Added a new SafeSPrintf() function that implements snprintf() in an async-safe-fashion

base/debug/format_unittest.cc

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Author: markus@chromium.org
+
+#include <stdio.h>
+#include <string.h>
+
+#include <limits>
+
+#include "base/debug/format.h"
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+#if defined(GTEST_HAS_DEATH_TEST)
+#define ALLOW_DEATH_TEST
+#endif
+
+namespace base {
+namespace debug {
+
+TEST(FormatTest, Empty) {
+  char buf[2] = { 'X', 'X' };
+
+  // Negative buffer size should always result in an error.
+  EXPECT_EQ(-1, FormatN(buf, -1, ""));
+  EXPECT_EQ('X', buf[0]);
+  EXPECT_EQ('X', buf[1]);
+
+  // Zero buffer size should always result in an error.
+  EXPECT_EQ(-1, FormatN(buf, 0, ""));
+  EXPECT_EQ('X', buf[0]);
+  EXPECT_EQ('X', buf[1]);
+
+  // A one-byte buffer should always print a single NUL byte.
+  EXPECT_EQ(0, FormatN(buf, 1, ""));
+  EXPECT_EQ(0, buf[0]);
+  EXPECT_EQ('X', buf[1]);
+  buf[0] = 'X';
+
+  // A larger buffer should leave the trailing bytes unchanged.
+  EXPECT_EQ(0, FormatN(buf, 2, ""));
+  EXPECT_EQ(0, buf[0]);
+  EXPECT_EQ('X', buf[1]);
+  buf[0] = 'X';
+
+  // The same test using Format() instead of FormatN().
+  EXPECT_EQ(0, Format(buf, ""));
+  EXPECT_EQ(0, buf[0]);
+  EXPECT_EQ('X', buf[1]);
+  buf[0] = 'X';
+}
+
+TEST(FormatTest, NoArguments) {
+  // Output a text message that doesn't require any substitutions. This
+  // is roughly equivalent to calling strncpy() (but unlike strncpy(), it does
+  // always add a trailing NUL; it always deduplicates '%' characters).
+  const char text[] = "hello world";
+  char ref[20], buf[20];
+  memset(ref, 'X', sizeof(buf));
+  memcpy(buf, ref, sizeof(buf));
+
+  // A negative buffer size should always result in an error.
+  EXPECT_EQ(-1, FormatN(buf, -1, text));
+  EXPECT_TRUE(!memcmp(buf, ref, sizeof(buf)));
+
+  // Zero buffer size should always result in an error.
+  EXPECT_EQ(-1, FormatN(buf, 0, text));
+  EXPECT_TRUE(!memcmp(buf, ref, sizeof(buf)));
+
+  // A one-byte buffer should always print a single NUL byte.
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1, FormatN(buf, 1, text));
+  EXPECT_EQ(0, buf[0]);
+  EXPECT_TRUE(!memcmp(buf+1, ref+1, sizeof(buf)-1));
+  memcpy(buf, ref, sizeof(buf));
+
+  // A larger (but limited) buffer should always leave the trailing bytes
+  // unchanged.
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1, FormatN(buf, 2, text));
+  EXPECT_EQ(text[0], buf[0]);
+  EXPECT_EQ(0, buf[1]);
+  EXPECT_TRUE(!memcmp(buf+2, ref+2, sizeof(buf)-2));
+  memcpy(buf, ref, sizeof(buf));
+
+  // A unrestricted buffer length should always leave the trailing bytes
+  // unchanged.
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1,
+            FormatN(buf, sizeof(buf), text));
+  EXPECT_EQ(std::string(text), std::string(buf));
+  EXPECT_TRUE(!memcmp(buf + sizeof(text), ref + sizeof(text),
+                      sizeof(buf) - sizeof(text)));
+  memcpy(buf, ref, sizeof(buf));
+
+  // The same test using Format() instead of FormatN().
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1, Format(buf, text));
+  EXPECT_EQ(std::string(text), std::string(buf));
+  EXPECT_TRUE(!memcmp(buf + sizeof(text), ref + sizeof(text),
+                      sizeof(buf) - sizeof(text)));
+  memcpy(buf, ref, sizeof(buf));
+
+  // Check for deduplication of '%' percent characters.
