My goal is to open the device in one process and then close it in another (not fork).
I don’t want to use UNIX socket. I found that starting kernel 5.6 there are two new syscalls: syscall(SYS_pidfd_open, pid, flags) and syscall(SYS_pidfd_getfd, pidfd, 0, 0).

I tried simple code to check this solution.

open_device.c:

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <errno.h>

#define pidfd_open(pid, flags) syscall(SYS_pidfd_open, pid, flags)

int main() {
    // Open the device
    int fd = open("/dev/null", O_RDWR);
    if (fd == -1) {
        perror("open");
        return EXIT_FAILURE;
    }

    printf("File descriptor for /dev/null: %dn", fd);

    // Get the current process ID
    pid_t pid = getpid();

    // Obtain a pidfd for this process
    int pidfd = pidfd_open(pid, 0);
    if (pidfd == -1) {
        perror("pidfd_open");
        close(fd);
        return EXIT_FAILURE;
    }

    printf("Generated pidfd: %dn", pidfd);

    // Write pidfd to a file
    FILE *pidfd_file = fopen("pidfd.txt", "w");
    if (pidfd_file == NULL) {
        perror("fopen");
        close(fd);
        close(pidfd);
        return EXIT_FAILURE;
    }
    fprintf(pidfd_file, "%dn", pidfd);
    fclose(pidfd_file);

    // Keep the process alive for testing
    while (1) {
        sleep(10);
    }

    // Clean up
    close(fd);
    close(pidfd);

    return 0;
}

close_device.c

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/syscall.h>
#include <errno.h>

#ifndef SYS_pidfd_getfd
#define SYS_pidfd_getfd 438  // Replace with the correct syscall number for your architecture
#endif

int main() {
    // Read the pidfd from the file
    FILE *pidfd_file = fopen("pidfd.txt", "r");
    if (pidfd_file == NULL) {
        perror("fopen");
        return EXIT_FAILURE;
    }

    int pidfd;
    if (fscanf(pidfd_file, "%d", &pidfd) != 1) {
        perror("fscanf");
        fclose(pidfd_file);
        return EXIT_FAILURE;
    }
    fclose(pidfd_file);

    printf("Read pidfd: %dn", pidfd);

    // Retrieve the file descriptor from the first process
    int target_fd = syscall(SYS_pidfd_getfd, pidfd, 0, 0);
    if (target_fd == -1) {
        perror("pidfd_getfd");
        close(pidfd);
        return EXIT_FAILURE;
    }

    printf("Successfully retrieved fd: %dn", target_fd);

    // Perform the close operation
    if (close(target_fd) == -1) {
        perror("close");
        close(pidfd);
        return EXIT_FAILURE;
    }

    printf("Device successfully closed.n");

    close(pidfd);
    return 0;
} 

The problem is that I receive error Bad file descriptor for pidfd_getfd.

Where can be an issue?