ENOTTY: Inappropriate ioctl for device: program works, but not when executed inside <()

Question

I have simple program that reads file line by line, and prints each line. The core of the program is:

while ((size = getline(&line, &len, f)) != -1)
    printf("%s", line);

Now I have following problem:

this works:

./test zz

this also works:

/bin/cat <(./test zz)

but when I pipe the output to other program, it does not work:

/bin/cat <(./test zz) | /bin/cat

there is no output. When I compare with strace the one that works, and the one that doesn't:

/bin/cat <(strace ./test zz)

/bin/cat <(strace ./test zz) | /bin/cat

I see this error: -1 ENOTTY (Inappropriate ioctl for device)

ioctl(0, TCGETS, 0x7ffc302104d0)        = -1 ENOTTY (Inappropriate ioctl for device)
brk(NULL)                               = 0x55b4e1fd3000
brk(0x55b4e1ff4000)                     = 0x55b4e1ff4000
fstat(0, {st_mode=S_IFCHR|0666, st_rdev=makedev(0x1, 0x3), ...}) = 0
ioctl(0, TCGETS, 0x7ffc30210350)        = -1 ENOTTY (Inappropriate ioctl for device)
read(0, "", 4096)                       = 0
close(0)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++

EDIT:

here is my proof-of-concept code:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <stdbool.h>
#include <unistd.h>


int main(int argc, char **argv) {

    FILE *f;
    char *line = NULL;
    size_t len = 0;
    ssize_t size, cut;


    if (!isatty(STDIN_FILENO) || argc < 2 || (argv[1] == "-"))
        f = stdin;
    else
        if (!(f = fopen(argv[1], "r"))) {
            perror(argv[1]);
            return 1;
        }

    while ((size = getline(&line, &len, f)) != -1)
    printf("%s", line);

    fflush(stdout);
    fclose(f);
    if (line)
        free(line);
    exit(EXIT_SUCCESS);
}

EDIT2:

I have removed the part: (!isatty(STDIN_FILENO) and that fixed the problem. But I don't understand what is happening.

I originally added this, so that my program can take filename as argument ./test zz or take input from pipe cat zz | ./test.

The `ioctl` is the implementation of the `isatty` function, which either your code or the standard library presumably calls. The "error" return simply indicates that fd 0 is not a terminal, which is correct. So I don't think that's directly related to your problem. You should post a [mcve]. — Nate Eldredge, Mar 16 '23 at 17:55
@Nate Eldredge thanks. I have added proof-of-concept code. Please see my edit. — 400 the Cat, Mar 16 '23 at 17:58
If you want each line to be written out to the pipe as soon as you print it, then you have to fflush immediately - i.e. *inside* the loop, not after it completes. Your program does work as it stands, in that all the input eventually gets echoed to the output - just not immediately after every line. — Nate Eldredge, Mar 16 '23 at 18:10
@Nate Eldredge - I moved the flush inside the while loop, but it makes no difference. — 400 the Cat, Mar 16 '23 at 18:12
Aside: `argv[1] == "-"` does not do what you want. Use `strcmp` to compare strings. — Ian Abbott, Mar 16 '23 at 18:24
It seems that standard input of the `./test` execution is not actually coming from a terminal. Are you running it from an IDE? — Ian Abbott, Mar 16 '23 at 18:36
Just to confirm, does `[ -t 0 ] && echo terminal` output `terminal`? — Ian Abbott, Mar 16 '23 at 18:53
@Ian Abbott - yes. all teh above was done in `zsh` and when I do as you say it returns `terminal`. But interestingly, when I use bash, instead of `zsh` the command works fine: `/bin/cat <(./test zz) | /bin/cat` — 400 the Cat, Mar 16 '23 at 19:01
@Ian Abbott - could you please explain why I should use `strcmp` instead of`argv[1] == "-"`? It seems to work fine. — 400 the Cat, Mar 16 '23 at 19:23
@400theCat, because the version with `==` doesn't mean what you think it means. See [How do I properly compare strings in C?](https://stackoverflow.com/q/8004237/2402272) — John Bollinger, Mar 16 '23 at 19:35
OK, so it seems to something to do with the way Zsh supplies standard input for that command. I don't know why it would supply standard input for `./test` differently in `/bin/cat <(./test)` and `/bin/cat <(./test) | /bin/cat`. — Ian Abbott, Mar 17 '23 at 11:27

ikegami · Accepted Answer · 2023-03-16T19:23:05.343

3

If any of the following conditions are true, the program uses stdin for input:

stdin isn't connected a terminal.
No arguments were provided.
The first argument is -.

In the scenario you presented, stdin isn't connected to a terminal^[1], so the test reads from stdin. You don't get the desired result since you want to read from the file zz.

The first condition (!isatty(STDIN_FILENO)) makes no sense. You were correct to remove it. The remaining two conditions are sufficient to make cat zz | ./test work.

I don't know why. But it's not important. There are plenty of times stdin isn't a terminal. For example, the stdin of daemons aren't terminals, and thus the stdin of program launched by deamons aren't terminals unless the daemon takes steps to make it so.

edited Mar 16 '23 at 19:23

answered Mar 16 '23 at 18:55

ikegami

367,544
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this solutiuon was suggested to me when I asked few days ago: https://stackoverflow.com/questions/75730197/error-handling-when-opening-either-file-or-stdin how else should I decide whether my program is on the receiving side of a pipe? – 400 the Cat Mar 16 '23 at 18:58
2

But you don't need to know whether it's on the receiving end of a pipe. – ikegami Mar 16 '23 at 18:58

score -3 · Answer 2 · answered Mar 16 '23 at 18:00

-3

The flush should be done to stdout, not the file you're reading.

answered Mar 16 '23 at 18:00

Solomon Ucko

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fixed that. But it makes no difference. – 400 the Cat Mar 16 '23 at 18:06

ENOTTY: Inappropriate ioctl for device: program works, but not when executed inside <()

EDIT:

EDIT2:

2 Answers2