Issue #13315 has been reported by nobu (Nobuyoshi Nakada).

----------------------------------------
Bug #13315: Single "%" at the end of `printf` format string appears in the result
https://bugs.ruby-lang.org/issues/13315

* Author: nobu (Nobuyoshi Nakada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Is this intentional?

```ruby
printf("%") #=> "%"
```

`printf` in glibc does print nothing, gcc warns though.

```c
#include <stdio.h>

int main(void)
{
  printf("%");
  return 0;
}
```

```sh
$ gcc fmt.c && ./a.out 
fmt.c: In function °∆main°«:
fmt.c:5:10: warning: spurious trailing °∆%°« in format [-Wformat=]
   printf("%");
          ^
```

`printf` command of bash errs.

```sh
$ printf "%"
-bash: printf: `%': missing format character
```



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