+  EXPECT_EQ(1, Format(buf, "%%"));
+  EXPECT_EQ(2, Format(buf, "%%%%"));
+  EXPECT_EQ(2, Format(buf, "%%X"));
+  EXPECT_EQ(3, Format(buf, "%%%%X"));
+#if defined(NDEBUG)
+  EXPECT_EQ(1, Format(buf, "%"));
+  EXPECT_EQ(2, Format(buf, "%%%"));
+  EXPECT_EQ(2, Format(buf, "%X"));
+  EXPECT_EQ(3, Format(buf, "%%%X"));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, "%"), "src.1. == '%'");
+  EXPECT_DEATH(Format(buf, "%%%"), "src.1. == '%'");
+  EXPECT_DEATH(Format(buf, "%X"), "src.1. == '%'");
+  EXPECT_DEATH(Format(buf, "%%%X"), "src.1. == '%'");
+#endif
+}
+
+TEST(FormatTest, OneArgument) {
+  // Test basic single-argument single-character substitution.
+  const char text[] = "hello world";
+  const char fmt[]  = "hello%cworld";
+  char ref[20], buf[20];
+  memset(ref, 'X', sizeof(buf));
+  memcpy(buf, ref, sizeof(buf));
+
+  // A negative buffer size should always result in an error.
+  EXPECT_EQ(-1, FormatN(buf, -1, fmt, ' '));
+  EXPECT_TRUE(!memcmp(buf, ref, sizeof(buf)));
+
+  // Zero buffer size should always result in an error.
+  EXPECT_EQ(-1, FormatN(buf, 0, fmt, ' '));
+  EXPECT_TRUE(!memcmp(buf, ref, sizeof(buf)));
+
+  // A one-byte buffer should always print a single NUL byte.
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1, FormatN(buf, 1, fmt, ' '));
+  EXPECT_EQ(0, buf[0]);
+  EXPECT_TRUE(!memcmp(buf+1, ref+1, sizeof(buf)-1));
+  memcpy(buf, ref, sizeof(buf));
+
+  // A larger (but limited) buffer should always leave the trailing bytes
+  // unchanged.
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1, FormatN(buf, 2, fmt, ' '));
+  EXPECT_EQ(text[0], buf[0]);
+  EXPECT_EQ(0, buf[1]);
+  EXPECT_TRUE(!memcmp(buf+2, ref+2, sizeof(buf)-2));
+  memcpy(buf, ref, sizeof(buf));
+
+  // A unrestricted buffer length should always leave the trailing bytes
+  // unchanged.
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1,
+            FormatN(buf, sizeof(buf), fmt, ' '));
+  EXPECT_EQ(std::string(text), std::string(buf));
+  EXPECT_TRUE(!memcmp(buf + sizeof(text), ref + sizeof(text),
+                      sizeof(buf) - sizeof(text)));
+  memcpy(buf, ref, sizeof(buf));
+
+  // The same test using Format() instead of FormatN().
+  EXPECT_EQ(static_cast<ssize_t>(sizeof(text))-1, Format(buf, fmt, ' '));
+  EXPECT_EQ(std::string(text), std::string(buf));
+  EXPECT_TRUE(!memcmp(buf + sizeof(text), ref + sizeof(text),
+                      sizeof(buf) - sizeof(text)));
+  memcpy(buf, ref, sizeof(buf));
+
+  // Check for deduplication of '%' percent characters.
+  EXPECT_EQ(1, Format(buf, "%%", 0));
+  EXPECT_EQ(2, Format(buf, "%%%%", 0));
+  EXPECT_EQ(2, Format(buf, "%Y", 0));
+  EXPECT_EQ(2, Format(buf, "%%Y", 0));
+  EXPECT_EQ(3, Format(buf, "%%%Y", 0));
+  EXPECT_EQ(3, Format(buf, "%%%%Y", 0));
+#if defined(NDEBUG)
+  EXPECT_EQ(1, Format(buf, "%", 0));
+  EXPECT_EQ(2, Format(buf, "%%%", 0));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, "%", 0), "ch");
+  EXPECT_DEATH(Format(buf, "%%%", 0), "ch");
+#endif
+}
+
+#if defined(NDEBUG)
+TEST(FormatTest, MissingArg) {
+  char buf[20];
+  EXPECT_EQ(3, Format(buf, "%c%c", 'A'));
+  EXPECT_EQ("A%c", std::string(buf));
+}
+#endif
+
+TEST(FormatTest, NArgs) {
+  // Pre-C++11 compilers have a different code path, that can only print
+  // up to ten distinct arguments.
