Pedro Larroy [random thoughts]

06.11.09

system in C++ like perl’s system

Small tricks, haven’t tested these yet, perhaps a bit of more error handling in fork or exec. I don’t like system (3) shell interpolation. In this case one can do system_exec(“ls”,”/tmp”,NULL) or special chars in args that won’t get interpolated by the shell.

    int system_exec(char* cmd, ...)
    {
        va_list ap;
        vector arglist;
        arglist.push_back(cmd);
        char* cur;
        va_start(ap,cmd);
        while( (cur=va_arg(ap,char*)) != NULL)
            arglist.push_back(strdup(cur));
        va_end(ap);
        arglist.push_back(NULL);
        int res;
        pid_t chld;
        if( (chld = fork()) ) {
            for(vector::iterator i = arglist.begin()+1; i != arglist.end(); ++i)
                free(*i);
            wait(&res);
        } else {
            execvp(cmd, &arglist[0]);
        }
        return res;
    }
#define W 1
#define R 0

    int stdout2stream_system_exec(std::ostream& os, char* cmd, ...)
    {
        va_list ap;
        vector arglist;
        arglist.push_back(cmd);
        char* cur;
        va_start(ap,cmd);
        while( (cur=va_arg(ap,char*)) != NULL)
            arglist.push_back(strdup(cur));
        va_end(ap);
        arglist.push_back(NULL);

        int fd[2];
        pipe(fd);

        int res;
        pid_t chld;
        if( (chld = fork()) ) {
            for(vector::iterator i = arglist.begin()+1; i != arglist.end(); ++i)
                free(*i);
            close(fd[W]);
            char buf[1024];
            ssize_t nread;
            while( (nread=read(fd[R],buf,1024)) > 0 && os.good() ) {
                os.write(buf,nread);

            }
            // TODO errors
            close(fd[R]);
            wait(&res);
        } else {
            close(fd[R]);
            // no need according to manpage
            //close(STDOUT_FILENO);
            if( dup2(fd[W], STDOUT_FILENO) != STDOUT_FILENO) {
                perror("dup2");
                exit(EXIT_FAILURE);
            }
            close(fd[W]);
            execvp(cmd, &arglist[0]);
                perror("execvp");
        }
        return res;
    }

06.3.09

Sony’s new motion sensor