stdout - How can I implement 'tee' programmatically in C?

Question

I'm looking for a way in C to programmatically (ie, not using redirection from the command line) implement 'tee' functionality such that my stdout goes to both stdout and a log file. This needs to work for both my code and all linked libraries that output to stdout. Any way to do this?

Answer 1

You could popen() the tee program.

Or you can fork() and pipe stdout through a child process such as this (adapted from a real live program I wrote, so it works!):

void tee(const char* fname) {
    int pipe_fd[2];
    check(pipe(pipe_fd));
    const pid_t pid = fork();
    check(pid);
    if(!pid) { // our log child
        close(pipe_fd[1]); // Close unused write end
        FILE* logFile = fname? fopen(fname,"a"): NULL;
        if(fname && !logFile)
            fprintf(stderr,"cannot open log file "%s": %d (%s)
",fname,errno,strerror(errno));
        char ch;
        while(read(pipe_fd[0],&ch,1) > 0) {
            //### any timestamp logic or whatever here
            putchar(ch);
            if(logFile)
                fputc(ch,logFile);
            if('
'==ch) {
                fflush(stdout);
                if(logFile)
                    fflush(logFile);
            }
        }
        putchar('
');
        close(pipe_fd[0]);
        if(logFile)
            fclose(logFile);
        exit(EXIT_SUCCESS);
    } else {
        close(pipe_fd[0]); // Close unused read end
        // redirect stdout and stderr
        dup2(pipe_fd[1],STDOUT_FILENO);  
        dup2(pipe_fd[1],STDERR_FILENO);  
        close(pipe_fd[1]);  
    }
}