how to declare a pointer in a function

micou, 

you need to have a declaration of  the I2C_write_data() function before using it in main().

Do you include the declaration in any header files included at the beginning of the code?

If not, just declare it like this:

// function declaration
int I2C_write_data(char nb_write, char* data_write, char adress);


int main (void)
{

  char data_read_write [3] = {0,12,0};

  char err = 0;

  err = I2C_write_data(3,data_read_write,0x40};

  ...
}

int I2C_write_data(char nb_write, char* data_write, char adress)
{

...

}