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

Question

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, receiveA, receiveB, receiveC, receiveD;
    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);
    receiveA = enrollment*0.30;
    receiveB = enrollment*0.25;
    receiveC = enrollment*0.15;
    receiveD = enrollment*0.30;
    printf("Students expected to receive an A: %f", receiveA);
    printf("Students expected to receive a B: %f", receiveB);
    printf("Students expected to receive a C: %f", receiveC);
    printf("Students expected to receive some other grade: %f", receiveD);
}

Answer 1

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;
              ~~~~~~~~~~^~~

Answer 2

To learn about the command line arguments for gcc, you can use the man page for gcc:

man gcc

The documentation is very daunting because there are a lot of options. Some experience with man pages will help you quickly scan for the information you need.

The first thing to look at is the SYNOPSIS section at the top. This shows the usage of gcc. Anything in [] is optional. Note at the very end of this section, there is infile.... This means that you must provide at least one file name for the compiler to process. I suspect you are getting an error because you are missing this.

So the correct command line should be

gcc -Wall -o <program> <program>.c -lm

This will compile your .c file to an executable with the same name without an extension.

For details about the options you are using, the following comes directly from the gcc man page.

-Wall

    This enables all the warnings about constructions that some users
    consider questionable, and that are easy to avoid (or modify to
    prevent the warning), even in conjunction with macros.

-llibrary
-l library
    Search the library named library when linking.

-o file
    Write output to file.

The errors you get are due to the -Wall option.

The -lm flag links in a library named "m". This is a math library which you probably don't need to worry about for now. For more details about this library use man libm.

Note that the -o option requires an argument. In your case, this tells gcc the name of the executable to create. (This is the reason for -o <program> in my suggested solution above.)