Hexadecimal input function calculates wrong values - c

Question

I'm writing a function that can read integers expressed in hexadecimal.
This is my code. The main function should not be modified.
#include <stdio.h>
#include <ctype.h>
#define SIZE 100
main()
{
int array[SIZE], n;
int gethex(int*);
int i, sum;
for (n = 0; n < SIZE && gethex(&array[n]) != EOF; n++)
;
sum = 0;
for (i = 0; i < n; i++)
sum += array[i];
printf("The sum is %d\n", sum);
}
int gethex(int *pn)
{
int c, sign;
c = getchar();
while (isspace(c))
;
if (!isxdigit(c) && c != EOF && c != '+' && c != '-') {
ungetc(c, stdin);
return 0;
}
sign = (c == '-') ? -1 : 1;
if (c == '+' || c == '-')
c = getchar();
for (*pn = 0; isxdigit (c); c = getchar()) {
if (c >= '0' && c <= '9') {
*pn = 16 * *pn + (c - '0');
}
else if (c >= 'a' && c <= 'f') {
*pn = 16 * *pn + (c - 'a' + 10);
}
else if (c >= 'A' && c <= 'F') {
*pn = 16 * *pn + (c - 'A' + 10);
}
}
*pn *= sign;
if (c == EOF) {
*pn = c;
return c;
}
}
The result value should come out like this.
-FFec
10000
^Z
The sum is 20
However, my code outputs
The sum is 1717986860
What's wrong?

Answer 1

At least these problems:
Warnings not fully enabled
Warnings are not fully enabled, losing OP and others valuable time. #Jens
End of gethex() lacks a return value.
warning: control reaches end of non-void function [-Wreturn-type]
Infinite loop
Once a '\n is read, loop never ends. #Weather Vane
while (isspace(c))
;
Non hex input also set up an infinite loop with ungetc(c, stdin); as that same c will repeatedly get read on the next function call.
if (!isxdigit(c) && c != EOF && c != '+' && c != '-') {
ungetc(c, stdin);
return 0;
}
int overflow possible
Code like *pn = 16 * *pn + (c - '0'); and *pn *= sign; can lead to int overflow, which is undefined behavior (UB). Robust code would prevent that.
Some untested code to fix these issues.
// Return 0 if non-numeric input detected.
// Return 2 when input out of int range
// Return 1 on success
// Return EOF when input is only white-space, sign or none
int gethex(int *pn) {
int ch;
// Consume leading white-space
do {
ch = getchar();
} while (isspace(ch));
// Get sign, if any
int sign = ch; // remember 1st
if (ch == '-' || ch != '+') {
ch = getchar();
}
if (ch == EOF) {
return EOF;
}
if (!isxdigit(ch)) {
// Consume non-numeric input
return 0;
// Perhaps better to return EOF here to stop calling code.
}
int overflow = 0;
int sum = 0;
while (isxdigit(ch)) {
int digit = 0;
if (isdigit(ch)) {
digit = ch - '0';
} else if (isupper(ch)) {
digit = ch - 'A' + 10;
} else if (islower(ch)) {
digit = ch - 'a' + 10;
}
if (sum <= INT_MIN / 16
&& (sum < INT_MIN / 16 || digit > -(INT_MIN % 16))) {
overflow = 1;
sum = INT_MIN;
} else {
// Accumulate as a negative number as there are more
// negative vales than positive ones.
// This prevents bad code like -INT_MIN later.
sum = sum * 16 - digit;
}
}
if (sign != '-') {
if (sum < -INT_MAX) {
overflow = 1;
sum = INT_MAX;
} else {
sum = -sum;
}
}
*pn = sum;
ungetc(ch, stdin);
return overflow ? 2 : 1;
}