+  // We test both Format() and FormatN(). This makes sure we don't have
+  // typos in the copy-n-pasted code that is needed to deal with various
+  // numbers of arguments.
+  char buf[12];
+  EXPECT_EQ(1, Format(buf, "%c", 1));
+  EXPECT_EQ("\1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%c%c", 1, 2));
+  EXPECT_EQ("\1\2", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%c%c%c", 1, 2, 3));
+  EXPECT_EQ("\1\2\3", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%c%c%c%c", 1, 2, 3, 4));
+  EXPECT_EQ("\1\2\3\4", std::string(buf));
+  EXPECT_EQ(5, Format(buf, "%c%c%c%c%c", 1, 2, 3, 4, 5));
+  EXPECT_EQ("\1\2\3\4\5", std::string(buf));
+  EXPECT_EQ(6, Format(buf, "%c%c%c%c%c%c", 1, 2, 3, 4, 5, 6));
+  EXPECT_EQ("\1\2\3\4\5\6", std::string(buf));
+  EXPECT_EQ(7, Format(buf, "%c%c%c%c%c%c%c", 1, 2, 3, 4, 5, 6, 7));
+  EXPECT_EQ("\1\2\3\4\5\6\7", std::string(buf));
+  EXPECT_EQ(8, Format(buf, "%c%c%c%c%c%c%c%c", 1, 2, 3, 4, 5, 6, 7, 8));
+  EXPECT_EQ("\1\2\3\4\5\6\7\10", std::string(buf));
+  EXPECT_EQ(9, Format(buf, "%c%c%c%c%c%c%c%c%c", 1, 2, 3, 4, 5, 6, 7, 8, 9));
+  EXPECT_EQ("\1\2\3\4\5\6\7\10\11", std::string(buf));
+  EXPECT_EQ(10, Format(buf, "%c%c%c%c%c%c%c%c%c%c",
+                       1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
+
+  // Repeat all the tests with FormatN() instead of Format().
+  EXPECT_EQ("\1\2\3\4\5\6\7\10\11\12", std::string(buf));
+  EXPECT_EQ(1, FormatN(buf, 11, "%c", 1));
+  EXPECT_EQ("\1", std::string(buf));
+  EXPECT_EQ(2, FormatN(buf, 11, "%c%c", 1, 2));
+  EXPECT_EQ("\1\2", std::string(buf));
+  EXPECT_EQ(3, FormatN(buf, 11, "%c%c%c", 1, 2, 3));
+  EXPECT_EQ("\1\2\3", std::string(buf));
+  EXPECT_EQ(4, FormatN(buf, 11, "%c%c%c%c", 1, 2, 3, 4));
+  EXPECT_EQ("\1\2\3\4", std::string(buf));
+  EXPECT_EQ(5, FormatN(buf, 11, "%c%c%c%c%c", 1, 2, 3, 4, 5));
+  EXPECT_EQ("\1\2\3\4\5", std::string(buf));
+  EXPECT_EQ(6, FormatN(buf, 11, "%c%c%c%c%c%c", 1, 2, 3, 4, 5, 6));
+  EXPECT_EQ("\1\2\3\4\5\6", std::string(buf));
+  EXPECT_EQ(7, FormatN(buf, 11, "%c%c%c%c%c%c%c", 1, 2, 3, 4, 5, 6, 7));
+  EXPECT_EQ("\1\2\3\4\5\6\7", std::string(buf));
+  EXPECT_EQ(8, FormatN(buf, 11, "%c%c%c%c%c%c%c%c", 1, 2, 3, 4, 5, 6, 7, 8));
+  EXPECT_EQ("\1\2\3\4\5\6\7\10", std::string(buf));
+  EXPECT_EQ(9, FormatN(buf, 11, "%c%c%c%c%c%c%c%c%c",
+                       1, 2, 3, 4, 5, 6, 7, 8, 9));
+  EXPECT_EQ("\1\2\3\4\5\6\7\10\11", std::string(buf));
+  EXPECT_EQ(10, FormatN(buf, 11, "%c%c%c%c%c%c%c%c%c%c",
+                        1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
+  EXPECT_EQ("\1\2\3\4\5\6\7\10\11\12", std::string(buf));
+
+
+  // C++11 is smart enough to handle variadic template arguments. It can
+  // deal with arbitrary numbers of arguments.
