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// The LLVM Linker
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file contains a few utility functions to handle files.
#include "Filesystem.h"
#include "Config.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/FileOutputBuffer.h"
#include <thread>
using namespace llvm;
using namespace lld;
using namespace lld::elf;
// Removes a given file asynchronously. This is a performance hack,
// so remove this when operating systems are improved.
// On Linux (and probably on other Unix-like systems), unlink(2) is a
// noticeably slow system call. As of 2016, unlink takes 250
// milliseconds to remove a 1 GB file on ext4 filesystem on my machine.
// To create a new result file, we first remove existing file. So, if
// you repeatedly link a 1 GB program in a regular compile-link-debug
// cycle, every cycle wastes 250 milliseconds only to remove a file.
// Since LLD can link a 1 GB binary in about 5 seconds, that waste
// actually counts.
// This function spawns a background thread to call unlink.
// The calling thread returns almost immediately.
void elf::unlinkAsync(StringRef Path) {
if (!Config->Threads || !sys::fs::exists(Config->OutputFile))
// First, rename Path to avoid race condition. We cannot remove
// Path from a different thread because we are now going to create
// Path as a new file. If we do that in a different thread, the new
// thread can remove the new file.
SmallString<128> TempPath;
if (sys::fs::createUniqueFile(Path + "tmp%%%%%%%%", TempPath))
if (sys::fs::rename(Path, TempPath)) {
// Remove TempPath in background.
std::thread([=] { ::remove(TempPath.str().str().c_str()); }).detach();
// Simulate file creation to see if Path is writable.
// Determining whether a file is writable or not is amazingly hard,
// and after all the only reliable way of doing that is to actually
// create a file. But we don't want to do that in this function
// because LLD shouldn't update any file if it will end in a failure.
// We also don't want to reimplement heuristics to determine if a
// file is writable. So we'll let FileOutputBuffer do the work.
// FileOutputBuffer doesn't touch a desitnation file until commit()
// is called. We use that class without calling commit() to predict
// if the given file is writable.
std::error_code elf::tryCreateFile(StringRef Path) {
if (Path.empty())
return std::error_code();
return FileOutputBuffer::create(Path, 1).getError();