How can I redirect output to a boost log?

问题:

I have a C++ program that uses boost log, and I load a user-provided dynamic link library. I'd like to redirect stderr to the boost log, so that anytime the user's library does:

std::cerr << "Some stuff";

It produces the same result** as:

BOOST_LOG_SEV(log,info) << "Some stuff";

Is this possible, and if so then how do I do it?

(Also, I'm not sure what to do about the severity... since cerr << doesn't privide severity information. I'm open to suggestions on that as well...)

** By "same result" I mean that it gets logged to the same log file as the rest of the log messages, and the same log formatter is applied to those lines.

回答1:

Here's my C++11 implementation. This class can be used for anything (not just boost) to capture stdout/stderr on a line-by-line basis and calling a user function (could be a lambda) to process it.

Warning: if you redirect stderr and stdout and are using Boost, then redirect stderr first, then stdout. Otherwise, Boost will write loopback the stderr message back to stdout, and you'll get a boost log entry inside another boost log entry.

Usage example

cout << "testing out before 1 2 3 " << endl;
cerr << "testing err before 1 2 3 " << endl;
{
    StdErrHandler err([](const char* line){ 
          BOOST_LOG_TRIVIAL(error) << "ERROR:" << strlen(line) << " " << line; 
    });
    StdOutHandler out([](const char* line){
          BOOST_LOG_TRIVIAL(info) << "OUT:" << strlen(line) << " " << line; 
    });
    cout << "cout testing 1 2 3 " << endl;
    cerr << "cerr testing 1 2 3 " << endl;
}
cout << "testing out after 1 2 3 " << endl;
cerr << "testing err after 1 2 3 " << endl;

Example output

pa-poca$ ./test
testing out before 1 2 3
testing err before 1 2 3
[2014-08-01 12:24:56.468335] [0x000007f89d8990d4] [error]   ERROR:19 cerr testing 1 2 3
[2014-08-01 12:24:56.468360] [0x000007f89d8990d4] [info]    OUT:19 cout testing 1 2 3
testing out after 1 2 3
testing err after 1 2 3

Code

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#include <signal.h>
#include <sys/wait.h>

class StdioHandler
{
private:
    pid_t pid = 0;
    int origfd;
    int streamid;
    int pipefd[2];
public:
    enum class Stream
    {
        stdout = STDOUT_FILENO,
        stderr = STDERR_FILENO
    };
    StdioHandler(Stream stream, std::function<void(const char*)> callback)
        :streamid(static_cast<int>(stream))
    {
            origfd = dup(streamid);

        pipe(pipefd); // create pipe
        pid = fork(); //spawn a child process to handle output of pipe
        if (pid == 0)
        {
            char line[256];
            FILE* output;

            close(pipefd[1]);
            output = fdopen(pipefd[0], "r");
            if (output)
            {
              while(fgets(line, sizeof(line), output)) 
              {

                 int n = strlen(line);
                 if (n > 0)
                     if (line[n-1] == '\n') line[n-1] = 0;
                 callback(line);
              }
              fclose(output);
            }
            abort();
        } else {
            // connect input of pipe to
            close(pipefd[0]);
            dup2(pipefd[1], streamid);
        }
    }

    ~StdioHandler()
    {
        int status;

        usleep(10000);

        close(pipefd[1]);
        kill(pid,SIGINT);

        waitpid(pid, &status, 0);

        dup2(origfd, streamid);
    }
};

class StdOutHandler : public StdioHandler
{
public:
    StdOutHandler(std::function<void(const char*)> callback) :
        StdioHandler(Stream::stdout, callback)
    {
    }
};

class StdErrHandler : public StdioHandler
{
public:
    StdErrHandler(std::function<void(const char*)> callback) :
        StdioHandler(Stream::stderr, callback)
    {
    }
};

回答2:

I am guessing , You can redirect File descriptor STDERR to your stream file descriptor [you have to get the file desc of your stream] using dup/dup2 api [its a posix api]