c-periphery

C Library for Linux Peripheral I/O (GPIO, LED, PWM, SPI, I2C, MMIO, Serial)

c-periphery is a small C library for GPIO, LED, PWM, SPI, I2C, MMIO, and Serial peripheral I/O interface access in userspace Linux. c-periphery simplifies and consolidate the native Linux APIs to these interfaces. c-periphery is useful in embedded Linux environments (including Raspberry Pi, BeagleBone, etc. platforms) for interfacing with external peripherals. c-periphery is re-entrant, has no dependencies outside the standard C library and Linux, compiles into a static library for easy integration with other projects, and is MIT licensed.

Using Python or Lua? Check out the python-periphery and lua-periphery projects.

Examples

GPIO

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#include "gpio.h"

int main(void) {
    gpio_t *gpio_in, *gpio_out;
    bool value;

    gpio_in = gpio_new();
    gpio_out = gpio_new();

    /* Open GPIO /dev/gpiochip0 line 10 with input direction */
    if (gpio_open(gpio_in, "/dev/gpiochip0", 10, GPIO_DIR_IN) < 0) {
        fprintf(stderr, "gpio_open(): %s\n", gpio_errmsg(gpio_in));
        exit(1);
    }

    /* Open GPIO /dev/gpiochip0 line 12 with output direction */
    if (gpio_open(gpio_out, "/dev/gpiochip0", 12, GPIO_DIR_OUT) < 0) {
        fprintf(stderr, "gpio_open(): %s\n", gpio_errmsg(gpio_out));
        exit(1);
    }

    /* Read input GPIO into value */
    if (gpio_read(gpio_in, &value) < 0) {
        fprintf(stderr, "gpio_read(): %s\n", gpio_errmsg(gpio_in));
        exit(1);
    }

    /* Write output GPIO with !value */
    if (gpio_write(gpio_out, !value) < 0) {
        fprintf(stderr, "gpio_write(): %s\n", gpio_errmsg(gpio_out));
        exit(1);
    }

    gpio_close(gpio_in);
    gpio_close(gpio_out);

    gpio_free(gpio_in);
    gpio_free(gpio_out);

    return 0;
}

Go to GPIO documentation.

LED

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#include "led.h"

int main(void) {
    led_t *led;
    unsigned int max_brightness;

    led = led_new();

    /* Open LED led0 */
    if (led_open(led, "led0") < 0) {
        fprintf(stderr, "led_open(): %s\n", led_errmsg(led));
        exit(1);
    }

    /* Turn on LED (set max brightness) */
    if (led_write(led, true) < 0) {
        fprintf(stderr, "led_write(): %s\n", led_errmsg(led));
        exit(1);
    }

    /* Get max brightness */
    if (led_get_max_brightness(led, &max_brightness) < 0) {
        fprintf(stderr, "led_get_max_brightness(): %s\n", led_errmsg(led));
        exit(1);
    }

    /* Set half brightness */
    if (led_set_brightness(led, max_brightness / 2) < 0) {
        fprintf(stderr, "led_set_brightness(): %s\n", led_errmsg(led));
        exit(1);
    }

    led_close(led);

    led_free(led);

    return 0;
}

Go to LED documentation.

PWM

#include <stdio.h>
#include <stdlib.h>

#include "pwm.h"

int main(void) {
    pwm_t *pwm;

    pwm = pwm_new();

    /* Open PWM chip 0, channel 10 */
    if (pwm_open(pwm, 0, 10) < 0) {
        fprintf(stderr, "pwm_open(): %s\n", pwm_errmsg(pwm));
        exit(1);
    }

    /* Set frequency to 1 kHz */
    if (pwm_set_frequency(pwm, 1e3) < 0) {
        fprintf(stderr, "pwm_set_frequency(): %s\n", pwm_errmsg(pwm));
        exit(1);
    }

    /* Set duty cycle to 75% */
    if (pwm_set_duty_cycle(pwm, 0.75) < 0) {
        fprintf(stderr, "pwm_set_duty_cycle(): %s\n", pwm_errmsg(pwm));
        exit(1);
    }

    /* Enable PWM */
    if (pwm_enable(pwm) < 0) {
        fprintf(stderr, "pwm_enable(): %s\n", pwm_errmsg(pwm));
        exit(1);
    }

    /* Change duty cycle to 50% */
    if (pwm_set_duty_cycle(pwm, 0.50) < 0) {
        fprintf(stderr, "pwm_set_duty_cycle(): %s\n", pwm_errmsg(pwm));
        exit(1);
    }

    pwm_close(pwm);

    pwm_free(pwm);

    return 0;
}

Go to PWM documentation.

