Turning main.c into an executable is four separate stages: preprocess → compile → assemble → link. Knowing them makes compiler and linker errors easy to read.

main.c → Preprocessor → main.i → Compiler → main.s → Assembler → main.o → Linker → a.out

The four stages

Preprocessing (cpp)
Text substitution: expands #include (pastes header files in), #define macros, #if conditionals, strips comments. Output: one big translation unit (-E).
Compilation (cc1)
Parses the C, type-checks it, optimises it and emits assembly for the target CPU (-S). Syntax and type errors happen here.
Assembly (as)
Converts assembly into machine code in an object file (.o) with a symbol table — unresolved references to functions defined elsewhere are left as holes (-c).
Linking (ld)
Combines object files and libraries (libc, -lm…), resolves every symbol to an address, and writes the executable. “undefined reference” errors happen here.

Terminal

$ gcc -E main.c -o main.i      # stop after preprocessing
$ gcc -S main.c -o main.s      # stop after compiling (assembly)
$ gcc -c main.c -o main.o      # stop after assembling (object file)
$ gcc main.o util.o -o app     # link object files into an executable
$ gcc -save-temps main.c       # keep main.i, main.s, main.o all at once
$ nm main.o                    # list symbols (U = undefined, T = defined text)

Essential gcc / clang flags

Flag	Meaning
`-o name`	Name of the output file (default `a.out`).
`-std=c17`	Language standard (`c89`, `c99`, `c11`, `c17`, `c2x`/`c23`, or `gnu17` with extensions).
`-Wall -Wextra`	Turn on the important warnings. Always use them.
`-Werror`	Treat warnings as errors (good in CI).
`-pedantic`	Warn about non-standard extensions.
`-g`	Include debug info (for `gdb` / `lldb`).
`-O0 -O1 -O2 -O3 -Os`	Optimisation level (none → aggressive; `-Os` for size).
`-I dir` / `-L dir` / `-lname`	Add a header search path / library search path / link `libname`.
`-D NAME=val`	Define a macro from the command line (like `#define NAME val`).
`-fsanitize=address,undefined`	Runtime checks for memory errors and undefined behaviour (ASan / UBSan).

Warnings are free bug reports

Every sample in these notes is compiled with -Wall -Wextra -Werror. This one is rejected at compile time because the format string does not match the argument:

#include <stdio.h>

int main(void) {
    double price = 9.99;
    printf("%d\n", price);   /* %d expects int, got double */
    return 0;
}

Compiler

main.c:5:20: error: format specifies type 'int' but the argument has type 'double' [-Werror,-Wformat]
    5 |     printf("%d\n", price);   /* %d expects int, got double */
      |             ~~     ^~~~~
      |             %f

Compile-time vs link-time errors

Error	Stage	Typical cause
`expected ';'`	Compile	Syntax error
`implicit declaration of function`	Compile	Missing `#include` or prototype
`undefined reference to 'foo'`	Link	Function declared but never defined / `.o` or `-l` library not linked
`multiple definition of 'x'`	Link	Global defined in a header included by two `.c` files
`undefined reference to 'sqrt'`	Link	Forgot `-lm` (math library) on some Linux toolchains

What is a translation unit?

What is the difference between a compiler error and a linker error?

Static vs dynamic linking?