 354                context->uc_mcontext->__ss.__ss,
 355                context->uc_mcontext->__ss.__eflags));
 356   write(STDERR_FILENO, buf, std::min(len, sizeof(buf) - 1));
 357
 358   len = static_cast<size_t>(
 359       snprintf(buf, sizeof(buf),
 360                "ip: %x, cs: %x, ds: %x, es: %x, fs: %x, gs: %x\n",
 361                context->uc_mcontext->__ss.__eip,
 362                context->uc_mcontext->__ss.__cs,
 363                context->uc_mcontext->__ss.__ds,
 364                context->uc_mcontext->__ss.__es,
 365                context->uc_mcontext->__ss.__fs,
 366                context->uc_mcontext->__ss.__gs));
 367   write(STDERR_FILENO, buf, std::min(len, sizeof(buf) - 1));
 368 #endif  // ARCH_CPU_32_BITS
 369 #endif  // defined(OS_MACOSX)
 370   _exit(1);
 371 }
 372
 373 class PrintBacktraceOutputHandler : public BacktraceOutputHandler {
 374  public:
 375   PrintBacktraceOutputHandler() {}
 376
 377   virtual void HandleOutput(const char* output) OVERRIDE {
 378     // NOTE: This code MUST be async-signal safe (it's used by in-process
 379     // stack dumping signal handler). NO malloc or stdio is allowed here.
 380     PrintToStderr(output);
 381   }
 382
 383  private:
 384   DISALLOW_COPY_AND_ASSIGN(PrintBacktraceOutputHandler);
 385 };
 386
 387 class StreamBacktraceOutputHandler : public BacktraceOutputHandler {
 388  public:
 389   explicit StreamBacktraceOutputHandler(std::ostream* os) : os_(os) {
 390   }
 391
 392   virtual void HandleOutput(const char* output) OVERRIDE {
 393     (*os_) << output;
 394   }
 395
 396  private:
 397   std::ostream* os_;
 398
 399   DISALLOW_COPY_AND_ASSIGN(StreamBacktraceOutputHandler);
 400 };
 401
 402 void WarmUpBacktrace() {
 403   // Warm up stack trace infrastructure. It turns out that on the first
 404   // call glibc initializes some internal data structures using pthread_once,
 405   // and even backtrace() can call malloc(), leading to hangs.
 406   //
 407   // Example stack trace snippet (with tcmalloc):
 408   //
 409   // #8  0x0000000000a173b5 in tc_malloc
 410   //             at ./third_party/tcmalloc/chromium/src/debugallocation.cc:1161
 411   // #9  0x00007ffff7de7900 in _dl_map_object_deps at dl-deps.c:517
 412   // #10 0x00007ffff7ded8a9 in dl_open_worker at dl-open.c:262
 413   // #11 0x00007ffff7de9176 in _dl_catch_error at dl-error.c:178
 414   // #12 0x00007ffff7ded31a in _dl_open (file=0x7ffff625e298 "libgcc_s.so.1")
 415   //             at dl-open.c:639
 416   // #13 0x00007ffff6215602 in do_dlopen at dl-libc.c:89
 417   // #14 0x00007ffff7de9176 in _dl_catch_error at dl-error.c:178
 418   // #15 0x00007ffff62156c4 in dlerror_run at dl-libc.c:48
 419   // #16 __GI___libc_dlopen_mode at dl-libc.c:165
 420   // #17 0x00007ffff61ef8f5 in init
 421   //             at ../sysdeps/x86_64/../ia64/backtrace.c:53
 422   // #18 0x00007ffff6aad400 in pthread_once
 423   //             at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_once.S:104
 424   // #19 0x00007ffff61efa14 in __GI___backtrace
 425   //             at ../sysdeps/x86_64/../ia64/backtrace.c:104
 426   // #20 0x0000000000752a54 in base::debug::StackTrace::StackTrace
 427   //             at base/debug/stack_trace_posix.cc:175
 428   // #21 0x00000000007a4ae5 in
 429   //             base::(anonymous namespace)::StackDumpSignalHandler
 430   //             at base/process_util_posix.cc:172
 431   // #22 <signal handler called>
 432   StackTrace stack_trace;
 433 }
 434
 435 }  // namespace
 436
 437 #if !defined(OS_IOS)
 438 bool EnableInProcessStackDumping() {
 439   // When running in an application, our code typically expects SIGPIPE
 440   // to be ignored.  Therefore, when testing that same code, it should run
 441   // with SIGPIPE ignored as well.
