Print with less(1) from C

Question

I would like to print a big matrix of data which I have in a linked list. It doesn't fit into a terminal (80 lines), so it is inconvenient to print it with standard printing functions; and less is already invented, so I wouldn't want to be reinventing it using ncurses; so I want to pass some printfd lines to less.

My first guess would be to write to a file, and then system("less -S file");, and then delete the file.

A more complicated solution would be to rewrite less so that its main() is converted to a less() function that I can call from C, and instead of a filename string I could provide it with a file descriptor or a stream.

Is there any way that doesn't involve needing to create a file and also not needing to rewrite (part of) less?

I assume the obvious way of external `./program > less` is unsuitable? — Neil, Feb 02 '20 at 23:02
I think the suggestion should have been `./program | less`. You could consider using `FILE *fp = popen("less", "w");` and then writing all output to `fp`. You'd use `pclose()` to close the stream when you've finished writing to it. — Jonathan Leffler, Feb 02 '20 at 23:09
@JonathanLeffler `popen()` is exactly what I was looking for! — alx - recommends codidact, Feb 02 '20 at 23:15

score 3 · Accepted Answer · answered Feb 02 '20 at 23:21

3

You could consider using POSIX functions popen() and pclose().

You'd use:

FILE *fp = popen("less", "w");

if (fp != NULL)
{
    …write output to fp…
    pclose(fp);
}
else
    …report error…

Note that the pclose() will wait for less to exit. If you want to, you can capture the return value from pclose() and analyze it. See How to detect if shell failed to execute a command after popen()? for discussion of this.

answered Feb 02 '20 at 23:21

Jonathan Leffler

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For those who read this question in the future: there is one little difference to shell `| less` in this code. Apparently, "you can't use Ctrl+C to stop skipping to the end of the output without killing both less and the original program". Source: [Piping into pager (less) from C program](https://bbs.archlinux.org/viewtopic.php?id=159460). It may or may not be a problem. – alx - recommends codidact Feb 02 '20 at 23:30
1

Use `q` instead of Control-C (or whatever your interrupt key is) to exit from `less`. You could consider adding signal handling to your program, but it isn't trivial to get right — you probably end up simulating `popen()` and `pclose()` rather than using them. – Jonathan Leffler Feb 02 '20 at 23:33
Actually that's not a problem for me, but after reading your answer, I googled it and found that difference. As I understood it, it's not a problem about closing `less`, but about stopping it from something that it's doing. From `man less`: *"Normally, an interrupt character causes less to stop whatever it is doing and return to its command prompt."*. I have never used that command in less before, however. For my usage, `popen()` is just perfect. I guess, as you said, that catching the signal in the program and passing the interrupt character through the stream would solve that. – alx - recommends codidact Feb 02 '20 at 23:49

Print with less(1) from C

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