Recommended C style and coding rules

This document describes C code style used by Tilen MAJERLE in his projects and libraries.

General rules

Here are listed most obvious and important general rules. Please check them carefully before you continue with other chapters.

Use C99 standard
Do not use tabs, use spaces instead
Use 4 spaces per indent level
Use 1 space between keyword and opening bracket
Do not use space between function name and opening bracket

int a = sum(4, 3);      /* OK */
int a = sum (4, 3);     /* Wrong */

Never use __ or _ prefix for variables/functions/macros/types. This is reserved for C language itself
Use only lowercase characters for variables/functions/macros/types with optional underscore _ char
Opening curly bracket is always at the same line as keyword (for, while, do, switch, if, ...)

int a;
for (a = 0; a < 5; ++i) {           /* OK */
}
for (a = 0; a < 5; ++i){            /* Wrong */
}
for (a = 0; a < 5; ++i)             /* Wrong */
{
}

Use single space before and after comparison and assignment operators

int a;
a = 3 + 4;              /* OK */
for (a = 0; a < 5; ++a) /* OK */
a=3+4;                  /* Wrong */
a = 3+4;                /* Wrong */
for (a=0;a<5;++a)       /* Wrong */

Use single space after every comma

func_name(5, 4);        /* OK */
func_name(4,3);         /* Wrong */

Do not initialize static and global variables to 0 (or NULL), let compiler do it for you

static int a;           /* OK */
static int b = 4;       /* OK */
static int a = 0;       /* Wrong */

void
my_func(void) {
    static int* ptr;    /* OK */
    static char abc = 0;/* Wrong */
}

Declare all local variables of the same type in the same line

void
my_func(void) {
    char a;             /* OK */
    char a, b;          /* OK */
    char b;             /* Wrong, variable with char type already exists */
}

Declare local variables in order
1. Custom structures and enumerations
2. Integer types, wider unsigned type first
3. Single/Double floating point

int
my_func(void) {
    /* 1 */
    my_struct_t my;     /* First custom structures */
    my_struct_ptr_t* p; /* Pointers too */

    /* 2 */
    uint32_t a;
    int32_t b;
    uint16_t c;
    int16_t g;
    /* ... */

    /* 3 */
    double d;
    float f;
}

Always declare local variables in beginning of the block, before first executable statement
Declare counter variables in for loop

/* OK */
for (int i = 0; i < 10; ++i)

/* OK, if you need counter variable later */
int i;
for (i = 0; i < 10; ++i) {
    if (...) {
        break;
    }
}
if (i == 10) {

}

/* Wrong */
int i;
for (i = 0; i < 10; ++i) ...

Avoid variable assignment with function call in declaration, except for single variables

void
a(void) {
    /* Avoid function calls when declaring variable */
    int a, b = sum(1, 2);
    
    /* Use this */
    int a, b;
    b = sum(1, 2);

    /* This is ok */
    uint8_t a = 3, b = 4;
}

Except char, float or double, always use types declared in stdint.h library, eg. uint8_t for unsigned 8-bit, etc.
Do not use stdbool.h library. Use 1 or 0 for true or false respectively

/* OK */
uint8_t status;
status = 0;

/* Wrong */
#include <stdbool.h>
bool status = true;

Never compare against true, eg. if (check_func() == 1), use if (check_func()) { ... }
Always compare pointers against NULL value

/* OK, compare against NULL */
void* ptr;
if (ptr == NULL || ptr != NULL) {

}

/* Wrong */
if (ptr || !ptr) {

}

Always use pre-increment (and decrement respectively) instead of post-increment (and decrement respectively)

int a = 0;
...

a++;            /* Wrong */
++a;            /* OK */

for (int j = 0; j < 10; ++j) {}     /* OK */

Always use size_t for length or size variables
Always use const for pointer if function should not modify memory pointed to by pointer
Always use const for function parameter or variable, if it should not be modified

/* When d could be modified, data pointed to by d could not be modified */
void
my_func(const void* d) {

}

/* When d and data pointed to by d both could not be modified */
void
my_func(const void* const d) {

}

/* Not required, it is advised */
void
my_func(const size_t len) {

}

/* When d should not be modified inside function, only data pointer to by d could be modified */
void
my_func(void* const d) {

