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-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "src/unistd/pread.h"
10
11#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
12#include "src/__support/common.h"
13#include "src/__support/libc_errno.h"
14#include "src/__support/macros/config.h"
15#include "src/__support/macros/sanitizer.h" // for MSAN_UNPOISON
16#include <stdint.h> // For uint64_t.
17#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 pointer
39 // 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_DECL
49

source code of libc/src/unistd/linux/pread.cpp