_EDC_FLUSH_STDOUT_PIPE

Instructs the C Runtime Library to flush the stdout stream when the stdin stream is being read from. Both stdin and stdout must be pipes. _EDC_FLUSH_STDOUT_PIPE is set with the command:

setenv("_EDC_FLUSH_STDOUT_PIPE","YES",1);

The purpose of this environment variable is to better facilitate communication between two processes that use pipes. The child process is using stdin for the read end of one pipe and stdout for the write end of a different pipe. The parent process has the opposite ends of the pipes.

Note the following usage notes and examples:

The child process wants to send a prompt to the parent process and then read the response. The parent process must see the prompt, capture a response, and then send the response back to the child.
If the child process sends the prompt without a trailing newline or without calling fflush, the prompt is never placed into the pipe for the parent process to read. The child process then tries to read the response from stdin. The application hangs, which means that the parent process cannot read the prompt that it needs to respond to and the child process cannot read the response from the parent process.
With the environment variable set to YES, the read from stdin will flush the stdout buffer. This allows the parent process to see the prompt, respond, and then send the response back to the child process who is waiting on the read. After the data comes back from the parent process, the child process reads the response and continues.
Note: The parent process must do a read by using read() from the pipe so that it can receive whatever data might be there without having to wait for a specific number of bytes or a newline character. Functions like fread() and fgets() will hang unless the child process wrote enough bytes or the newline character to stdout.