blob: 693295a09f1e4313b57aee7ce91f7c0d3b58af99 [file] [log] [blame]
// Copyright 2016 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include <stdarg.h>
#include <stddef.h>
#include <stdio.h>
#include <zircon/assert.h>
#include <array>
#include <memory>
#include <fbl/string_printf.h>
namespace fbl {
String StringPrintf(const char* format, ...) {
va_list ap;
va_start(ap, format);
String rv = StringVPrintf(format, ap);
return rv;
String StringVPrintf(const char* format, va_list ap) {
// Size of the small stack buffer to use first. This should be kept in sync
// with the numbers in StringPrintfTest.StringPrintf_Boundary.
constexpr size_t kStackBufferSize = 1024U;
// First, try with a small buffer on the stack.
std::array<char, kStackBufferSize> stack_buf;
// Copy |ap| (which can only be used once), in case we need to retry.
va_list ap_copy;
va_copy(ap_copy, ap);
int result = vsnprintf(, kStackBufferSize, format, ap_copy);
if (result < 0) {
// As far as I can tell, we'd only get |EOVERFLOW| if the result is so large
// that it can't be represented by an |int| (in which case retrying would be
// futile), so Chromium's implementation is wrong.
return String();
// |result| should be the number of characters we need, not including the
// terminating null. However, |vsnprintf()| always null-terminates!
size_t output_size = static_cast<size_t>(result);
// Check if the output fit into our stack buffer. This is "<" not "<=", since
// |vsnprintf()| will null-terminate.
if (output_size < kStackBufferSize) {
// It fit.
return String(, static_cast<size_t>(result));
// Since we have the required output size, we can just heap allocate that.
// (Add 1 because |vsnprintf()| will always null-terminate.)
size_t heap_buf_size = output_size + 1U;
// Allocate a buffer to print into.
std::unique_ptr<char[]> heap_buf(new char[heap_buf_size]);
result = vsnprintf(heap_buf.get(), heap_buf_size, format, ap);
ZX_ASSERT(result >= 0 && static_cast<size_t>(result) == output_size);
return String(heap_buf.get(), static_cast<size_t>(result));
} // namespace fbl