Re: pty: childs don't always react on close(2)

[Dirk Gouders <dirk@gouders.net>]

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Al Viro <viro@zeniv.linux.org.uk> writes:

> On Fri, Dec 05, 2025 at 10:37:44PM +0100, Dirk Gouders wrote:
>
>> 	child_pid1 = forkpty(&pty_fd1, NULL, NULL, NULL);
>
> You do realize that it will inherit all your opened descriptors,
> including pty_fd, right?
>
> ...

>> 	close(pty_fd);
>
> ... which doesn't do anything to the second child's descriptor
> table, including the descriptor that refers to the same opened file.
> IOW, the IO channel (== opened file) is very much opened after
> that close() - descriptors refering to it still exist.

Oh yes, thank you very much!

I use FD_CLOEXEC nearly everywhere in the mentioned program but exactly
not for the pty file descriptors.  Oh well...

For completenes, I'll attach the modified test-case.
Would be interesting to hear if there are other possible fixes.

Thank you very much, again,

Dirk


[-- Attachment #2: PTY test program --]
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/*
 * Test closing file descriptors opened via forkpty() when not all data has been
 * read.  A following waitpid() blocks, when we opened two childs and try to
 * close the file descriptor and then waitpid() for that child...
 */
#include <stdlib.h>
#include <stdio.h>
#include <pty.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdint.h>
#include <fcntl.h>

#define READ_SIZE 4096

/*
 * Set PAGER variables and start a man(1) process.
 */
void do_child(void);
void do_child()
{
	char *e_argv[3] = {"man", "groff_ms", NULL};

	putenv("PAGER=cat");
	putenv("MANPAGER=cat");
	execvp("man", e_argv);
}

int main()
{
	unsigned char read_buffer[READ_SIZE];
	int pty_fd;
	int pty_fd1;
	int wstatus;
	pid_t child_pid;
	pid_t child_pid1;
	pid_t ret_pid;
	ssize_t ret;
	/*
	 * Start a child to send us a manual page.
	 */
	child_pid = forkpty(&pty_fd, NULL, NULL, NULL);

	if (child_pid == -1) {
		fprintf(stderr, "forkpty(): %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}

	ret = fcntl(pty_fd, F_SETFD, FD_CLOEXEC);

	if (child_pid == 0)
		do_child();

	printf("child_pid = %jd\n", (intmax_t) child_pid);

	memset(read_buffer, '\0', READ_SIZE);
	ret = read(pty_fd, read_buffer, READ_SIZE);

	if (ret == -1) {
		fprintf(stderr, "read(): %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}

	printf("%s\n", read_buffer);

	printf("%ld bytes read.\n", ret);


	/*
	 * Start another child to send us a manual page.
	 */
	child_pid1 = forkpty(&pty_fd1, NULL, NULL, NULL);

	if (child_pid1 == -1) {
		fprintf(stderr, "forkpty(): %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}

	ret = fcntl(pty_fd1, F_SETFD, FD_CLOEXEC);

	if (child_pid1 == 0)
		do_child();

	printf("child_pid1 = %jd\n", (intmax_t) child_pid1);

	memset(read_buffer, '\0', READ_SIZE);
	ret = read(pty_fd1, read_buffer, READ_SIZE);

	if (ret == -1) {
		fprintf(stderr, "read(): %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	}

	printf("%s\n", read_buffer);

	printf("%ld bytes read.\n", ret);


	close(pty_fd);

	ret_pid = waitpid(child_pid, &wstatus, 0);

	printf("ret_pid = %jd\n", (intmax_t) ret_pid);

	exit(EXIT_SUCCESS);
}