Header Files

module = Module(
    code="ELEE1147",
    name="Programming for Engineers",
    credits=15,
    module_leader="Seb Blair BEng(H) PGCAP MIET MIHEEM FHEA"
)
Header Files

Why Use Header Files?

  • Modularity: Separate interface from implementation.

  • Reusability: Share functions and data structures.

  • Readability: Enhance code organisation.

  • Function Prototypes: Allows the compiler to check function signatures during compilation.

  • Precompiled Headers: Speeds up compilation by avoiding redundant parsing of headers in multiple source files.

Header Files

How Does It Work?

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Header Files > How does it work?

Example

// main.c
#include "header.h"

int main() {
    greeter();

    printf("PI: %.5f\n", PI);
    printf("Golden Ratio: %.5f\n", GR);

    struct Student s1 = {"Ada Lovelace", 42, 1.0f};
    printf("Student: %s, ID: %d, Grade: %.1f\n",
        s1.name, s1.studentId, s1.classification);

    return 0;
}

Compile command for reference:

gcc main.c header.c -o main.exe

// header.h
#ifndef HEADER_H // Header guard
#define HEADER_H // Macro

#include <stdio.h> // Other libraries

void greeter(); // Function prototype

#define PI 3.14159 // Shared constant
#define GR ((double)1.61803) // Golden Ratio

// Shared DataStorage
struct Student {
  char name[50];
  int studentId;
  float classification;
};

#endif // HEADER_H

// header.c
#include "header.h"

void greeter(){
    printf("Hello World!")!
}
Header Files > How does it work?

What are Header Guards?

  • Purpose: Prevent multiple inclusions of the same header file.
  • Issue: Without guards, redefinitions can occur during multiple inclusions.
  • Solution: Use preprocessor directives to conditionally include the contents.
Header Files > How does it work?

Why Use Header Guards?

  • Avoid Redefinitions: Prevent compilation errors due to duplicate declarations.

  • Ensure Once-Only Inclusion: Each header is included only once in a translation unit.

  • Improve Compilation Efficiency: Reduce redundant parsing of header contents.

#ifndef HEADER_H
#define HEADER_H

...

#endif //end of HEADER_H

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Header Files > How does it work?

Example

// main.c
#include "header.h"

int main() {
    greeter();

    printf("PI: %.5f\n", PI);
    printf("Golden Ratio: %.5f\n", GR);

    struct Student s1 = {"Ada Lovelace", 42, 1.0f};
    printf("Student: %s, ID: %d, Grade: %.1f\n",
        s1.name, s1.studentId, s1.classification);

    return 0;
}

Compile command for reference:

gcc main.c header.c -o main.exe

// header.h
#ifndef HEADER_H // Header guard
#define HEADER_H // Macro

#include <stdio.h> // Other libraries

void greeter(); // Function prototype

#define PI 3.14159 // Shared constant
#define GR ((double)1.61803) // Golden Ratio

// Shared DataStorage
struct Student {
  char name[50];
  int studentId;
  float classification;
};

#endif // HEADER_H

// header.c
#include "header.h"

void greeter(){
    printf("Hello World!")!
}
Header Files > How does it work?

Preprocessor Directive: #include "" vs <>

  • Use #include ""

    • for including header files that are part of your project or are in the current directory.

  • Use #include <> for

    • including standard library header files or other headers that are part of the system include directories.
Header Files > How does it work?

Standardised Header Examples
stdio.h:

23. #ifndef _STDIO_H
24. #define _STDIO_H	1
25. 
26. #define __GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION
27. #include <bits/libc-header-start.h>
...
878. 
879. __END_DECLS
880. 
881. #endif /* <stdio.h> included.  */
Header Files > How does it work?

