Languages/C/SplitCH

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Source and header file split

It is sometimes complicated to split the header and the source file, here are some hints:

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

  • (1) The header MUST contain what is necessary from the outside of the source file.
  • (2) The header SHOULD NOT contain, what is not necessary.
    • There is no reason to pollute the namespace of the whole project.
    • Keeping the interface small and as clean as possible will make team work simplier.
    • Including only the necessary headers will shorten (re-)compilation time.
  • (3) The header file MUST prevent multiple inclusion, by using include guards.
  • (4) A header file SHOULD be complete (it defines or includes all it needs).
    • Example, if you use a uint32_t parameter in a function, the header should include <stdint.h>.
    • This can be tested by compiling a empty C file only including the header.
  • (5) A header file SHOULD not include files only used in source file.
  • (6) The header SHOULD be commented enough so there is no need to open the source file.
    • A consistent commenting SHOULD be used, for instance doxygen style.
  • (7) The source file MUST include the header, preferably first.
    • This will ensure consistency and (4)
  • (8) All symbols not exported by the header SHOULD be static (functions and variables).
    • This will prevent namespace pollution
    • Free benefit: the compiler can do better optimization (automatic inline, ...)
  • (9) C headers SHOULD be C++ friendly ("export C")
    • Without this C++ application can't link against C files

Temperature sensor example

Here is a small example, using main.c, temp_sensor.h, and temp_sensor.c.

  • temp_sensor.c Contains the code for accessing a fictional temperature sensor.
  • temp_sensor.h Defines the software interface for the temperature sensor.
  • main.c Will display the current temperature.

main.c

#include "temp_sensor.h"
#include <stdio.h>
 
int main(int argc, char *argv[])
{
	printf("temperature is : %d", temp_sensor_get());
	return 0;
}

temp_sensor.h

/**
 * \file temp_sensor.h
 *
 * \brief Temperature sensor driver
 */
#ifndef TEMP_SENSOR_H // (3) prevent multiple inclusion, see the #endif at the end of the file
#define TEMP_SENSOR_H
 
#ifdef __cplusplus // (9) C++ friendly export, see the corresponding closing brace
extern "C" { // This brace must contain every functions and variables declaration
#endif
 
#include <stdint.h> // (4) We use stdint in the interface !
 
// (6) Full comments, no need to open temp_sensor.c
// (1) This function is needed in main
/**
 * \brief Get the temperature
 *
 * \return the temperature in K.
 *
 * \warning the value will be 0 if the sensor read fails.
 */
uint16_t temp_sensor_get(void);
 
#ifdef __cplusplus
}
#endif
 
#endif /* TEMP_SENSOR_H */

temp_sensor.c

/**
 * \file temp_sensor.c
 *
 * \brief Temperature sensor driver implementation
 */
 
#include "temp_sensor.h" // (7) Include the header for compile-time checks
#include <super_os/i2c.h> // (5) The i2c bus is not used in main, so it is included in this .c file, not in the .h
 
// (2) These defines are private, we don't want to see them in main
#define SENSOR_ADDRESS 0x12
#define SENSOR_REG 0x33
 
// (8) this function is not exported -> static
// (6) this function MUST be commented here
/**
 * \brief Do the real work here
 * \param chip, the i2c address of the chip
 * \param address, the address in the chip
 * \param value, where to store the value
 * \return 0 for no problem
 */
static uint8_t read(uint8_t chip, uint8_t address, uint16_t *value)
{
	if (i2c_read(chip, address, value, 2) != 0)
	{
		return 1;
	}
 
	return 0;
}
 
// (6) This function is already commented in the header
uint16_t temp_sensor_get(void)
{
	uint16_t temp;
	uint8_t status = read(SENSOR_ADDRESS, SENSOR_REG, &temp);
 
	// Do what is said in temp_sensor.h
	if (status != 0)
	{
		temp = 0;
	}
 
	return temp;
}
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