+#if __cplusplus >= 201103  // C++11
+  EXPECT_EQ(11, Format(buf, "%c%c%c%c%c%c%c%c%c%c%c",
+                       1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11));
+  EXPECT_EQ("\1\2\3\4\5\6\7\10\11\12\13", std::string(buf));
+  EXPECT_EQ(11, FormatN(buf, 12, "%c%c%c%c%c%c%c%c%c%c%c",
+                        1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11));
+  EXPECT_EQ("\1\2\3\4\5\6\7\10\11\12\13", std::string(buf));
+#endif
+}
+
+TEST(FormatTest, DataTypes) {
+  char buf[40];
+
+  // Bytes
+  EXPECT_EQ(1, Format(buf, "%d", (uint8_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%d", (uint8_t)-1));
+  EXPECT_EQ("255", std::string(buf));
+  EXPECT_EQ(1, Format(buf, "%d", (int8_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%d", (int8_t)-1));
+  EXPECT_EQ("-1", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%d", (int8_t)-128));
+  EXPECT_EQ("-128", std::string(buf));
+
+  // Half-words
+  EXPECT_EQ(1, Format(buf, "%d", (uint16_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(5, Format(buf, "%d", (uint16_t)-1));
+  EXPECT_EQ("65535", std::string(buf));
+  EXPECT_EQ(1, Format(buf, "%d", (int16_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%d", (int16_t)-1));
+  EXPECT_EQ("-1", std::string(buf));
+  EXPECT_EQ(6, Format(buf, "%d", (int16_t)-32768));
+  EXPECT_EQ("-32768", std::string(buf));
+
+  // Words
+  EXPECT_EQ(1, Format(buf, "%d", (uint32_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(10, Format(buf, "%d", (uint32_t)-1));
+  EXPECT_EQ("4294967295", std::string(buf));
+  EXPECT_EQ(1, Format(buf, "%d", (int32_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%d", (int32_t)-1));
+  EXPECT_EQ("-1", std::string(buf));
+  // Work-around for an limitation of C90
+  EXPECT_EQ(11, Format(buf, "%d", (int32_t)-2147483647-1));
+  EXPECT_EQ("-2147483648", std::string(buf));
+
+  // Quads
+  EXPECT_EQ(1, Format(buf, "%d", (uint64_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(20, Format(buf, "%d", (uint64_t)-1));
+  EXPECT_EQ("18446744073709551615", std::string(buf));
+  EXPECT_EQ(1, Format(buf, "%d", (int64_t)1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%d", (int64_t)-1));
+  EXPECT_EQ("-1", std::string(buf));
+  // Work-around for an limitation of C90
+  EXPECT_EQ(20, Format(buf, "%d", (int64_t)-9223372036854775807LL-1));
+  EXPECT_EQ("-9223372036854775808", std::string(buf));
+
+  // Strings (both const and mutable).
+  EXPECT_EQ(4, Format(buf, "test"));
+  EXPECT_EQ("test", std::string(buf));
+  EXPECT_EQ(4, Format(buf, buf));
+  EXPECT_EQ("test", std::string(buf));
+
+  // Pointer
+  char addr[20];
+  sprintf(addr, "0x%llX", (unsigned long long)(uintptr_t)buf);
+  Format(buf, "%p", buf);
+  EXPECT_EQ(std::string(addr), std::string(buf));
+  Format(buf, "%p", (const char *)buf);
+  EXPECT_EQ(std::string(addr), std::string(buf));
+  sprintf(addr, "0x%llX", (unsigned long long)(uintptr_t)sprintf);
+  Format(buf, "%p", sprintf);
+  EXPECT_EQ(std::string(addr), std::string(buf));
+
+  // Padding for pointers is a little more complicated because of the "0x"
+  // prefix. Padding with '0' zeros is relatively straight-forward, but
+  // padding with ' ' spaces requires more effort.
+  sprintf(addr, "0x%017llX", (unsigned long long)(uintptr_t)buf);
+  Format(buf, "%019p", buf);
+  EXPECT_EQ(std::string(addr), std::string(buf));
+  sprintf(addr, "0x%llX", (unsigned long long)(uintptr_t)buf);
+  memset(addr, ' ',
+         (char*)memmove(addr + sizeof(addr) - strlen(addr) - 1,
+                        addr, strlen(addr)+1) - addr);
+  Format(buf, "%19p", buf);
+  EXPECT_EQ(std::string(addr), std::string(buf));
+}
+
+namespace {
+void PrintLongString(char* buf, size_t sz) {
+  // Output a reasonably complex expression into a limited-size buffer.