}

When function may accept pointer of any type, always use void *, do not use uint8_t *
- Function must take care of proper casting in implementation

/*
 * To send data, function should not modify memory pointed to by `data` variable
 * thus `const` keyword is important
 *
 * To send generic data (or to write them to file)
 * any type may be passed for data,
 * thus use `void *`
 */
/* OK example */
void
send_data(const void* data, size_t len) { /* OK */
    /* Do not cast `void *` or `const void *` */
    const uint8_t* d = data;/* Function handles proper type for internal usage */
}

void
send_data(const void* data, int len) {    /* Wrong */
}

Never use Variable Length Array (VLA). Use dynamic memory allocation instead with standard C malloc and free functions or if library/project provides custom memory allocation, use its implementation
Always use brackets with sizeof operator.

/* OK */
#include <stdlib.h>
void my_func(size_t size) {
    int* arr;
    arr = malloc(sizeof(*arr) * n); /* OK, Allocate memory */
    arr = malloc(sizeof *arr * n);  /* Wrong, brackets for sizeof operator are missing */
    if (arr == NULL) {
        /* FAIL, no memory */
    }
    
    free(arr);  /* Free memory after usage */
}

/* Wrong */
void
my_func(int size) {
    int arr[size];      /* Wrong, do not use VLA */
}

Always compare variable against zero, except if it is treated as boolean type
Never compare boolean-treated variables against zero or one. Use NOT (!) instead

size_t length = 5;  /* Counter variable */
uint8_t is_ok = 0;  /* Boolean-treated variable */
if (length)         /* Wrong, length is not treated as boolean */
if (length > 0)     /* OK, length is treated as counter variable containing multi values, not only 0 or 1 */
if (length == 0)    /* OK, length is treated as counter variable containing multi values, not only 0 or 1 */

if (is_ok)          /* OK, variable is treated as boolean */
if (!is_ok)         /* OK, -||- */
if (is_ok == 1)     /* Wrong, never compare boolean variable against 1! */
if (is_ok == 0)     /* Wrong, use ! for negative check */

Always use /* comment */ for comments, even for single-line comment
Always include check for C++ with extern keyword in header file
Every function must include doxygen-enabled comment, even if function is static
Use English names/text for functions, variables, comments
Use lowercase characters for variables
Use underscore if variable contains multiple names, eg. force_redraw. Do not use forceRedraw
Never cast function returning void *, eg. uint8_t* ptr = (uint8_t *)func_returning_void_ptr(); as void * is safely promoted to any other pointer type
- Use uint8_t* ptr = func_returning_void_ptr(); instead
Always use < and > for C Standard Library include files, eg. #include <stdlib.h>
Always use "" for custom libraries, eg. #include "my_library.h"
When casting to pointer type, always add space between type and asterisk, eg. uint8_t* t = (uint8_t *)var_width_diff_type
Always respect code style already used in project or library

Comments

Comments starting with // are not allowed. Always use /* comment */, even for single-line comment

//This is comment (wrong)
/* This is comment (ok) */

For multi-line comments use space+asterisk for every line

/*
 * This is multi-line comments,
 * written in 2 lines (ok)
 */
 
/**
 * Wrong, use double-asterisk only for doxygen documentation
 */
 
/*
* Single line comment without space before asterisk (wrong)
*/

/*
 * Single line comment in multi-line configuration (wrong)
 */

/* Single line comment (ok) */

Use 12 indents (12 * 4 spaces) offset when commenting. If statement is larger than 12 indents, make comment 4-spaces aligned (examples below)

void
my_func(void) {
    char a, b;
                                                
    a = call_func_returning_char_a(a);          /* This is comment with 12*4 spaces indent from beginning of line */
    b = call_func_returning_char_a_but_func_name_is_very_long(a);   /* This is comment, aligned to 4-spaces indent */
}

Functions

Every function which may have access from outside its module, must include function prototype (or declaration)
Function name must be lowercase, optionally separated with underscore _ character

/* OK */
void my_func(void);
void myfunc(void);

/* Wrong */
void MYFunc(void);
void myFunc();

When function returns pointer, add space between asterisk and data type

/* OK */
const char * my_func(void);
my_struct_t * my_func(int a, int b);

/* Wrong */
const char *my_func(void);
my_struct_t* my_func(void);

