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