+  // At least one byte is available for writing the NUL character.
+  CHECK_GT(sz, static_cast<size_t>(0));
+
+  // Allocate slightly more space, so that we can verify that Format()
+  // never writes past the end of the buffer.
+  scoped_ptr<char[]> tmp(new char[sz+2]);
+  memset(tmp.get(), 'X', sz+2);
+
+  // Use Format() to output a complex list of arguments:
+  // - test padding and truncating %c single characters.
+  // - test truncating %s simple strings.
+  // - test mismatching arguments and truncating (for %d != %s).
+  // - test zero-padding and truncating %x hexadecimal numbers.
+  // - test outputting and truncating %d MININT.
+  // - test outputting and truncating %p arbitrary pointer values.
+  // - test outputting, padding and truncating NULL-pointer %s strings.
+  char* out = tmp.get();
+  size_t out_sz = sz;
+  size_t len;
+  for (scoped_ptr<char[]> perfect_buf;;) {
+    size_t needed = FormatN(out, out_sz,
+#if defined(NDEBUG)
+                            "A%2cong %s: %d %010X %d %p%7s", 'l', "string", "",
+#else
+                            "A%2cong %s: %%d %010X %d %p%7s", 'l', "string",
+#endif
+                            0xDEADBEEF, std::numeric_limits<intptr_t>::min(),
+                            PrintLongString, static_cast<char*>(NULL)) + 1;
+
+    // Various sanity checks:
+    // The numbered of characters needed to print the full string should always
+    // be bigger or equal to the bytes that have actually been output.
+    len = strlen(tmp.get());
+    CHECK_GE(needed, len+1);
+
+    // The number of characters output should always fit into the buffer that
+    // was passed into Format().
+    CHECK_LT(len, out_sz);
+
+    // The output is always terminated with a NUL byte (actually, this test is
+    // always going to pass, as strlen() already verified this)
+    EXPECT_FALSE(tmp[len]);
+
+    // ASAN can check that we are not overwriting buffers, iff we make sure the
+    // buffer is exactly the size that we are expecting to be written. After
+    // running FormatN() the first time, it is possible to compute the correct
+    // buffer size for this test. So, allocate a second buffer and run the
+    // exact same FormatN() command again.
+    if (!perfect_buf.get()) {
+      out_sz = std::min(needed, sz);
+      out = new char[out_sz];
+      perfect_buf.reset(out);
+    } else {
+      break;
+    }
+  }
+
+  // All trailing bytes are unchanged.
+  for (size_t i = len+1; i < sz+2; ++i)
+    EXPECT_EQ('X', tmp[i]);
+
+  // The text that was generated by Format() should always match the
+  // equivalent text generated by sprintf(). Please note that the format
+  // string for sprintf() is nor complicated, as it does not have the
+  // benefit of getting type information from the C++ compiler.
+  //
+  // N.B.: It would be so much cleaner to use snprintf(). But unfortunately,
+  //       Visual Studio doesn't support this function, and the work-arounds
+  //       are all really awkward.
+  char ref[256];
+  CHECK_LE(sz, sizeof(ref));
+  sprintf(ref, "A long string: %%d 00DEADBEEF %lld 0x%llX <NULL>",
+          static_cast<long long>(std::numeric_limits<intptr_t>::min()),
+          (long long)PrintLongString);
+  ref[sz-1] = '\000';
+
+#if defined(NDEBUG)
+  const size_t kSSizeMax = std::numeric_limits<ssize_t>::max();
+#else
+  const size_t kSSizeMax = internal::GetFormatSSizeMax();
+#endif
+
+  // Compare the output from Format() to the one from sprintf().
+  EXPECT_EQ(std::string(ref).substr(0, kSSizeMax-1), std::string(tmp.get()));
+
+  // We allocated a slightly larger buffer, so that we could perform some
+  // extra sanity checks. Now that the tests have all passed, we copy the
+  // data to the output buffer that the caller provided.