Align all function prototypes (with the same/similar functionality) for better readability

/* OK, function names aligned */
void        set(int a);
my_type_t   get(void);
my_ptr_t *  get_ptr(void);

/* Wrong */
void set(int a);
const char* get(void);

Function implementation must include return type and optional other keywords in separate line

/* OK */
int
foo(void) {
    return 0;
}

/* OK */
static const char *
get_string(void) {
    return "Hello world!\r\n";
}

/* Wrong */
int foo(void) {
    return 0;
}

When function returns pointer, asterisk character must include space between type and character (char *)

/* OK */
const char *
foo(void) {
    return "test";
}

/* Wrong */
const char*
foo(void) {
    return "test";
}

Variables

Make variable name all lowercase with optional underscore _ character

/* OK */
int a;
int my_var;
int myvar;

/* Wrong */
int A; 
int myVar;
int MYVar;

Group local variables together by type

void
foo(void) {
    int a, b;   /* OK */
    char a;
    char b;     /* Wrong, char type already exists */
}

Do not declare variable after first executable statement

void
foo(void) {
    int a;
    a = bar();
    int b;      /* Wrong, there is already executable statement */
}

You may declare new variables inside next indent level

int a, b;
a = foo();
if (a) {
    int c, d;   /* OK, c and d are in if-statement scope */
    c = foo();
    int e;      /* Wrong, there was already executable statement inside block */
}

Declare pointer variables with asterisk aligned to type

/* OK */
char* a;

/* Wrong */
char *a;
char * a;

When declaring multiple pointer variables, you may declare them with asterisk aligned to variable name

/* OK */
char *p, *n;

Structures, enumerations, typedefs

Structure or enumeration name must be lowercase with optional underscore _ character between words
Structure or enumeration may contain typedef keyword
All structure members must be lowercase
All enumeration members must be uppercase
Declare every member in its own line, even if they share the same type, eg. do not do int a, b
Structure/enumeration must follow doxygen documentation syntax

When structure is declared, it may use one of 3 different options:

When structure is declared with name only, it must not contain _t suffix after its name.

struct struct_name {
    char* a;
    char b;
};

When structure is declared with typedef only, it has to contain _t suffix after its name.

typedef struct {
    char* a;
    char b;
} struct_name_t;

When structure is declared with name and typedef, it must not contain _t for basic name and it has to contain _t suffix after its name for typedef part.

typedef struct struct_name {
    char* a;
    char b;
    char c;
} struct_name_t;

Examples of bad declarations and their suggested corrections

/* a and b must be separated to 2 lines */
/* Name of structure with typedef must include _t suffix */
typedef struct {
    int a, b;
} a;

/* Corrected version */
typedef struct {
    int a;
    int b;
} a_t;

/* Wrong name, it must not include _t suffix */
struct name_t {
    int a;
    int b;
};

/* Wrong parameters, must be all uppercase */
typedef enum {
    MY_ENUM_TESTA,
    my_enum_testb,
} my_enum_t;

When initializing structure on declaration, use C99 initialization style

/* OK */
a_t a = {
    .a = 4,
    .b = 5,
};

/* Wrong */
a_t a = {1, 2};

When new typedef is introduced for function handles, use _fn suffix

/* Function accepts 2 parameters and returns uint8_t */
/* Name of typedef has `_fn` suffix */
typedef uint8_t (*my_func_typedef_fn)(uint8_t p1, const char* p2);

Compound statements

Every compound statement must include opening and closing curly bracket, even if it includes only 1 nested statement
Every compound statement must include single indent; when nesting statements, include 1 indent size for each nest

/* OK */
if (c) {
    do_a();
} else {
    do_b();
}

/* Wrong */
if (c)
    do_a();
else
    do_b();
    
/* Wrong */
if (c) do_a();
else do_b();

In case of if or if-else-if statement, else must be in the same line as closing bracket of first statement

/* OK */
if (a) {

} else if (b) {

} else {

}

/* Wrong */
if (a) {

} 
else {

}

/* Wrong */
if (a) {

} 
else
{

}

In case of do-while statement, while part must be in the same line as closing bracket of do part

/* OK */
do {
    int a;
    a = do_a();
    do_b(a);
} while (check());

