[TUHS] Unix APIs: elegant or not?
wkt at tuhs.org
Wed Oct 31 14:38:10 AEST 2018
I was just reading this book review:
and came across these paragraphs:
The mechanism for file IO provided by the Unix operating system
and its descendants, such as Linux, is a beautiful example of a
deep interface. There are only five basic system calls for I/O,
with simple signatures:
int open(const char* path, int flags, mode_t permissions);
ssize_t read(int fd, void* buffer, size_t count);
ssize_t write(int fd, const void* buffer, size_t count);
off_t lseek(int fd, off_t offset, int referencePosition);
int close(int fd);
The POSIX file API is a great example, but not of a deep
interface. Rather, it’s a great example of how code with a very
complicated interface may look deceptively simple when reduced to C-style
function signatures. It’s a stateful API with interesting orderings
and interactions between calls. The flags and permissions parameters
of open hide an enormous amount of complexity, with hidden requirements
like “exactly one of these five bits should be specified.” open may
return 20 different error codes, each with their own meaning, and many
with references to specific implementations.
The authors of SibylIFS tried to write down an exact description of the
open interface. Their annotated version of the POSIX standard is over
3000 words. Not counting basic machinery, it took them over 200 lines
to write down the properties of open in higher-order logic, and another
70 to give the interactions between open and close.
I just thought it was a thought-provoking comment on the apparent elegance
of the Unix file API that actually has some subtle complexity.
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