...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Now let's revisit our c++filt example and we will put in an obvious mistake. This might however be not as obvious for more complex applications.
vector<string> demangle(vector<string> in) { ipstream is; opstream os; child c("c++filt", std_out > is, std_in < os); vector<string> data; for (auto & elem : data) { string line; getline(is, line); os << elem; } }
We switched the read and write operation up, so that's causing a dead-lock.
This locks immediately. This is because c++filt
expects input, before outputting any data. The launching process on the other
hand waits for its output.
Now for another example, which might look correct, let's consider you want
to use ls
to read the current
directory.
ipstream is; child c("ls", std_out > is); std::string file; while (is >> file) cout << "File: " << file << endl;
This will also deadlock, because the pipe does not close when the subprocess
exits. So the ipstream
will
still look for data even though the process has ended.
Note | |
---|---|
It is not possible to use automatic pipe-closing in this library, because a pipe might be a file-handle (as for async pipes on windows). |
But, since pipes are buffered, you might get incomplete data if you do this:
ipstream is; child c("ls", std_out > is); std::string file; while (c.running()) { is >> file; cout << "File: " << file << endl; }
It is therefore highly recommended that you use the asynchronous API if you are not absolutely sure how the output will look.
Since windows does not use UTF-8 it is sometimes unavoidable to use the
wchar_t
version of the WinApi.
To keep this library consistent it provides wchar_t
support on posix also.
Since the posix api is purely char
every wchar_t
based type will
be converted into char
.
Windows on the other hand is more selective; the default is to use char
, but if any parameter requires wchar_t
, everything will be converted to
wchar_t
. This also includes
boost::filesystem::path
. Additionally, if the system does
not provide the char
api (as
is the case with Windows CE) everything will also be converted.