How to synchronously call a child process in CasperJS?

Question

the code:

var casper = require('casper').create({
    waitTimeout: 120000,
    stepTimeout: 120000,
    verbose: true,
    logLevel: "debug"
});

var process = require("child_process")
var spawn = process.spawn

casper.start('http://baidu.com')

casper.then(function () {
    casper.echo(httpCall('baidu.com', 'GET'))
})

function httpCall(url, method) {
    var para = ["-X", method, url]
    casper.log('curl with parameters: ' + para)
    var child = spawn("curl", para)
    var result = "abc"
    child.stdout.on("data", function (data) {
        casper.echo("curl ended.")
        result = data;
    })
    return result;
}

casper.then(function () {
    casper.wait(2000)
})
casper.run()

Run with casperjs test_child_process.js, output:

[info] [phantom] [2016-05-05T10:23:36.196Z] Starting...
[info] [phantom] [2016-05-05T10:23:36.203Z] Running suite: 4 steps
[debug] [phantom] [2016-05-05T10:23:36.226Z] opening url: http://baidu.com/, HTTP GET
[debug] [phantom] [2016-05-05T10:23:36.227Z] Navigation requested: url=http://baidu.com/, type=Other, willNavigate=true, isMainFrame=true
[debug] [phantom] [2016-05-05T10:23:36.461Z] Navigation requested: url=https://www.baidu.com/, type=Other, willNavigate=true, isMainFrame=true
[debug] [phantom] [2016-05-05T10:23:36.684Z] url changed to "https://www.baidu.com/"
[debug] [phantom] [2016-05-05T10:23:37.364Z] Successfully injected Casper client-side utilities
[debug] [phantom] [2016-05-05T10:23:37.382Z] start page is loaded
[info] [phantom] [2016-05-05T10:23:37.406Z] Step anonymous 3/4 https://www.baidu.com/ (HTTP 200)
[debug] [phantom] [2016-05-05T10:23:37.406Z] curl with parameters: -X,GET,baidu.com
abc <- I want it to be the stdout of curl...
[info] [phantom] [2016-05-05T10:23:37.413Z] Step anonymous 3/4: done in 1216ms.
[info] [phantom] [2016-05-05T10:23:37.429Z] Step anonymous 4/4 https://www.baidu.com/ (HTTP 200)
[info] [phantom] [2016-05-05T10:23:37.430Z] Step anonymous 4/4: done in 1233ms.
[info] [phantom] [2016-05-05T10:23:37.451Z] Step _step 5/5 https://www.baidu.com/ (HTTP 200)
[info] [phantom] [2016-05-05T10:23:37.451Z] Step _step 5/5: done in 1254ms.
curl ended.
[info] [phantom] [2016-05-05T10:23:39.452Z] wait() finished waiting for 2000ms.
[info] [phantom] [2016-05-05T10:23:39.453Z] Done 5 steps in 3256ms
[debug] [phantom] [2016-05-05T10:23:39.455Z] Navigation requested: url=about:blank, type=Other, willNavigate=true, isMainFrame=true
[debug] [phantom] [2016-05-05T10:23:39.456Z] url changed to "about:blank"

I just want httpCall()stdout curl to return (it will take some time, but it can save our legacy codes from many callbacks ...), but I have no idea how to do this. Can someone give me some advice?