Printf.printf and related functions do not take precision argument for strings

[vicuna]

Original bug ID: 6942
Reporter: acascella
Status: acknowledged (set by @damiendoligez on 2016-11-08T10:19:06Z)
Resolution: open
Priority: low
Severity: feature
Platform: x86_64
OS: GNU/Linux
OS Version: 3.13.0-57-generi
Version: 4.02.2
Category: standard library
Monitored by: @gasche

Bug description

I noticed that precision in formatters for strings is ignored. For instance Printf.printf "%.2" "Hello" will print "Hello".

In C, the equivalent call would print only the first two chars, "He". The precision is interpreted as the maximum number of bytes to be written to the output.

I know that the OCaml documentation for Printf doesn't specify the behavior of precision for strings, so this is not a bug. I was only wondering if there is a reason for not handling precision for strings, or if it will be a desirable feature to add.

Steps to reproduce

OCaml version 4.02.2

Printf.printf "%s" "Hello";;

Hello- : unit = ()

Printf.printf "%.2s" "Hello" ;;

Hello- : unit = ()

[vicuna]

Comment author: @gasche

Note that both "%10s" "%.10s" pad when the input string is less than 10 characters (and you can use "-" for padding to the right, but 0-padding is ignored). The part that was never implemented is cutting the input when the width specification is smaller than the string size.

[vicuna]

Comment author: acascella

The part that was never implemented is cutting the input when the width specification is smaller than the string size.

Actually specifying a width smaller than the size of the string has no effect:

printf("%2s", "Hello") will print "Hello".

The width gives the minimum width of the field.
The precision the maximum number of bytes to write.

The format '%x.ys' will print at most y chars into a field of size x, padding with spaces if needed.

On the other side, if a precision is given, the cutting takes place whatever the width specified, before eventually padding to the minimum width.

printf("%10.2s", "Hello") will print "________He". (where _ = space).

So it would be useful to have both width and precision in string formats.

[vicuna]

Comment author: bvaugon

This feature may be easy to implement, I think, and sometimes useful.
Another useful feature may be to print ellipsis when the string is
truncated.

However, remarks that truncating a string "anywhere" is "dangerous"
when you print (for example) some utf8 data because you may truncate a
multi-byte character.

[vicuna]

Comment author: @gasche

One think I like with this proposal is that at least it gives a clear specification of what the padding and precision numbers do for non-floating-point parameters.

[vicuna]

Comment author: acascella

@bvaugon: you're right, truncation should be done by number of chars and not bytes, in order to avoid that kind of problem with multi-byte characters.

@gasche: I think it is a good idea to use the C specification, which is clear and precise, when possible. (You can have a look at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf starting page 274 for the printf family functions).

[github-actions]

This issue has been open one year with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. Comments that provide new information on the issue are especially welcome: is it still reproducible? did it appear in other contexts? how critical is it? etc.