How to log POST body data?

[shawnxzhou]

I'm trying to use $request_body but get '-' in my log file for this field
here is my configure file, is there sth wrong or the $request_body has other
deps to work?

http {
log_format client '$remote_addr - $remote_user $request_time
$upstream_response_time '
                  '[$time_local] "$request" $status $body_bytes_sent
$request_body "$http_referer" '
                  '"$http_user_agent" "$http_x_forwarded_for"';
......
server {
......
location = /c.gif {  
empty_gif;  
access_log logs/uaa_access.log client;  
}
......
}
}

I'm using linux command 'curl -d name=xxxx myip/my_location' to fire a POST
request, and just get '-' for $request_body field.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,240828,240828#msg-240828