 442   struct sigaction sigpipe_action;
 443   memset(&sigpipe_action, 0, sizeof(sigpipe_action));
 444   sigpipe_action.sa_handler = SIG_IGN;
 445   sigemptyset(&sigpipe_action.sa_mask);
 446   bool success = (sigaction(SIGPIPE, &sigpipe_action, NULL) == 0);
 447
 448   // Avoid hangs during backtrace initialization, see above.
 449   WarmUpBacktrace();
 450
 451   struct sigaction action;
 452   memset(&action, 0, sizeof(action));
 453   action.sa_flags = SA_RESETHAND | SA_SIGINFO;
 454   action.sa_sigaction = &StackDumpSignalHandler;
 455   sigemptyset(&action.sa_mask);
 456
 457   success &= (sigaction(SIGILL, &action, NULL) == 0);
 458   success &= (sigaction(SIGABRT, &action, NULL) == 0);
 459   success &= (sigaction(SIGFPE, &action, NULL) == 0);
 460   success &= (sigaction(SIGBUS, &action, NULL) == 0);
 461   success &= (sigaction(SIGSEGV, &action, NULL) == 0);
 462   success &= (sigaction(SIGSYS, &action, NULL) == 0);
 463
 464   return success;
 465 }
 466 #endif  // !defined(OS_IOS)
 467
 468 StackTrace::StackTrace() {
 469   // NOTE: This code MUST be async-signal safe (it's used by in-process
 470   // stack dumping signal handler). NO malloc or stdio is allowed here.
 471
 472   // Though the backtrace API man page does not list any possible negative
 473   // return values, we take no chance.
 474   count_ = std::max(backtrace(trace_, arraysize(trace_)), 0);
 475 }
 476
 477 void StackTrace::PrintBacktrace() const {
 478   // NOTE: This code MUST be async-signal safe (it's used by in-process
 479   // stack dumping signal handler). NO malloc or stdio is allowed here.
 480
 481   PrintBacktraceOutputHandler handler;
 482   ProcessBacktrace(trace_, count_, &handler);
 483 }
 484
 485 void StackTrace::OutputToStream(std::ostream* os) const {
 486   StreamBacktraceOutputHandler handler(os);
 487   ProcessBacktrace(trace_, count_, &handler);
 488 }
 489
 490 namespace internal {
 491
 492 // NOTE: code from sandbox/linux/seccomp-bpf/demo.cc.
 493 char *itoa_r(intptr_t i, char *buf, size_t sz, int base, size_t padding) {
 494   // Make sure we can write at least one NUL byte.
 495   size_t n = 1;
 496   if (n > sz)
 497     return NULL;
 498
 499   if (base < 2 || base > 16) {
 500     buf[0] = '\000';
 501     return NULL;
 502   }
 503
 504   char *start = buf;
 505
 506   uintptr_t j = i;
 507
 508   // Handle negative numbers (only for base 10).
 509   if (i < 0 && base == 10) {
 510     j = -i;
 511
 512     // Make sure we can write the '-' character.
 513     if (++n > sz) {
 514       buf[0] = '\000';
 515       return NULL;
 516     }
 517     *start++ = '-';
 518   }
 519
 520   // Loop until we have converted the entire number. Output at least one
 521   // character (i.e. '0').
 522   char *ptr = start;
 523   do {
 524     // Make sure there is still enough space left in our output buffer.
 525     if (++n > sz) {
 526       buf[0] = '\000';
 527       return NULL;
 528     }
 529
 530     // Output the next digit.
 531     *ptr++ = "0123456789abcdef"[j % base];
 532     j /= base;
 533
 534     if (padding > 0)
 535       padding--;
 536   } while (j > 0 || padding > 0);
 537
 538   // Terminate the output with a NUL character.
 539   *ptr = '\000';
 540
 541   // Conversion to ASCII actually resulted in the digits being in reverse
 542   // order. We can't easily generate them in forward order, as we can't tell
 543   // the number of characters needed until we are done converting.
 544   // So, now, we reverse the string (except for the possible "-" sign).
 545   while (--ptr > start) {
 546     char ch = *ptr;
 547     *ptr = *start;
 548     *start++ = ch;
 549   }
 550   return buf;
 551 }
 552
 553 }  // namespace internal
 554
 555 }  // namespace debug
 556 }  // namespace base