`::dup2(fd, -1)` does not return -1 or set `errno`

[hly2019]

Please include the following in your bug report:

Version of emscripten/emsdk:
3.1.54

Failing command line in full:

# wasm:
emcc dup2.cpp -o main.js 
node main.js

# native:
g++ dup2.cpp -o main
./main

Hi, it seems the return value of int dup2(int oldfd, int newfd) is not correct and the errno is not set when the newfd is -1, as shown in the code and results below.

According to https://man7.org/linux/man-pages/man2/dup.2.html#RETURN_VALUE, dup2 should return -1 and set errno on error, and so it does with native execution.
But in wasm execution, the return value is not -1, and the errno is not set when the second argument, new_fd is -1. This issue may lead to problems with error checking.

I'd appreciate it so much if you could check it. Thanks!

Code:

Here shows the case I faced.
I also tried the case when the fd is got from calling ::open with a regular file (compiled with --preload-file), and the same problem also happens.

#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
int main() {
    int fds[2];
    pipe(fds);
    int fd = fds[0];
    // int fd = ::open("temp.txt", O_RDWR); // compile with --preload-file temp.txt
    int ret = ::dup2(fd, -1); // the problem remains if here the fd is from ::open with regular file
    printf("ret of dup2: %d, error num: %d, error info: %s\n", ret, errno, strerror(errno));
    return 0;
}

Results:

In wasm, the errno is not set and the return value is also not -1, which is not as expected.

# in wasm:
ret of dup2: 5, error num: 0, error info: No error information

# in native:
ret of dup2: -1, error num: 9, error info: Bad file descriptor