1//===-- Linux implementation of select ------------------------------------===//
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/sys/select/select.h"
10
11#include "hdr/types/sigset_t.h"
12#include "hdr/types/struct_timespec.h"
13#include "src/__support/CPP/limits.h"
14#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
15#include "src/__support/common.h"
16#include "src/errno/libc_errno.h"
17
18#include <stddef.h> // For size_t
19#include <sys/syscall.h> // For syscall numbers.
20
21namespace LIBC_NAMESPACE {
22
23struct pselect6_sigset_t {
24 sigset_t *ss;
25 size_t ss_len;
26};
27
28LLVM_LIBC_FUNCTION(int, select,
29 (int nfds, fd_set *__restrict read_set,
30 fd_set *__restrict write_set, fd_set *__restrict error_set,
31 struct timeval *__restrict timeout)) {
32 // Linux has a SYS_select syscall but it is not available on all
33 // architectures. So, we use the SYS_pselect6 syscall which is more
34 // widely available. However, SYS_pselect6 takes a struct timespec argument
35 // instead of a struct timeval argument. Also, it takes an additional
36 // argument which is a pointer to an object of a type defined above as
37 // "pselect6_sigset_t".
38 struct timespec ts {
39 .tv_sec: 0, .tv_nsec: 0
40 };
41 if (timeout != nullptr) {
42 // In general, if the tv_sec and tv_usec in |timeout| are correctly set,
43 // then converting tv_usec to nanoseconds will not be a problem. However,
44 // if tv_usec in |timeout| is more than a second, it can lead to overflows.
45 // So, we detect such cases and adjust.
46 constexpr time_t TIME_MAX = cpp::numeric_limits<time_t>::max();
47 if ((TIME_MAX - timeout->tv_sec) < (timeout->tv_usec / 1000000)) {
48 ts.tv_sec = TIME_MAX;
49 ts.tv_nsec = 999999999;
50 } else {
51 ts.tv_sec = timeout->tv_sec + timeout->tv_usec / 1000000;
52 ts.tv_nsec = timeout->tv_usec * 1000;
53 }
54 }
55 pselect6_sigset_t pss{.ss: nullptr, .ss_len: sizeof(sigset_t)};
56#if SYS_pselect6
57 int ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_pselect6, ts: nfds, ts: read_set,
58 ts: write_set, ts: error_set, ts: &ts, ts: &pss);
59#elif defined(SYS_pselect6_time64)
60 int ret = LIBC_NAMESPACE::syscall_impl<int>(
61 SYS_pselect6_time64, nfds, read_set, write_set, error_set, &ts, &pss);
62#else
63#error "SYS_pselect6 and SYS_pselect6_time64 syscalls not available."
64#endif
65 if (ret < 0) {
66 libc_errno = -ret;
67 return -1;
68 }
69 return ret;
70}
71
72} // namespace LIBC_NAMESPACE
73

source code of libc/src/sys/select/linux/select.cpp