How do I get warn()'s output into a string?

Question

I'm using the non-standard function warn() (provided by BSD) to output an error message if a file can't be opened, like so:

std::string path = get_path() ;
std::ifstream file(path) ;
if (file.is_open()) { /* do something */ }
else {
    warn("%s", path.c_str()) ;
    // uses errno to figure out what the error was and outputs it nicely along with the filename
}

That's all very well for outputting it, but what if I want to use the entire string somewhere else, in addition to printing it? The warn() functions don't seem to have a form that writes the error to a string. I've tried rolling my own, but it seems awfully cumbersome in comparison (besides not getting the program's name):

this->foo((boost::format("%s: %s") % path % strerror(errno)).str()) ;

So how do I get warn()'s output as a string?

Where does '`warn` come from? It certainly is not a standard c++ function, so maybe the documentation of the API you use it from helps more... — PlasmaHH, Feb 26 '13 at 15:58
@PlasmaHH Its manual page says it's in the standard C library… — Blacklight Shining, Feb 26 '13 at 16:01
@BlacklightShining: Which section and paragraph of the standard mentions it? — PlasmaHH, Feb 26 '13 at 16:03
@LightnessRacesinOrbit `err(3)`—there's about a dozen functions dealing with formatted error messages. — Blacklight Shining, Feb 26 '13 at 16:04
@PlasmaHH I couldn't tell you. o.o If there's a better way to be printing error messages, I'm open to switching… — Blacklight Shining, Feb 26 '13 at 16:04
@BlacklightShining "These functions are nonstandard BSD extensions." — Joseph Mansfield, Feb 26 '13 at 16:06
@BlacklightShining: When you claim they are standard C, you better should. Tthere are many, depending on your exact requirements, if you want format strings then wrapping up boost::format in variadic templates seems to be a good idea to me. — PlasmaHH, Feb 26 '13 at 16:07
@BlacklightShining I see no mention of the C standard library in `err(3)`. This appears to be a BSD function. Anyway, the answer is what you already wrote. — Lightness Races in Orbit, Feb 26 '13 at 16:07

score 1 · Answer 1 · edited May 23 '17 at 11:57

warn puts its output on the standard error output. So you would have to create a mechanism to redirect standard error output to a location that you can read back into a string. The most straight forward way may be to redirect standard error to a file, and then read the file back as a string. You could, for instance, try to use dup2() to accomplish this (as explained in the answer to this question).

However, wrapping your own version of warn is probably a better choice. You may consider the C vsnprintf() function to implement it, though. There are answers to this question that address both using boost::format and vsnprintf().

score 0 · Answer 2 · answered Feb 26 '13 at 16:08

0

You're right — there's no sprintf analog (i.e. that is, no hypothetical swarn function).

Your approach seems viable.

answered Feb 26 '13 at 16:08

Lightness Races in Orbit

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Jerry Coffin · Answer 3 · 2013-02-26T16:28:45.717

It would appear that your gyrations produce a result similar to:

path + ": " +  strerror(errno);

At a guess, the "program's name" that it's including is probably just argv[0], so you could apparently produce a roughly equivalent of your warn that just returns a std::string with something on this general order:

std::string warn_s(std::string const &path) { 
    char *pname = strrchr(argv[0], '/');
    if (pname == NULL)
        pname = argv[0];
    return path + pname + ":  " + strerror(errno);
}

The major difficulty here is that argv is local to main, so you'll probably need to either save it into an accessible location in main, or else use some non-standard mechanism to re-retrieve that data in your function.

Unfortunately, the documentation for warn I was able to find was poor enough that a bit of testing/trial and error will probably be needed if you want to duplicate its output precisely.

How do I get warn()'s output into a string?

3 Answers3