How to attach() to container and capture output of running function?

[xfbs]

Dear all,
I'm trying to use the attach() function to attach to a container and run a function inside it, capturing the output. My idea was to create a pipe() of two file descriptors, and set the lxc_attach_options standard output file descriptor to the write end of the pipe, and read from the read end of it. However, that doesn't work, as this code here demonstrates. Compile it with

gcc -o lxctest lxctest.c -llxc

and run it with

sudo ./lxctest running_container_name

How can I make this work and actually capture data from the running function? Any help would be appreciated.

#include <lxc/lxccontainer.h>
#include <lxc/attach_options.h>
#include <errno.h>                                                                                                                                                                                                                                                                   [0/89]
#include <string.h>
#include <assert.h>
#include <unistd.h>

// Run a function inside the container and capture the output.
void run_function(struct lxc_container *container, lxc_attach_exec_t function, void *data) {
    // default options and PID
    lxc_attach_options_t options = LXC_ATTACH_OPTIONS_DEFAULT;
    pid_t pid;

    // create a pipe to collect stdout
    int pipefd[2];
    assert(0 == pipe(pipefd));
    options.stdout_fd = pipefd[1];
    //options.attach_flags |= LXC_ATTACH_TERMINAL;

    // attach to container, run function
    assert(0 == container->attach(container, function, data, &options, &pid));
    //close(pipefd[1]);

    // read in data
    size_t buffer_size = 1024;
    size_t buffer_pos = 0;
    char *buffer = malloc(buffer_size);

    while(true) {
        if(!(buffer_size - buffer_pos)) {
            buffer_size *= 2;
            buffer = realloc(buffer, buffer_size);
            assert(buffer);
        }
        ssize_t ret = read(pipefd[0], buffer + buffer_pos, buffer_size - buffer_pos);
        assert(ret >= 0);
        printf("just read something\n");

        // read end of file
        if(!ret) {
            break;
        } else {
            buffer_pos += ret;
        }
    }

    buffer[buffer_pos] = 0;
    printf("Read size: %zu\n", buffer_pos);
    printf("Read: %s\n", buffer);
}

int hello_world(void *data) {
    printf("Hello, World!\n");
}

int main(int argc, char *argv[]) {
    if(argc != 2) {
        printf("Usage: %s <container>\n", argv[0]);
        return 0;
    }

    char **names;
    struct lxc_container **containers;
    int amount = list_all_containers("/var/lib/lxc", &names, &containers);

    assert(amount > 0);
    size_t i;
    for(i = 0; i < amount; i++) {
        if(0 == strcmp(argv[1], names[i])) {
            break;
        }
    }

    assert(i < amount);

    run_function(containers[i], hello_world, NULL);

    return 0;
}