base/safe_strerror_posix.cc

   1 // Copyright (c) 2006-2009 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 #if defined(__ANDROID__)
   6 // Post-L versions of bionic define the GNU-specific strerror_r if _GNU_SOURCE
   7 // is defined, but the symbol is renamed to __gnu_strerror_r which only exists
   8 // on those later versions. To preserve ABI compatibility with older versions,
   9 // undefine _GNU_SOURCE and use the POSIX version.
  10 #undef _GNU_SOURCE
  11 #endif
  12
  13 #include "build/build_config.h"
  14 #include "base/safe_strerror_posix.h"
  15
  16 #include <errno.h>
  17 #include <stdio.h>
  18 #include <string.h>
  19
  20 #define USE_HISTORICAL_STRERRO_R (defined(__GLIBC__) || defined(OS_NACL))
  21
  22 #if USE_HISTORICAL_STRERRO_R && defined(__GNUC__)
  23 // GCC will complain about the unused second wrap function unless we tell it
  24 // that we meant for them to be potentially unused, which is exactly what this
  25 // attribute is for.
  26 #define POSSIBLY_UNUSED __attribute__((unused))
  27 #else
  28 #define POSSIBLY_UNUSED
  29 #endif
  30
  31 #if USE_HISTORICAL_STRERRO_R
  32 // glibc has two strerror_r functions: a historical GNU-specific one that
  33 // returns type char *, and a POSIX.1-2001 compliant one available since 2.3.4
  34 // that returns int. This wraps the GNU-specific one.
  35 static void POSSIBLY_UNUSED wrap_posix_strerror_r(
  36     char *(*strerror_r_ptr)(int, char *, size_t),
  37     int err,
  38     char *buf,
  39     size_t len) {
  40   // GNU version.
  41   char *rc = (*strerror_r_ptr)(err, buf, len);
  42   if (rc != buf) {
  43     // glibc did not use buf and returned a static string instead. Copy it
  44     // into buf.
  45     buf[0] = '\0';
  46     strncat(buf, rc, len - 1);
  47   }
  48   // The GNU version never fails. Unknown errors get an "unknown error" message.
  49   // The result is always null terminated.
  50 }
  51 #endif  // USE_HISTORICAL_STRERRO_R
  52
  53 // Wrapper for strerror_r functions that implement the POSIX interface. POSIX
  54 // does not define the behaviour for some of the edge cases, so we wrap it to
  55 // guarantee that they are handled. This is compiled on all POSIX platforms, but
  56 // it will only be used on Linux if the POSIX strerror_r implementation is
  57 // being used (see below).
  58 static void POSSIBLY_UNUSED wrap_posix_strerror_r(
  59     int (*strerror_r_ptr)(int, char *, size_t),
  60     int err,
  61     char *buf,
  62     size_t len) {
  63   int old_errno = errno;
  64   // Have to cast since otherwise we get an error if this is the GNU version
  65   // (but in such a scenario this function is never called). Sadly we can't use
  66   // C++-style casts because the appropriate one is reinterpret_cast but it's
  67   // considered illegal to reinterpret_cast a type to itself, so we get an
  68   // error in the opposite case.
  69   int result = (*strerror_r_ptr)(err, buf, len);
  70   if (result == 0) {
  71     // POSIX is vague about whether the string will be terminated, although
  72     // it indirectly implies that typically ERANGE will be returned, instead
  73     // of truncating the string. We play it safe by always terminating the
  74     // string explicitly.
  75     buf[len - 1] = '\0';
  76   } else {
  77     // Error. POSIX is vague about whether the return value is itself a system
  78     // error code or something else. On Linux currently it is -1 and errno is
  79     // set. On BSD-derived systems it is a system error and errno is unchanged.
  80     // We try and detect which case it is so as to put as much useful info as
  81     // we can into our message.
  82     int strerror_error;  // The error encountered in strerror
  83     int new_errno = errno;
  84     if (new_errno != old_errno) {
  85       // errno was changed, so probably the return value is just -1 or something
  86       // else that doesn't provide any info, and errno is the error.
  87       strerror_error = new_errno;
  88     } else {
  89       // Either the error from strerror_r was the same as the previous value, or
  90       // errno wasn't used. Assume the latter.
  91       strerror_error = result;
  92     }
  93     // snprintf truncates and always null-terminates.
  94     snprintf(buf,
  95              len,
  96              "Error %d while retrieving error %d",
  97              strerror_error,
  98              err);
  99   }
 100   errno = old_errno;
 101 }
 102
 103 void safe_strerror_r(int err, char *buf, size_t len) {
 104   if (buf == NULL || len <= 0) {
 105     return;
 106   }
 107   // If using glibc (i.e., Linux), the compiler will automatically select the
 108   // appropriate overloaded function based on the function type of strerror_r.
 109   // The other one will be elided from the translation unit since both are
 110   // static.
 111   wrap_posix_strerror_r(&strerror_r, err, buf, len);
 112 }
 113
 114 std::string safe_strerror(int err) {
 115   const int buffer_size = 256;
 116   char buf[buffer_size];
 117   safe_strerror_r(err, buf, sizeof(buf));
 118   return std::string(buf);
 119 }