The return value of and the value read by sscanf are misleading/wrong when an integer greater than INT_MAX is read.

sscanf_overflow.c

//
// File: sscanf_overflow.c
// Author: Costava
//
// If sscanf is given a string of an integer greater than INT_MAX,
// sscanf will still return 1 even though the integer cannot have been
// read correctly into the given variable.
//
// errno gets set to ERANGE
// errno can be checked to see if sscanf actually succeeded.
//
// Example compilation:
// gcc sscanf_overflow.c -o sscanf_overflow -std=c99 -Wall -Wextra -Wconversion
//

#include <errno.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>

// Call printf. If error, exit(1)
#define CHECKED_PRINTF(...) if (printf(__VA_ARGS__) < 0) { exit(1); }

int main(void) {
    CHECKED_PRINTF("INT_MAX            : %d\n", INT_MAX); // 2147483647

    int num;
    CHECKED_PRINTF("errno before sscanf: %d\n", errno);  // 0
    const int sscanf_return_value = sscanf("99999999999999999999999999999999999999999999999999999999999999999999999999999999", "%d", &num);
    CHECKED_PRINTF("errno after sscanf : %d\n", errno);  // 34
    CHECKED_PRINTF("ERANGE             : %d\n", ERANGE); // 34

    // The read was not successful, but sscanf still returned 1.
    // Unfortunate. Footgun.
    CHECKED_PRINTF("sscanf_return_value: %d\n", sscanf_return_value); // 1
    CHECKED_PRINTF("sscanf value read  : %d\n", num);                 // -1

    return 0;
}