A Layered Service With Honest Errors

Assemble the module: a three-layer pipeline where each layer translates errors, cleans up its own resources, and never lies. The boilerplate declares: ``c /* layer 3 (top) codes */ #define APP_OK 0 #define APP_ECONFIG 1 /* config stage failed */ #define APP_EDATA 2 /* data stage failed */ #define APP_EARGS (-1) /* contract violation by caller */ /* layer 2 stages: 0 ok, -1 fail. Records what happened: */ /* config: parses "k=v ..." lines (max 8, key/value <= 7 chars) */ int cfg_stage(const char *input, char *report, size_t cap); /* data: sums "n " lines; all lines must be nonneg ints */ int data_stage(const char *input, long *total, char *report, size_t cap); /* layer 3: runs both stages in order; first failure aborts and returns the APP_E* code for the FAILING stage (args bad -> APP_EARGS). report (cap chars) gets "config:..." then "data:..." details. */ int app_run(const char *cfg_in, const char *data_in, char *report, size_t cap); `` The error taxonomy is the test: config failures and data failures must be DISTINGUISHABLE at the top, and caller errors (NULL report) must be a third thing.

Difficulty: intermediate