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Thank you for interest in the Intel® Edison Breakout Board.
The mraa_gpio_init() function initializes a general purpose Input/Output based on the board number pin (number inside the parenthesis). It helps relate a specific pin from the board to a certain task, via coding.
This link provides the mraa number for each pin of your Edison board: https://iotdk.intel.com/docs/master/mraa/edison.html.
For more detailed information, please check the following links:
For example, this code: https://github.com/intel-iot-devkit/mraa/blob/master/examples/gpio_read6.c, helps the programmer read data form pin 6 (pin J17-7/GP27 of the Edison board) through the mraa_gpio_init(6) function.
I hope you find the previous information useful.