C Compile Process

Overview
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This post explains the C compilation process using practical examples with multiple source files, object files, and static libraries.

1. Example Files
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math_utils.h
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#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);
int multiply(int a, int b);
int subtract(int a, int b);

#endif

add.c
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#include "math_utils.h"

int add(int a, int b) {
    return a + b;
}

multiply.c
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#include "math_utils.h"

int multiply(int a, int b) {
    return a * b;
}

subtract.c
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#include "math_utils.h"

int subtract(int a, int b) {
    return a - b;
}

main.c
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#include <stdio.h>
#include "math_utils.h"

int main() {
    int x = 10, y = 5;

    printf("Add: %d\n", add(x, y));
    printf("Multiply: %d\n", multiply(x, y));

    return 0;
}

2. Compilation to Object Files
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Compile each source file separately:

gcc -c add.c -o add.o
gcc -c multiply.c -o multiply.o
gcc -c subtract.c -o subtract.o
gcc -c main.c -o main.o

3. Using `nm` Tool
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Check symbol table with nm command:

nm add.o
nm main.o

Key symbols:

T - Functions defined in that object file
U - Undefined symbols to be resolved elsewhere

4. Using `objdump` Tool
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objdump -r main.o    # Relocation entries
objdump -d main.o    # Disassemble
objdump -h main.o    # Section headers
objdump -t myprogram # Symbol table

Flag	Description
`-d`	Disassemble executable sections (assembly code)
`-r`	Display relocation entries (e.g., `R_X86_64_PLT32`)
`-h`	Section headers and memory layout
`-t`	Symbol table

5. Creating Static Libraries
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Bundle multiple object files into a static library:

ar rcs libmath.a add.o multiply.o subtract.o

6. Linking Process
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Link objects with libraries to create final executable:

gcc main.o -L. -lmath -o myprogram

Real addresses are assigned to functions at this stage.

7. Optimization Levels
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gcc -O0 main.c -o program_slow      # No optimization
gcc -O1 main.c -o program_basic     # Basic optimization
gcc -O2 main.c -o program_standard  # Recommended optimization
gcc -O3 main.c -o program_fast      # Aggressive optimization
gcc -Os main.c -o program_small     # Size optimization

Flag	Description
`-O0`	No optimization (for debugging)
`-O1`	Basic optimization
`-O2`	Recommended optimization
`-O3`	Aggressive optimization
`-Os`	Size optimization

Overview#

1. Example Files#

math_utils.h#

add.c#

multiply.c#

subtract.c#

main.c#

2. Compilation to Object Files#

3. Using nm Tool#

4. Using objdump Tool#

5. Creating Static Libraries#

6. Linking Process#

7. Optimization Levels#