Example

// main.c
#include "header.h"

int main() {
    greeter();

    printf("PI: %.5f\n", PI);
    printf("Golden Ratio: %.5f\n", GR);

    struct Student s1 = {"Ada Lovelace", 42, 1.0f};
    printf("Student: %s, ID: %d, Grade: %.1f\n",
        s1.name, s1.studentId, s1.classification);

    return 0;
}

Compile command for reference:

gcc main.c header.c -o main.exe

// header.h
#ifndef HEADER_H // Header guard
#define HEADER_H // Macro

#include <stdio.h> // Other libraries

void greeter(); // Function prototype

#define PI 3.14159 // Shared constant
#define GR ((double)1.61803) // Golden Ratio

// Shared DataStorage
struct Student {
  char name[50];
  int studentId;
  float classification;
};

#endif // HEADER_H

// header.c
#include "header.h"

void greeter(){
    printf("Hello World!")!
}
Header Files > How does it work?

Example

// main.c
#include "header.h"

int main() {
    greeter();

    printf("PI: %.5f\n", PI);
    printf("Golden Ratio: %.5f\n", GR);

    struct Student s1 = {"Ada Lovelace", 42, 1.0f};
    printf("Student: %s, ID: %d, Grade: %.1f\n",
        s1.name, s1.studentId, s1.classification);

    return 0;
}

Compile command for reference:

gcc main.c header.c -o main.exe

// header.h
#ifndef HEADER_H // Header guard
#define HEADER_H // Macro

#include <stdio.h> // Other libraries

void greeter(); // Function prototype

#define PI 3.14159 // Shared constant
#define GR ((double)1.61803) // Golden Ratio

// Shared DataStorage
struct Student {
  char name[50];
  int studentId;
  float classification;
};

#endif // HEADER_H

// header.c
#include "header.h"

void greeter(){
    printf("Hello World!")!
}
Header Files > How does it work?

Macros

Macros in C are a way to define constants or simple functions using the #define directive. They are preprocessor directives, meaning they are processed before the actual compilation of the code.

// example_macros.h

#ifndef EXAMPLE_MACROS_H
#define EXAMPLE_MACROS_H

#define PI 3.14159 // Shared Constant
#define SQUARE(x) ((x) * (x)) // Function

#ifdef _MSC_VER
// Code specific to Microsoft Version C/C++
#endif //end of _MSC_VER

#endif // end of EXAMPLE_MACROS_H
Header Files > How does it work?

Standardised Header Examples:
[math.h]

...
130. #define	M_E		((double)2.7182818284590452354)  /* e */
131. #define	M_LOG2E		((double)1.4426950408889634074)  /* log 2e */
132. #define	M_LOG10E	((double)0.43429448190325182765) /* log 10e */
133. #define	M_LN2		((double)0.69314718055994530942) /* log e2 */
134. #define	M_LN10		((double)2.30258509299404568402) /* log e10 */
135. #define	M_PI		((double)3.14159265358979323846) /* pi */
...
494. int __signbitl(long double);
495. __END_DECLS
496. 
497. #endif /* !_MATH_H_ */
Header Files > How does it work?

Example

// main.c
#include "header.h"

int main() {
    greeter();

    printf("PI: %.5f\n", PI);
    printf("Golden Ratio: %.5f\n", GR);

    struct Student s1 = {"Ada Lovelace", 42, 1.0f};
    printf("Student: %s, ID: %d, Grade: %.1f\n",
        s1.name, s1.studentId, s1.classification);

    return 0;
}

Compile command for reference:

gcc main.c header.c -o main.exe

// header.h
#ifndef HEADER_H // Header guard
#define HEADER_H // Macro

#include <stdio.h> // Other libraries

void greeter(); // Function prototype

#define PI 3.14159 // Shared constant
#define GR ((double)1.61803) // Golden Ratio

// Shared DataStorage
struct Student {
  char name[50];
  int studentId;
  float classification;
};

#endif // HEADER_H

// header.c
#include "header.h"

void greeter(){
    printf("Hello World!")!
}
Header Files > How does it work?

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