[LLVMdev] raw_ostream behavior

[Shankar Easwaran]

On 8/14/2013 2:53 PM, Shankar Easwaran 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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>>>
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>
>


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