usage mesasge

src/main.rs

@@ -8,16 +8,35 @@ mod work;
 
 mod consts;
 
+fn usage() -> !
+{
+    eprintln!("uasge: {} <output file> <input files...>", std::env::args().next().unwrap());
+
+    eprintln!();
+    eprintln!("mapcat v{}: mmap() based file concatenation", env!("CARGO_PKG_VERSION"));
+    eprintln!(" by {} with <3 (licensed GPL3+)", env!("CARGO_PKG_AUTHORS"));
+    eprintln!();
+
+    eprintln!("NOTES:");
+    eprintln!("The output file is required, if one is not provided, we will just exit with error code `2`. If there are no input files, it will just create an empty output and return with error code `0` (unless there were other errors)");
+    eprintln!("Output files are always clobbered regardless if there are any inputs, or if the concatenation operation succeeds / fails");
+    eprintln!();
+    eprintln!(" (compiled with write parallelisation enabled)"); //TODO: we will eventually feature gate the threaded scheduler based solution. but for now, it is the only one implemented so we'll just print this always
+    
+    std::process::exit(0)
+}
+
 fn main() {
-    println!("Hello, world!");
+    usage()
     // todo:
     //  - create and open output file
-    //  - open and stat all input files (`job::create_from_file`).
+    //  - open and stat all input files in order (`job::create_from_file`).
     //  - `fallocate` the output file fd to the sum of all input file sizes
     //  - `mmap` the output file as writable
     //
     //  - spawn the task thread pool
-    //  - dispatch jobs with their fds, stats, and calculated output offsets along with a reference to the output mapped file (`job::Prelude::start`)
+    //  - move the output mapped file to the thread-safe refcounted `state::State`.
+    //  - dispatch jobs to the pool with their fds, stats, and calculated output offsets; along with a reference to the output mapped file (`job::Prelude::start`)
     //  - wait on all worker threads to complete.
     //  - ensure all data was written.
 }