+  memcpy(buf, tmp.get(), len+1);
+}
+
+#if !defined(NDEBUG)
+class ScopedFormatSSizeMaxSetter {
+ public:
+  ScopedFormatSSizeMaxSetter(size_t sz) {
+    old_ssize_max_ = internal::GetFormatSSizeMax();
+    internal::SetFormatSSizeMax(sz);
+  }
+
+  ~ScopedFormatSSizeMaxSetter() {
+    internal::SetFormatSSizeMax(old_ssize_max_);
+  }
+
+ private:
+  size_t old_ssize_max_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedFormatSSizeMaxSetter);
+};
+#endif
+
+}  // anonymous namespace
+
+TEST(FormatTest, Truncation) {
+  // We use PrintLongString() to print a complex long string and then
+  // truncate to all possible lengths. This ends up exercising a lot of
+  // different code paths in Format() and IToASCII(), as truncation can
+  // happen in a lot of different states.
+  char ref[256];
+  PrintLongString(ref, sizeof(ref));
+  for (size_t i = strlen(ref)+1; i; --i) {
+    char buf[sizeof(ref)];
+    PrintLongString(buf, i);
+    EXPECT_EQ(std::string(ref, i - 1), std::string(buf));
+  }
+
+  // When compiling in debug mode, we have the ability to fake a small
+  // upper limit for the maximum value that can be stored in an ssize_t.
+  // Format() uses this upper limit to determine how many bytes it will
+  // write to the buffer, even if the caller claimed a bigger buffer size.
+  // Repeat the truncation test and verify that this other code path in
+  // Format() works correctly, too.
+#if !defined(NDEBUG)
+  for (size_t i = strlen(ref)+1; i > 1; --i) {
+    ScopedFormatSSizeMaxSetter ssize_max_setter(i);
+    char buf[sizeof(ref)];
+    PrintLongString(buf, sizeof(buf));
+    EXPECT_EQ(std::string(ref, i - 1), std::string(buf));
+  }
+
+  // kSSizeMax is also used to constrain the maximum amount of padding, before
+  // Format() detects an error in the format string.
+  ScopedFormatSSizeMaxSetter ssize_max_setter(100);
+  char buf[256];
+  EXPECT_EQ(99, Format(buf, "%99c", ' '));
+  EXPECT_EQ(std::string(99, ' '), std::string(buf));
+  *buf = '\000';
+#if defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, "%100c", ' '), "padding <= max_padding");
+#endif
+  EXPECT_EQ(0, *buf);
+#endif
+}
+
+TEST(FormatTest, Padding) {
+  char buf[40], fmt[40];
+
+  // Chars %c
+  EXPECT_EQ(1, Format(buf, "%c", 'A'));
+  EXPECT_EQ("A", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%2c", 'A'));
+  EXPECT_EQ(" A", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%02c", 'A'));
+  EXPECT_EQ(" A", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%-2c", 'A'));
+  EXPECT_EQ("%-2c", std::string(buf));
+  Format(fmt, "%%%dc", std::numeric_limits<ssize_t>::max() - 1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, Format(buf, fmt, 'A'));
+  Format(fmt, "%%%dc",
+         static_cast<size_t>(std::numeric_limits<ssize_t>::max()));
+#if defined(NDEBUG)
+  EXPECT_EQ(2, Format(buf, fmt, 'A'));
+  EXPECT_EQ("%c", std::string(buf));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, fmt, 'A'), "padding <= max_padding");
+#endif
+
+  // Octal %o
+  EXPECT_EQ(1, Format(buf, "%o", 1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%2o", 1));
+  EXPECT_EQ(" 1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%02o", 1));
+  EXPECT_EQ("01", std::string(buf));
+  EXPECT_EQ(12, Format(buf, "%12o", -1));
+  EXPECT_EQ(" 37777777777", std::string(buf));
+  EXPECT_EQ(12, Format(buf, "%012o", -1));
+  EXPECT_EQ("037777777777", std::string(buf));
+  EXPECT_EQ(23, Format(buf, "%23o", -1LL));
+  EXPECT_EQ(" 1777777777777777777777", std::string(buf));
+  EXPECT_EQ(23, Format(buf, "%023o", -1LL));
+  EXPECT_EQ("01777777777777777777777", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%2o", 0111));
+  EXPECT_EQ("111", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%-2o", 1));
+  EXPECT_EQ("%-2o", std::string(buf));
