49 lines · cpp
1//===-- Linux implementation of pread -------------------------------------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8 9#include "src/unistd/pread.h"10 11#include "hdr/stdint_proxy.h" // For uint64_t.12#include "src/__support/OSUtil/syscall.h" // For internal syscall function.13#include "src/__support/common.h"14#include "src/__support/libc_errno.h"15#include "src/__support/macros/config.h"16#include "src/__support/macros/sanitizer.h" // for MSAN_UNPOISON17#include <sys/syscall.h> // For syscall numbers.18 19namespace LIBC_NAMESPACE_DECL {20 21LLVM_LIBC_FUNCTION(ssize_t, pread,22 (int fd, void *buf, size_t count, off_t offset)) {23 ssize_t ret;24 if constexpr (sizeof(long) == sizeof(uint32_t) &&25 sizeof(off_t) == sizeof(uint64_t)) {26 // This is a 32-bit system with a 64-bit offset, offset must be split.27 const uint64_t bits = cpp::bit_cast<uint64_t>(offset);28 const uint32_t lo = bits & UINT32_MAX;29 const uint32_t hi = bits >> 32;30 const long offset_low = cpp::bit_cast<long>(static_cast<long>(lo));31 const long offset_high = cpp::bit_cast<long>(static_cast<long>(hi));32 ret = LIBC_NAMESPACE::syscall_impl<ssize_t>(SYS_pread64, fd, buf, count,33 offset_low, offset_high);34 } else {35 ret = LIBC_NAMESPACE::syscall_impl<ssize_t>(SYS_pread64, fd, buf, count,36 offset);37 }38 // The cast is important since there is a check that dereferences the pointer39 // which fails on void*.40 MSAN_UNPOISON(reinterpret_cast<char *>(buf), count);41 if (ret < 0) {42 libc_errno = static_cast<int>(-ret);43 return -1;44 }45 return ret;46}47 48} // namespace LIBC_NAMESPACE_DECL49