/* Wrong */
do
{
/* ... */
} while (check());

/* Wrong */
do {
/* ... */
}
while (check());

Indentation is required for every opening bracket

if (a) {
    do_a();
} else {
    do_b();
    if (c) {
        do_c();
    }
}

Never do compound statement without curly bracket, even in case of single statement. Examples below show bad practices

if (a) do_b();
else do_c();

if (a) do_a(); else do_b();

Empty while, do-while or for loops must include brackets

/* OK */
while (is_register_bit_set()) {}

/* Wrong */
while (is_register_bit_set());
while (is_register_bit_set()) { }
while (is_register_bit_set()) {
}

If while (or for, do-while, etc) is empty (it can be the case in embedded programming), use empty single-line brackets

/* Wait for bit to be set in embedded hardware unit
uint32_t* addr = HW_PERIPH_REGISTER_ADDR;

/* Wait bit 13 to be ready */
while (*addr & (1 << 13)) {}        /* OK, empty loop contains no spaces inside curly brackets */
while (*addr & (1 << 13)) { }       /* Wrong */
while (*addr & (1 << 13)) {         /* Wrong */

}
while (*addr & (1 << 13));          /* Wrong, curly brackets are missing. Can lead to compiler warnings or unintentional bugs */

Always prefer using loops in this order: for, do-while, while
Avoid incrementing variables inside loop block if possible, see examples

/* Not recommended */
int a = 0;
while (a < 10) {
    .
    ..
    ...
    ++a;
}

/* Better */
for (int a = 0; a < 10; ++a) {

}

/* Better, if inc may not happen in every cycle */
for (int a = 0; a < 10; ) {
    if (...) {
        ++a;
    }
}

Switch statement

Add single indent for every case statement
Use additional single indent for break statement in each case or default

/* OK, every case has single indent */
/* OK, every break has additional indent */
switch (check()) {
    case 0:
        do_a();
        break;
    case 1:
        do_b();
        break;
    default:
        break;
}

/* Wrong, case indent missing */
switch (check()) {
case 0:
    do_a();
    break;
case 1:
    do_b();
    break;
default:
    break;
}

/* Wrong */
switch (check()) {
    case 0:
        do_a();
    break;      /* Wrong, break must have indent as it is under case */
    case 1:
    do_b();     /* Wrong, indent under case is missing */
    break;
    default:
        break;
}

Always include default statement

/* OK */
switch (var) {
    case 0: 
        do_job(); 
        break;
    default: break;
}

/* Wrong, default is missing */
switch (var) {
    case 0: 
        do_job(); 
        break;
}

If local variables are required, use curly brackets and put break statement inside.
- Put opening curly bracket in the same line as case statement

switch (a) {
    /* OK */
    case 0: {
        int a, b;
        char c;
        a = 5;
        /* ... */
        break;
    }
    
    /* Wrong */
    case 1:
    {
        int a;
        break;    
    }
    
    /* Wrong */
    case 2: {
        int a;   
    }
    break; 
}

Macros and preprocessor directives

Always use macros instead of literal constants, specially for numbers
All macros must be fully uppercase, with optional underscore _ character, except if they are clearly marked as function which may be in the future replaced with regular function syntax

/* OK */
#define MY_MACRO(x)         ((x) * (x))

/* Wrong */
#define square(x)           ((x) * (x))

Always protect input parameters with parentheses

/* OK */
#define MIN(x, y)           ((x) < (y) ? (x) : (y))

/* Wrong */
#define MIN(x, y)           x < y ? x : y

Always protect final macro evaluation with parenthesis

/* Wrong */
#define MIN(x, y)           (x) < (y) ? (x) : (y)
#define SUM(x, y)           (x) + (y)

/* Imagine result of this equation using wrong SUM implementation */
int x = 5 * SUM(3, 4);      /* Expected result is 5 * 7 = 35 */
int x = 5 * (3) + (4);      /* It is evaluated to this, final result = 19 which is not what we expect */

/* Correct implementation */
#define MIN(x, y)           ((x) < (y) ? (x) : (y))
#define SUM(x, y)           ((x) + (y))

When macro uses multiple statements, protect it using do-while (0) statement

typedef struct {
    int px, py;
} point_t;
point_t p;                  /* Define new point */