+  Format(fmt, "%%%do", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, 1));
+  EXPECT_EQ("   ", std::string(buf));
+  Format(fmt, "%%0%do", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, 1));
+  EXPECT_EQ("000", std::string(buf));
+  Format(fmt, "%%%do",
+         static_cast<size_t>(std::numeric_limits<ssize_t>::max()));
+#if defined(NDEBUG)
+  EXPECT_EQ(2, Format(buf, fmt, 1));
+  EXPECT_EQ("%o", std::string(buf));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, fmt, 1), "padding <= max_padding");
+#endif
+
+  // Decimals %d
+  EXPECT_EQ(1, Format(buf, "%d", 1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%2d", 1));
+  EXPECT_EQ(" 1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%02d", 1));
+  EXPECT_EQ("01", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%3d", -1));
+  EXPECT_EQ(" -1", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%03d", -1));
+  EXPECT_EQ("-01", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%2d", 111));
+  EXPECT_EQ("111", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%2d", -111));
+  EXPECT_EQ("-111", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%-2d", 1));
+  EXPECT_EQ("%-2d", std::string(buf));
+  Format(fmt, "%%%dd", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, 1));
+  EXPECT_EQ("   ", std::string(buf));
+  Format(fmt, "%%0%dd", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, 1));
+  EXPECT_EQ("000", std::string(buf));
+  Format(fmt, "%%%dd",
+         static_cast<size_t>(std::numeric_limits<ssize_t>::max()));
+#if defined(NDEBUG)
+  EXPECT_EQ(2, Format(buf, fmt, 1));
+  EXPECT_EQ("%d", std::string(buf));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, fmt, 1), "padding <= max_padding");
+#endif
+
+  // Hex %X
+  EXPECT_EQ(1, Format(buf, "%X", 1));
+  EXPECT_EQ("1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%2X", 1));
+  EXPECT_EQ(" 1", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%02X", 1));
+  EXPECT_EQ("01", std::string(buf));
+  EXPECT_EQ(9, Format(buf, "%9X", -1));
+  EXPECT_EQ(" FFFFFFFF", std::string(buf));
+  EXPECT_EQ(9, Format(buf, "%09X", -1));
+  EXPECT_EQ("0FFFFFFFF", std::string(buf));
+  EXPECT_EQ(17, Format(buf, "%17X", -1LL));
+  EXPECT_EQ(" FFFFFFFFFFFFFFFF", std::string(buf));
+  EXPECT_EQ(17, Format(buf, "%017X", -1LL));
+  EXPECT_EQ("0FFFFFFFFFFFFFFFF", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%2X", 0x111));
+  EXPECT_EQ("111", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%-2X", 1));
+  EXPECT_EQ("%-2X", std::string(buf));
+  Format(fmt, "%%%dX", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, 1));
+  EXPECT_EQ("   ", std::string(buf));
+  Format(fmt, "%%0%dX", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, 1));
+  EXPECT_EQ("000", std::string(buf));
+  Format(fmt, "%%%dX",
+         static_cast<size_t>(std::numeric_limits<ssize_t>::max()));
+#if defined(NDEBUG)
+  EXPECT_EQ(2, Format(buf, fmt, 1));
+  EXPECT_EQ("%X", std::string(buf));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, fmt, 1), "padding <= max_padding");
+#endif
+
+  // Pointer %p
+  EXPECT_EQ(3, Format(buf, "%p", (void*)1));
+  EXPECT_EQ("0x1", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%4p", (void*)1));
+  EXPECT_EQ(" 0x1", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%04p", (void*)1));
+  EXPECT_EQ("0x01", std::string(buf));
+  EXPECT_EQ(5, Format(buf, "%4p", (void*)0x111));
+  EXPECT_EQ("0x111", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%-2p", (void*)1));
+  EXPECT_EQ("%-2p", std::string(buf));
+  Format(fmt, "%%%dp", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1,
+            FormatN(buf, 4, fmt, (void*)1));
+  EXPECT_EQ("   ", std::string(buf));
+  Format(fmt, "%%0%dp", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1,
+            FormatN(buf, 4, fmt, (void*)1));
+  EXPECT_EQ("0x0", std::string(buf));
