[SOLVED] Difference between a straight 'gcc' compile and 'gcc -lm -Wall -o'

Issue

I am working on a homework assignment for my computer science class and, we are required to develop this simple program for our first use of C.

I was able to create the program through a straight gcc compile, but while using gcc -lm -Wall -o compile, my program crashes and returns

Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

What would cause my program to crash when entering it into the second compile?

My code

#include <stdio.h>

void main() {
    float enrollment, fullsec, leftover, recieveA, recieveB, recieveC, recieveD;
    printf("Program #1, masc0865, Tom Bachar");
    printf("Enter enrollments on one line: ");
    scanf("%f", &enrollment);
    fullsec = enrollment/25;
    leftover = enrollment%25;
    printf("Enrollment = %f", enrollment);
    printf("Amount of Full Sections = %f", fullsec);
    printf("Students Left Over = %0.2f", leftover);
    recieveA = enrollment*.30;
    recieveB = enrollment*.25;
    recieveC = enrollment*.15;
    recieveD = enrollment*.30;
    printf("Students expected to receive an A: %f", recieveA);
    printf("Students expected to receive a B: %f", recieveB);
    printf("Students expected to receive a C: %f", recieveC);
    printf("Students expected to receive some other grade: %f", recieveD);
}

Solution

To compile your code with gcc, you also need to pass in the names of your source files. So if you had a source file named project1.c, you could compile it by running:

gcc -lm -Wall -o a.out project1.c

Assuming your code successfully compiled (no errors), you could run it by then doing:

./a.out

That being said, your code won't compile. You can't use the modulus operator against a floating point number.

foo.c:9:25: error: invalid operands to binary expression ('float' and 'float')
    leftover= enrollment%25;
              ~~~~~~~~~~^~~

Answered By - Bill Lynch

Answer Checked By - Mary Flores (WPSolving Volunteer)