[LLVMdev] raw_ostream behavior

[Nick Kledzik]

On Aug 14, 2013, at 3:00 PM, Dan Gohman <dan433584 at gmail.com> wrote:
> Standard error is what many tools, including clang for example, use for their --verbose output. This is appropriate because it leaves standard output available for regular output data, which the user may wish to capture in a file or a pipe.
Well, at least with the darwin linker, the linker main output goes to a file (specified by -o or “a.out”). So, since there is normally nothing going to stdout, -v and -t output goes to stdout.

Shankar, although not quite as convenient, we could change the core linker test cases from:
      lld … | FileCheck 
to:
     lld …  -o %t && FileCheck < %t
Then change the core linker to not use “-“, but error if -o is not supplied.

-Nick

> 
> On Wed, Aug 14, 2013 at 2:48 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:
> Hi Dan,
> 
> The other messages, need to go to stdout as well. Using stderr to output for a switch like --verbose is wrong in my opinion.
> 
> 
> On 8/14/2013 4:44 PM, Dan Gohman wrote:
> Would it be appropriate for lld's other output to go to standard error
> instead of standard output, since standard output may be used for YAML
> output?
> 
> Dan
> 
> On Wed, Aug 14, 2013 at 12:53 PM, Shankar Easwaran
> <shankare at codeaurora.org>wrote:
> 
> Hi Dan,
> 
> lld takes a -emit-yaml option, which emits the intermediate
> representation(atoms) in YAML form.
> 
> By default output goes to stdout, the user can control it by using the -o
> option too.
> 
> The way its handled is, similiar to this piece of pseudo-code
> 
> if (dash_o_option)
>      outputFile = dash_o_option->value()
> else
>     outputFile = "-"
> 
> When lld tries to mix things that go to stdout using llvm::outs() and
> -emit-yaml, it starts to get this error.
> 
> If there is a restriction like this, possibly it has to be mentioned
> somewhere for usecases (or) raw_fd_ostream should not be accepting a '-' in
> the argument.
> 
> What do you think ?
> 
> Thanks
> 
> Shankar Easwaran
> 
> 
> On 8/14/2013 2:02 PM, Dan Gohman wrote:
> 
> The high-level answer is that library code shouldn't hardcode output
> paths,
> including "-" and including using llvm::outs(). Library code should (in
> general) instead accept an ostream in its API and write to wherever it is
> being asked to write. If this is followed, the only place in any program
> using llvm::outs() is code which is not part of a "library" -- i.e. it is
> the "main" code, and it should only do so if it is a program which doesn't
> open file names which could be "-".
> 
> Do you have a use-case where this restriction is too restrictive?
> 
> As a random aside, a more aggressive answer could be that "-" is a kind of
> hack, and that it would be a better approach to use /dev/stdout and
> /dev/stdin instead of "-", because they wouldn't require special-case
> logic. Unfortunately, this may not be sufficiently portable.
> 
> Dan
> 
> 
> On Wed, Aug 14, 2013 at 8:28 AM, Shankar Easwaran
> <shankare at codeaurora.org>**wrote:
> 
>   Hi,
> When I run the below example, it results in :-
> 
> hello
> world
> LLVM ERROR: IO failure on output stream.
> 
> Testcase :-
> 
> #include <llvm/Support/raw_ostream.h>
> 
> int fn() {
>     std::string errorInfo;
>     llvm::raw_fd_ostream out("-", errorInfo);
>     out << "world\n";
>     return 0;
> }
> int main(int argc, char **argv) {
>     llvm::outs() << "hello\n";
>     fn();
>     return 0;
> }
> 
> I tried to fix this by making llvm::outs(), not close the stdout
> descriptor, but I think its wrong. Recommendations ?
> 
> Thanks
> 
> Shankar Easwaran
> 
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