In this post there is described how to start a program and then to send data to the stdin of the program (with FIFOs). But what to do if the program is already running, and I cannot re/start the program?
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I once wrote a program to do the reverse (monitor output of an already started program). It's not a simple operation; it relies on using ptrace() to attach to the process and intercept system calls. When you detect a read() system call, inject the data.