# "Unget" char from stream

**URL:** <https://discourse.julialang.org/t/unget-char-from-stream/68588>\
**Category:** General Usage\
**Tags:** question\
**Created:** [September 22, 2021, 3:16pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588 "2021-09-22T15:16:32Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![ess3sq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ess3sq/32/26834_2.png) [@ess3sq](https://discourse.julialang.org/u/ess3sq)\
**Post date:** [September 22, 2021, 3:16pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/1 "2021-09-22T15:16:32Z")

</div>

Hello there, I was wondering whether the Julia IO API provided a built-in facility for _ungetting characters_ from an ` ::IO` stream in a way analogous to C’s `ungetc()` function.

If not I am aware I can implement it by caching them in some stack and popping the stack instead of reading from the actual stream, but it would be good to know if such facilities are provided.

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [September 22, 2021, 5:14pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/2 "2021-09-22T17:14:58Z")

</div>

From one I can tell, in Julia one typically uses `peek` and/or `seek` and/or `mark` and `reset` for situations where one might otherwise use `ungetc`. Did you have a specific application in mind?

---

<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [September 22, 2021, 5:31pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/3 "2021-09-22T17:31:54Z")

</div>

(I’ve changed the title to fix the typo `Unger` → `Unget`.)

Agreed with @stevengj - usually, instead of directly reading a character (if desired), you’d `peek` first to avoid the sort of push-back associated with `ungetc`.

---

<div class="post-metadata">

**Author:** ![ess3sq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ess3sq/32/26834_2.png) [@ess3sq](https://discourse.julialang.org/u/ess3sq)\
**Post date:** [September 22, 2021, 8:15pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/4 "2021-09-22T20:15:18Z")

</div>

Thanks for the suggestion. `peek()` is indeed an acceptable solution in my case (I’m tokenizing some file) and will probably be what I’ll end up using. The motivations for using `ungetc()` as opposed to `peek()` _in C_, however, are not satisfied here. They are not necessary in my case, but may be in others, which warrants interest.

The first is that ungetc reduces the amount of system calls. This is because usual implementations don’t actually put the char back onto the stream, but rather in a side stack, and when stuff is read fr the stream, first the stack is popped until it’s empty, and then the system calls (`read()`) are resumed. If a file is large enough and contains a lot of peek situations, peek calls will lead to the same character being read again, whereas a char that was previously ungetc’d will not be read through a system call the next time. I am not entirely sure what kind of benchmarking this yields but I might try this out both in C and Julia to see if my argument holds water.

The second motivation is that ungetc is not simply an alternative to peek becausw the char that is pushed back needs not be the actual char that comes from the previous read operation. You can push back anything you like, which is not at all what I intend to do in my problem, but may be for others.

---

<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [September 22, 2021, 8:46pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/5 "2021-09-22T20:46:13Z")

</div>

The default `IO` objects already buffer read data and read in batches, so there isn’t necessarily a `read` syscall just because you’ve written `read` in julia (you can check `src/support/ios.[c/h]` if you want to find out more about the default `IOStream` you get when opening a file. You can find this by doing `@edit peek(open("file.txt"), UInt8)`). Moreover, the underlying implementation also has an `ios_ungetc` defined. It isn’t exposed on the top level in general, most likely because that doesn’t translate well to anything other than single bytes - and even that isn’t guaranteed to actually work on all systems for more than one byte. E.g. on my system, only one pushback is guaranteed per the man page:

> `ungetc()` pushes `c` back to stream, cast to `unsigned char`, where it is available for subsequent read operations. Pushed-back characters will be returned in reverse order; only one pushback is guaranteed.

---

<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [September 22, 2021, 8:53pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/6 "2021-09-22T20:53:33Z")

</div>

> [@ess3sq](#):
>
> I’m tokenizing some file

May I suggest the excellent [Parsers.jl](https://juliahub.com/ui/Packages/Parsers/833b9/2.0.3) package? I think if you’re doing tokenization/parsing, you’ll see great benefits.

---

<div class="post-metadata">

**Author:** ![ess3sq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ess3sq/32/26834_2.png) [@ess3sq](https://discourse.julialang.org/u/ess3sq)\
**Post date:** [September 22, 2021, 9:29pm UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/7 "2021-09-22T21:29:23Z")

</div>

That looks very enticing, I’ll check it out!

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [September 23, 2021, 12:42am UTC](https://discourse.julialang.org/t/unget-char-from-stream/68588/8 "2021-09-23T00:42:44Z")

</div>

> [@Sukera](#):
>
> Moreover, the underlying implementation also has an `ios_ungetc` defined. It isn’t exposed on the top level in general,

It would be easy enough to expose this (just change `ios.h` to export it with `JL_DLLEXPORT` and then write a thin wrapper), but it would only work for `IOStream` objects (what you get when you open files). For several other stream types Julia uses [libuv](https://libuv.org/), which doesn’t implement `ungetc` as far as I can tell, though we support a buffering layer on top of libuv that might be used to provide this functionality.

> [@ess3sq](#):
>
> The second motivation is that ungetc is not simply an alternative to peek becausw the char that is pushed back needs not be the actual char that comes from the previous read operation.

That’s true, but I’m unclear on the usefulness of that functionality.

See also the discussion around the implementation of `mark`/`reset` in Julia (way back in 2014) — the main motivation was that it was so much more general than `peek` and `ungetc` (and subsequently allowed `peek` to be implemented for multi-byte objects): [https://github.com/JuliaLang/julia/pull/3656](https://github.com/JuliaLang/julia/pull/3656)