Read a fixed-size record

Challenge

Read the Nth fixed-size record from a binary buffer with bounds checking — how databases, MFT entries, and packet logs index their records.

Task

Implement int read_record(const uint8_t *buf, size_t n, size_t recsize, int idx, unsigned char *out) that copies the idx-th recsize-byte record from buf into out.

Input

• buf: a byte buffer holding back-to-back fixed-size records.
• n: total number of valid bytes in buf.
• recsize: size of each record in bytes.
• idx: 0-based record index to read.
• out: destination buffer, at least recsize bytes.

Output

Return 0 on success (the record is copied into out). Return -1 if buf/out is NULL, recsize == 0, idx < 0, or the record would run past n.

Example

buf = {0,1,2,3, 4,5,6,7, 8,9,10,11} (three 4-byte records)
read_record(buf, 12, 4, 0, out)   ->   0,  out = {0,1,2,3}
read_record(buf, 12, 4, 2, out)   ->   0,  out = {8,9,10,11}
read_record(buf, 12, 4, 3, out)   ->   -1  (past the end)

Edge cases

• Last valid record vs. one past the end.
• recsize == 0 or negative idx: -1.

Rules

• The record starts at idx * recsize; verify start + recsize <= n before copying (guards against overrun and overflow).

Why this matters

Binary formats (databases, MFT, packet logs) are arrays of fixed-size records; reading record N is just bounds-checked indexing.

Input format

buf/n: record buffer and its byte length. recsize: record size. idx: 0-based index. out: destination.

Output format

0 on success (record copied to out); -1 on NULL / recsize==0 / idx<0 / out-of-range.

Constraints

Bounds-check idx*recsize + recsize <= n before copying.

Starter code

#include <stdint.h>
#include <stddef.h>
int read_record(const uint8_t *buf, size_t n, size_t recsize, int idx, unsigned char *out) {
    /* TODO */
    (void)buf; (void)n; (void)recsize; (void)idx; (void)out;
    return -1;
}

Common mistakes

Integer overflow on idx×recsize (guard with the bound check). Off-by-one letting the last record read past n.

Edge cases to handle

Last valid record. idx one past the end. recsize 0.

Complexity

O(recsize).

Background lessons

• Binary files
• fread for binary I/O