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Writing to stdout file and file

I have a parent process that issues a child to execute execv ().

I need the output given by execv () to stdout, which will be displayed on the screen, also copied to the log file.

How to write the same output for both stdout and file, without using pipes or tees ?

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7 answers

If you do not want to use the tee before writing data, write it to a file, and then send it to standard output.

You must write a registration function that will do this for you to make it cleaner.

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tee.

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dup2() -

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, , pipe().

, , tee, :

int child_pipe[2];
pid_t pid_exec_child, pid_output_child;

pipe(child_pipe);

pid_exec_child = fork();
if (pid_exec_child == 0)
{
    dup2(child_pipe[1], STDOUT_FILENO);
    close(child_pipe[0]);
    close(child_pipe[1]);
    execve(/* ... */);
    _exit(127);
}

close(child_pipe[1]);

pid_output_child = fork();
if (pid_output_child == 0)
{
    /* This child just loops around, reading from the other child and writing everything
     * to both stdout and the log file. */
    int logfd = open("logfile", O_WRONLY);
    char buffer[4096];
    ssize_t nread;

    while ((nread = read(child_pipe[0], buffer, sizeof buffer) != 0)
    {
        size_t nwritten_total;
        ssize_t nwritten;

        if (nread < 0)
        {
            if (errno == EINTR)
                continue;

            perror("read");
            _exit(1);
        }

        /* Copy data to stdout */
        nwritten_total = 0;
        while (nwritten_total < nread)
        {
            nwritten = write(STDOUT_FILENO, buffer + nwritten_total, nread - nwritten_total);

            if (nwritten < 0)
            {
                if (errno == EINTR)
                    continue;

                perror("write(stdout)");
                _exit(2);
            }

            nwritten_total += nwritten;
        }

        /* Copy data to logfile */
        nwritten_total = 0;
        while (nwritten_total < nread)
        {
            nwritten = write(logfd, buffer + nwritten_total, nread - nwritten_total);

            if (nwritten < 0)
            {
                if (errno == EINTR)
                    continue;

                perror("write(logfile)");
                _exit(3);
            }

            nwritten_total += nwritten;
        }
    }
    _exit(0);
}

close(child_pipe[0]);

/* Parent continues here */

, exec tee ...

( , _exit(), - ).

+2

, fifo. mkfifo my.fifo; execv: program > my.fifo; fifo , . , stdout, .

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, ?

tee unix , : .

0

Tee :

./myprog | tee outputfile
0

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