+  Format(fmt, "%%%dp",
+         static_cast<size_t>(std::numeric_limits<ssize_t>::max()));
+#if defined(NDEBUG)
+  EXPECT_EQ(2, Format(buf, fmt, 1));
+  EXPECT_EQ("%p", std::string(buf));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, fmt, 1), "padding <= max_padding");
+#endif
+
+  // String
+  EXPECT_EQ(1, Format(buf, "%s", "A"));
+  EXPECT_EQ("A", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%2s", "A"));
+  EXPECT_EQ(" A", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%02s", "A"));
+  EXPECT_EQ(" A", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%2s", "AAA"));
+  EXPECT_EQ("AAA", std::string(buf));
+  EXPECT_EQ(4, Format(buf, "%-2s", "A"));
+  EXPECT_EQ("%-2s", std::string(buf));
+  Format(fmt, "%%%ds", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, "A"));
+  EXPECT_EQ("   ", std::string(buf));
+  Format(fmt, "%%0%ds", std::numeric_limits<ssize_t>::max()-1);
+  EXPECT_EQ(std::numeric_limits<ssize_t>::max()-1, FormatN(buf, 4, fmt, "A"));
+  EXPECT_EQ("   ", std::string(buf));
+  Format(fmt, "%%%ds",
+         static_cast<size_t>(std::numeric_limits<ssize_t>::max()));
+#if defined(NDEBUG)
+  EXPECT_EQ(2, Format(buf, fmt, "A"));
+  EXPECT_EQ("%s", std::string(buf));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, fmt, "A"), "padding <= max_padding");
+#endif
+}
+
+TEST(FormatTest, EmbeddedNul) {
+  char buf[] = { 'X', 'X', 'X', 'X' };
+  EXPECT_EQ(2, Format(buf, "%3c", 0));
+  EXPECT_EQ(' ', buf[0]);
+  EXPECT_EQ(' ', buf[1]);
+  EXPECT_EQ(0,   buf[2]);
+  EXPECT_EQ('X', buf[3]);
+
+  // Check handling of a NUL format character. N.B. this takes two different
+  // code paths depending on whether we are actually passing arguments. If
+  // we don't have any arguments, we are running in the fast-path code, that
+  // looks (almost) like a strncpy().
+#if defined(NDEBUG)
+  EXPECT_EQ(2, Format(buf, "%%%"));
+  EXPECT_EQ("%%", std::string(buf));
+  EXPECT_EQ(2, Format(buf, "%%%", 0));
+  EXPECT_EQ("%%", std::string(buf));
+#elif defined(ALLOW_DEATH_TEST)
+  EXPECT_DEATH(Format(buf, "%%%"), "src.1. == '%'");
+  EXPECT_DEATH(Format(buf, "%%%", 0), "ch");
+#endif
+}
+
+TEST(FormatTest, EmitNULL) {
+  char buf[40];
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wconversion-null"
+#endif
+  EXPECT_EQ(1, Format(buf, "%d", NULL));
+  EXPECT_EQ("0", std::string(buf));
+  EXPECT_EQ(3, Format(buf, "%p", NULL));
+  EXPECT_EQ("0x0", std::string(buf));
+  EXPECT_EQ(6, Format(buf, "%s", NULL));
+  EXPECT_EQ("<NULL>", std::string(buf));
+#if defined(__GCC__)
+#pragma GCC diagnostic pop
+#endif
+}
+
+TEST(FormatTest, PointerSize) {
+  // The internal data representation is a 64bit value, independent of the
+  // native word size. We want to perform sign-extension for signed integers,
+  // but we want to avoid doing so for pointer types. This could be a
+  // problem on systems, where pointers are only 32bit. This tests verifies
+  // that there is no such problem.
+  char *str = reinterpret_cast<char *>(0x80000000u);
+  void *ptr = str;
+  char buf[40];
+  EXPECT_EQ(10, Format(buf, "%p", str));
+  EXPECT_EQ("0x80000000", std::string(buf));
+  EXPECT_EQ(10, Format(buf, "%p", ptr));
+  EXPECT_EQ("0x80000000", std::string(buf));
+}

[jln (very slow on Chromium), 2013/08/14 04:18:06]

Do you mind adding a dead simple test that would catch off-by ones with ASAN ?

I've seen that your more complicated test was amended for this, but it would
made certain bug immediately obvious.

something like:

const char kTestString[] = "This is a test";
scoped_ptr.... (new ...[sizeof(kTestString)]);

Format(buf, %s, kTestString);

EXPECT_EQ()..

It's redundant, but it would go a long way making a simple test fail on
off-by-one.

Other format characters would be appreciated.

+
+}  // namespace debug
+}  // namespace base