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Q054FreeFirmware

Measure the final UART command token

Difficulty
Easy
Topic
Firmware Algorithms
Language
C
Interview prompt

Question

Measure the last token in an explicitly sized UART command buffer after ignoring trailing ASCII space bytes. Nonnull pointers refer to valid storage for their stated extents. The token_len output object must not overlap the input byte span, and the input is immutable during the call. These storage preconditions preserve the explicit null/empty-input cases below.

Sixteen-byte UART buffer containing STATUS followed by two spaces and unused capacity
The example token STATUS occupies indices 0 through 5; the following two ASCII spaces are trimmed. Scan within the explicit input length, ignoring unused capacity.
Candidate starting point

Implementation scaffold

#include <stddef.h>
#include <stdint.h>

typedef enum { TOKEN_OK, TOKEN_EINVAL } token_rc_t;

token_rc_t last_token_len(const uint8_t *buf, size_t len,
                          size_t *token_len) {
  // Implement here: validate arguments, find the final bounded token,
  // and publish its length only after complete success.
}
Reviewed example

Trace one case

Input
buffer bytes="run test   ", explicit_length=11
Expected output
last_token_length=4 ("test")

Trailing ASCII spaces are skipped, then the scan stops at the separator before test without relying on a terminator.

What to cover

Requirements

  1. Treat only byte 0x20 as a separator; embedded zero and nonprinting bytes remain token data.
  2. Return zero for an empty or all-space input.
  3. Scan with unsigned indexes without decrementing below zero, calling strlen, allocating, or mutating the buffer.
  4. Reject a null output, length above 128, or null nonempty input without changing the caller's old length.
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