Operating System

The interface every program has whether it asked for one or not: three descriptors it did not open, an environment it inherited, and a status code it owes its parent. Four of these chapters run in your browser, because WASI kept the descriptor numbering even though there is no operating system underneath.

1. A file descriptor is an integer and nothing more. A File built by hand out of the number 1 writes to standard output exactly as the one stdout() returns does.
2. Buffering belongs to your writer, not to the descriptor. Two writers on the same descriptor can deliver their bytes in the order you did not write them.
3. There is no std.posix.pipe any more. The portable way to hold one end of a pipe is to spawn a process on the other, and the way to end the conversation is to close your end.
4. A signal handler runs between two arbitrary instructions of whatever you were doing. Set an atomic flag and return; anything that allocates or takes a lock can deadlock the program that was holding it.
5. std.process.exit runs no defer and drains no buffer. Bytes still in a writer when it is called are simply lost.

7 chapters.

• A File Descriptor Is a NumberEverything the operating system hands you to read or write is an integer.
• The Three Standard StreamsBuffering belongs to your writer, not to the descriptor underneath it.
• The EnvironmentA string-to-string map the parent process chose, handed over at startup.
• Spawning a ProcessA child is a program, an argument list, and a status code coming back.
• Pipes Are the Child's StreamsA pipe is a descriptor at each end and an end-of-file in the middle.
• SignalsA handler interrupts you between two instructions. Set a flag and return.
• Exitingexit is the process stopping, not your program returning.