Implicit declaration of function x

Question

I'm well aware of function prototypes, this error seems to be a function declaration error, which means I'm really bewildered as to why I'm see this warning and thus error.

It's almost like gcc completely ignores my function prototype. Is this a compiler bug?

In the interest of brevity, I did not declare this function in a separate header file, though it should make no difference.

gcc output:

$ gcc -Wall -std=c99 -pedantic primefactors.c
primefactors.c: In function ‘main’:
primefactors.c:8:5: warning: implicit declaration of function ‘largestprime’ [-Wimplicit-function-declaration]
primefactors.c: At top level:
primefactors.c:12:6: error: conflicting types for ‘largestprime’
primefactors.c:8:20: note: previous implicit declaration of ‘largestprime’ was here

code:

#include <stdio.h>
#include <math.h>

long largetsprime(long);

int main()
{
    printf("%d\n", largestprime(600851475143));
    return 0;
}

long largestprime(long num)
{
    int highest;
    int mid = sqrt(num);
    for (int i = 2; i < mid; i++) {
        if (mid % i == 0) {
            if (i % 1 == 0 && i % i == 0)
                highest = i;
        }
    }
    return highest;
}

Answer 1

Point-1
You have misspelled largest in function name

long largetsprime(long)
           ^
           s is wrong here 

In declaration It should be

long largestprime(long)
          ^ before t

Point-2
You are using sqrt() library function from math.h, you should compile your program with -lm as:

gcc -Wall -std=c99 -pedantic primefactors.c -lm

Point-3
You are returning int whereas return type of your function is long.

Point-4 One more mistake suggestion in call of printf() you forgot adding suffix for long int.

largestprime(600851475143)

should be:

largestprime(600851475143L)
      //                 ^ added  suffix  L for long 

If you are not aware of suffix L then read: What does the “L” mean at the end of an integer literal?

Thanks to @Eric Postpischil:

point-5: printf() in main() function is printing long type integer whereas you have used %d format specifier to print it:

printf("%d\n", largestprime(600851475143));
                 ^
                 | 
                 returns long

use %ld instead.

point-6:

if-condition in largest prime function i % 1 == 0 and i % i == 0 are each always true (except the latter is undefined if i is zero) because i % 1 = 0 (every number is divisible by 1).

Answer 2

Typo. The declaration says largeTSprime. Change it to the correct largestprime and it will work.

ProTip #1: use camelCapsOnWordBoundaries or under_scores for readability.

ProTip #2: it's almost never a compiler bug.

Answer 3

You have a typo in the prototype. It should be largestprime instead of largetsprime.

Answer 4

You have a typo in the prototype:

largetsprime != largestprime