Stupid Question about C

Dev Zone Code
1

hey guys I have a library that I want to use for the raspberry pi 2 in c that I downloaded. I installed it but how do I include that project. Explain this to me in an abstract way please?

2

Abstract you say... Well if it's a compiled library you will have your header files and dll's (or so's). You use the library via headers and link the compiled things into your final executable.

3

Typically, libraries live in /lib and /usr/lib and headers live in /usr/include. So if the library is installed correctly it will have the library itself (.a file) under /lib or /usr/lib and the header files in /usr/include.

Now you can include the header files you need, by using a preprocessor directive called include that will look like this:

#include <headername.h>

To compile succesfuly you have to "link" the library. Recall that compiling C happens in two steps, compiling and linking. Compiling only looks at header files (.h files) and only cares about -I directives; it output object code (.o files). Linking only cares about -L and -l options and attempts to resolve the symbols in the object code.

So in case the header file you need is in /usr/include you don't need to use the -I directive, you'll only use it to specify a different path to look for header files. But you still need to link the library using -l followed by the library name.

An example command would be:
gcc -o hello hello.c -lncurses
where ncurses is the name of the library. Or
gcc -o hello hello.c -lm
m is the name of the math library, that you would need to link if you include math.h

4

I have a more specific question. I am using this library.

bcm2835 by Mike McCauley

I am kind of new to c not to programming. I am wondering how to turn on different pins. I am looking at this example. I am wondering if there is a way to select which pin to turn on?

`/ blink.c
//
// Example program for bcm2835 library
// Blinks a pin on an off every 0.5 secs
//
// After installing bcm2835, you can build this
// with something like:
// gcc -o blink blink.c -l bcm2835
// sudo ./blink
//
// Or you can test it before installing with:
// gcc -o blink -I ../../src ../../src/bcm2835.c blink.c
// sudo ./blink
//
// Author: Mike McCauley
// Copyright (C) 2011 Mike McCauley
// $Id: RF22.h,v 1.21 2012/05/30 01:51:25 mikem Exp $

include

include

// Blinks on RPi Plug P1 pin 11 (which is GPIO pin 17)

define PIN RPI_GPIO_P1_11

int main(int argc, char **argv)
{
// If you call this, it will not actually access the GPIO
// Use for testing
// bcm2835_set_debug(1);

if (!bcm2835_init())
  return 1;

// Set the pin to be an output
bcm2835_gpio_fsel(PIN, BCM2835_GPIO_FSEL_OUTP);

// Blink
while (1)
{
// Turn it on
bcm2835_gpio_write(PIN, HIGH);

// wait a bit
bcm2835_delay(500);

// turn it off
bcm2835_gpio_write(PIN, LOW);

// wait a bit
bcm2835_delay(500);
}
bcm2835_close();
return 0;

}
`

5 

#define PIN RPI_GPIO_P1_11

Defines a symbolic constant that identify the 17th PIN (pin map here http://elinux.org/RPi_BCM2835_GPIOs). To turn on a pin you first set it as output, then turn its state to HIGH:

bcm2835_gpio_fsel(PIN, BCM2835_GPIO_FSEL_OUTP);
...
bcm2835_gpio_write(PIN, HIGH);

To select another and turn on another pin, do the same using another PIN by changing the PIN parameter in both calls to a different PIN.
Same for turning the pin off.