/* Wrong implementation */

/* Define macro to set point */
#define SET_POINT(p, x, y)  (p)->px = (x); (p)->py = (y)    /* 2 statements. Last one should not implement semicolon */

SET_POINT(&p, 3, 4);        /* Set point to position 3, 4. This evaluates to... */
(&p)->px = (3); (&p)->py = (4); /* ... to this. In this example this is not a problem. */

/* Consider this ugly code, however it is valid by C standard (not recommended) */
if (a)                      /* If a is true */
    if (b)                  /* If b is true */
        SET_POINT(&p, 3, 4);/* Set point to x = 3, y = 4 */
    else
        SET_POINT(&p, 5, 6);/* Set point to x = 5, y = 6 */

/* Evaluates to code below. Do you see the problem? */
if (a)
    if (b) 
        (&p)->px = (3); (&p)->py = (4);
    else
        (&p)->px = (5); (&p)->py = (6);

/* Or if we rewrite it a little */
if (a)
    if (b) 
        (&p)->px = (3);
        (&p)->py = (4);
    else
        (&p)->px = (5);
        (&p)->py = (6);

/*
 * Ask yourself a question: To which `if` statement `else` keyword belongs?
 *
 * Based on first part of code, answer is straight-forward. To inner `if` statement when we check `b` condition
 * Actual answer: Compilation error as `else` belongs nowhere
 */

/* Better and correct implementation of macro */
#define SET_POINT(p, x, y)  do { (p)->px = (x); (p)->py = (y); } while (0)    /* 2 statements. No semicolon after while loop */
/* Or even better */
#define SET_POINT(p, x, y)  do {    \   /* Backslash indicates statement continues in new line */
    (p)->px = (x);                  \
    (p)->py = (y);                  \
} while (0)                             /* 2 statements. No semicolon after while loop */

/* Now original code evaluates to */
if (a)
    if (b) 
        do { (&p)->px = (3); (&p)->py = (4); } while (0);
    else
        do { (&p)->px = (5); (&p)->py = (6); } while (0);

/* Every part of `if` or `else` contains only `1` inner statement (do-while), hence this is valid evaluation */

/* To make code perfect, use brackets for every if-ifelse-else statements */
if (a) {                    /* If a is true */
    if (b) {                /* If b is true */
        SET_POINT(&p, 3, 4);/* Set point to x = 3, y = 4 */
    } else {
        SET_POINT(&p, 5, 6);/* Set point to x = 5, y = 6 */
    }
}

Always write macro documentation as regular function with additional hideinitializer doxygen keyword

#define MY_MACRO(x)         ((x) * 2)

Avoid using #ifdef or #ifndef. Use defined() or !defined() instead

#ifdef XYZ
/* do something */
#endif /* XYZ */

Always document if/elif/else/endif statements

/* OK */
#if defined(XYZ)
/* Do if XYZ defined */
#else /* defined(XYZ) */
/* Do if XYZ not defined */
#endif /* !defined(XYZ) */

/* Wrong */
#if defined(XYZ)
/* Do if XYZ defined */
#else
/* Do if XYZ not defined */
#endif

Do not indent sub statements inside #if statement

/* OK */
#if defined(XYZ)
#if defined(ABC)
/* do when ABC defined */
#endif /* defined(ABC) */
#else /* defined(XYZ) */
/* Do when XYZ not defined */
#endif /* !defined(XYZ) */

/* Wrong */
#if defined(XYZ)
    #if defined(ABC)
        /* do when ABC defined */
    #endif /* defined(ABC) */
#else /* defined(XYZ) */
    /* Do when XYZ not defined */
#endif /* !defined(XYZ) */

Documentation

Documented code allows doxygen to parse and general html/pdf/latex output, thus it is very important to do it properly.

