base/posix/safe_strerror.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 "base/posix/safe_strerror.h"
  14
  15 #include <errno.h>
  16 #include <stdio.h>
  17 #include <string.h>
  18
  19 #include "build/build_config.h"
  20
  21 namespace base {
  22
  23 #define USE_HISTORICAL_STRERRO_R (defined(__GLIBC__) || defined(OS_NACL))
  24
  25 #if USE_HISTORICAL_STRERRO_R && defined(__GNUC__)
  26 // GCC will complain about the unused second wrap function unless we tell it
  27 // that we meant for them to be potentially unused, which is exactly what this
  28 // attribute is for.
  29 #define POSSIBLY_UNUSED __attribute__((unused))
  30 #else
  31 #define POSSIBLY_UNUSED
  32 #endif
  33
  34 #if USE_HISTORICAL_STRERRO_R
  35 // glibc has two strerror_r functions: a historical GNU-specific one that
  36 // returns type char *, and a POSIX.1-2001 compliant one available since 2.3.4
  37 // that returns int. This wraps the GNU-specific one.
  38 static void POSSIBLY_UNUSED wrap_posix_strerror_r(
  39     char *(*strerror_r_ptr)(int, char *, size_t),
  40     int err,
  41     char *buf,
  42     size_t len) {
  43   // GNU version.
  44   char *rc = (*strerror_r_ptr)(err, buf, len);
  45   if (rc != buf) {
  46     // glibc did not use buf and returned a static string instead. Copy it
  47     // into buf.
  48     buf[0] = '\0';
  49     strncat(buf, rc, len - 1);
  50   }
  51   // The GNU version never fails. Unknown errors get an "unknown error" message.
  52   // The result is always null terminated.
  53 }
  54 #endif  // USE_HISTORICAL_STRERRO_R
  55
  56 // Wrapper for strerror_r functions that implement the POSIX interface. POSIX
  57 // does not define the behaviour for some of the edge cases, so we wrap it to
  58 // guarantee that they are handled. This is compiled on all POSIX platforms, but
  59 // it will only be used on Linux if the POSIX strerror_r implementation is
  60 // being used (see below).
  61 static void POSSIBLY_UNUSED wrap_posix_strerror_r(
  62     int (*strerror_r_ptr)(int, char *, size_t),
  63     int err,
  64     char *buf,
  65     size_t len) {
  66   int old_errno = errno;
  67   // Have to cast since otherwise we get an error if this is the GNU version
  68   // (but in such a scenario this function is never called). Sadly we can't use
  69   // C++-style casts because the appropriate one is reinterpret_cast but it's
  70   // considered illegal to reinterpret_cast a type to itself, so we get an
  71   // error in the opposite case.
  72   int result = (*strerror_r_ptr)(err, buf, len);
  73   if (result == 0) {
  74     // POSIX is vague about whether the string will be terminated, although
  75     // it indirectly implies that typically ERANGE will be returned, instead
  76     // of truncating the string. We play it safe by always terminating the
  77     // string explicitly.
  78     buf[len - 1] = '\0';
  79   } else {
  80     // Error. POSIX is vague about whether the return value is itself a system
  81     // error code or something else. On Linux currently it is -1 and errno is
  82     // set. On BSD-derived systems it is a system error and errno is unchanged.
  83     // We try and detect which case it is so as to put as much useful info as
  84     // we can into our message.
  85     int strerror_error;  // The error encountered in strerror
  86     int new_errno = errno;
  87     if (new_errno != old_errno) {
  88       // errno was changed, so probably the return value is just -1 or something
  89       // else that doesn't provide any info, and errno is the error.
  90       strerror_error = new_errno;
  91     } else {
  92       // Either the error from strerror_r was the same as the previous value, or
  93       // errno wasn't used. Assume the latter.
  94       strerror_error = result;
  95     }
  96     // snprintf truncates and always null-terminates.
  97     snprintf(buf,
  98              len,
  99              "Error %d while retrieving error %d",
 100              strerror_error,
 101              err);
 102   }
 103   errno = old_errno;
 104 }
 105
 106 void safe_strerror_r(int err, char *buf, size_t len) {
 107   if (buf == NULL || len <= 0) {
 108     return;
 109   }
 110   // If using glibc (i.e., Linux), the compiler will automatically select the
 111   // appropriate overloaded function based on the function type of strerror_r.
 112   // The other one will be elided from the translation unit since both are
 113   // static.
 114   wrap_posix_strerror_r(&strerror_r, err, buf, len);
 115 }
 116
 117 std::string safe_strerror(int err) {
 118   const int buffer_size = 256;
 119   char buf[buffer_size];
 120   safe_strerror_r(err, buf, sizeof(buf));
 121   return std::string(buf);
 122 }
 123
 124 }  // namespace base