Use doxygen-enabled documentation style for variables, functions and structures/enumerations
Always use \ for doxygen, do not use @
Always use 5x4 spaces (5 tabs) offset from beginning of line for text

/**
 * \brief           Holds pointer to first entry in linked list
 *                  Beginning of this text is 5 tabs (20 spaces) from beginning of line
 */
static
type_t* list;

Every structure/enumeration member must include documentation
Use 12x4 spaces offset for beginning of comment

/**
 * \brief           This is point struct
 * \note            This structure is used to calculate all point 
 *                      related stuff
 */
typedef struct {
    int x;                                      /*!< Point X coordinate */
    int y;                                      /*!< Point Y coordinate */
    int size;                                   /*!< Point size.
                                                    Since comment is very big,
                                                    you may go to next line */
} point_t;

/**
 * \brief           Point color enumeration
 */
typedef enum {
    COLOR_RED,                                  /*!< Red color. This comment has 12x4
                                                    spaces offset from beginning of line */
    COLOR_GREEN,                                /*!< Green color */
    COLOR_BLUE,                                 /*!< Blue color */
} point_color_t;

Documentation for functions must be written in function implementation (source file usually)
Function must include brief and all parameters documentation
Every parameter must be noted if it is in or out for input and output respectively
Function must include return parameter if it returns something. This does not apply for void functions
Function can include other doxygen keywords, such as note or warning
Use colon : between parameter name and its description

/**
 * \brief           Sum `2` numbers
 * \param[in]       a: First number
 * \param[in]       b: Second number
 * \return          Sum of input values
 */
int
sum(int a, int b) {
    return a + b;
}

/**
 * \brief           Sum `2` numbers and write it to pointer
 * \note            This function does not return value, it stores it to pointer instead
 * \param[in]       a: First number
 * \param[in]       b: Second number
 * \param[out]      result: Output variable used to save result
 */
void
void_sum(int a, int b, int* result) {
    *result = a + b;
}

If function returns member of enumeration, use ref keyword to specify which one

/**
 * \brief           My enumeration
 */
typedef enum {
    MY_ERR,                                     /*!< Error value */
    MY_OK                                       /*!< OK value */
} my_enum_t;

/**
 * \brief           Check some value
 * \return          \ref MY_OK on success, member of \ref my_enum_t otherwise
 */
my_enum_t
check_value(void) {
    return MY_OK;
}

Use notation (`NULL` => NULL) for constants or numbers

/**
 * \brief           Get data from input array
 * \param[in]       in: Input data
 * \return          Pointer to output data on success, `NULL` otherwise
 */
const void *
get_data(const void* in) {
    return in;
}

Documentation for macros must include hideinitializer doxygen command

/**
 * \brief           Get minimal value between `x` and `y`
 * \param[in]       x: First value
 * \param[in]       y: Second value
 * \return          Minimal value between `x` and `y`
 * \hideinitializer
 */
#define MIN(x, y)       ((x) < (y) ? (x) : (y))

Header/source files

Leave single empty line at the end of file
Every file must include doxygen annotation for file and brief description followed by empty line (when using doxygen)

/**
 * \file            template.h
 * \brief           Template include file
 */
                    /* Here is empty line */

Every file (header or source) must include license (opening comment includes single asterisk as this must be ignored by doxygen)
Use the same license as already used by project/library

/**
 * \file            template.h
 * \brief           Template include file
 */

/*
 * Copyright (c) year FirstName LASTNAME
 *  
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without restriction,
 * including without limitation the rights to use, copy, modify, merge,
 * publish, distribute, sublicense, and/or sell copies of the Software, 
 * and to permit persons to whom the Software is furnished to do so, 
 * subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
 * AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 *
 * This file is part of library_name.
 *
 * Author:          FirstName LASTNAME <optional_email@example.com>
 */

Header file must include guard #ifndef
Header file must include C++ check
Header file must include only every other header file in order to compile correctly, but not more (.c should include the rest if required)
Header file must only expose module public variables/types/functions
Use extern for global module variables in header file, define them in source file later

/* file.h ... */
#ifndef ...

extern int my_variable; /* This is global variable declaration in header */

#endif 

/* file.c ... */
int my_variable;        /* Actually defined in source */

Never include .c files in another .c file
.c file should first include corresponding .h file, later others, unless otherwise explicitly necessary
Do not include module private declarations in header file
Header file example (no license for sake of an example)

/* License comes here */
#ifndef TEMPLATE_HDR_H
#define TEMPLATE_HDR_H

#ifdef __cplusplus
extern "C" {
#endif /* __cplusplus */

/* Include other headers */

/* File content here */

#ifdef __cplusplus
}
#endif /* __cplusplus */

#endif /* TEMPLATE_HDR_H */

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Recommended C style and coding rules

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