SPI

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#include "spi.h"

int main(void) {
    spi_t *spi;
    uint8_t buf[4] = { 0xaa, 0xbb, 0xcc, 0xdd };

    spi = spi_new();

    /* Open spidev1.0 with mode 0 and max speed 1MHz */
    if (spi_open(spi, "/dev/spidev1.0", 0, 1000000) < 0) {
        fprintf(stderr, "spi_open(): %s\n", spi_errmsg(spi));
        exit(1);
    }

    /* Shift out and in 4 bytes */
    if (spi_transfer(spi, buf, buf, sizeof(buf)) < 0) {
        fprintf(stderr, "spi_transfer(): %s\n", spi_errmsg(spi));
        exit(1);
    }

    printf("shifted in: 0x%02x 0x%02x 0x%02x 0x%02x\n", buf[0], buf[1], buf[2], buf[3]);

    spi_close(spi);

    spi_free(spi);

    return 0;
}

Go to SPI documentation.

I2C

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#include "i2c.h"

#define EEPROM_I2C_ADDR 0x50

int main(void) {
    i2c_t *i2c;

    i2c = i2c_new();

    /* Open the i2c-0 bus */
    if (i2c_open(i2c, "/dev/i2c-0") < 0) {
        fprintf(stderr, "i2c_open(): %s\n", i2c_errmsg(i2c));
        exit(1);
    }

    /* Read byte at address 0x100 of EEPROM */
    uint8_t msg_addr[2] = { 0x01, 0x00 };
    uint8_t msg_data[1] = { 0xff, };
    struct i2c_msg msgs[2] =
        {
            /* Write 16-bit address */
            { .addr = EEPROM_I2C_ADDR, .flags = 0, .len = 2, .buf = msg_addr },
            /* Read 8-bit data */
            { .addr = EEPROM_I2C_ADDR, .flags = I2C_M_RD, .len = 1, .buf = msg_data},
        };

    /* Transfer a transaction with two I2C messages */
    if (i2c_transfer(i2c, msgs, 2) < 0) {
        fprintf(stderr, "i2c_transfer(): %s\n", i2c_errmsg(i2c));
        exit(1);
    }

    printf("0x%02x%02x: %02x\n", msg_addr[0], msg_addr[1], msg_data[0]);

    i2c_close(i2c);

    i2c_free(i2c);

    return 0;
}

Go to I2C documentation.

MMIO

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <byteswap.h>

#include "mmio.h"

struct am335x_rtcss_registers {
    uint32_t seconds;       /* 0x00 */
    uint32_t minutes;       /* 0x04 */
    uint32_t hours;         /* 0x08 */
    /* ... */
};

int main(void) {
    mmio_t *mmio;
    uint32_t mac_id0_lo, mac_id0_hi;
    volatile struct am335x_rtcss_registers *regs;

    mmio = mmio_new();

    /* Open Control Module */
    if (mmio_open(mmio, 0x44E10000, 0x1000) < 0) {
        fprintf(stderr, "mmio_open(): %s\n", mmio_errmsg(mmio));
        exit(1);
    }

    /* Read lower 2 bytes of MAC address */
    if (mmio_read32(mmio, 0x630, &mac_id0_lo) < 0) {
        fprintf(stderr, "mmio_read32(): %s\n", mmio_errmsg(mmio));
        exit(1);
    }

    /* Read upper 4 bytes of MAC address */
    if (mmio_read32(mmio, 0x634, &mac_id0_hi) < 0) {
        fprintf(stderr, "mmio_read32(): %s\n", mmio_errmsg(mmio));
        exit(1);
    }

    printf("MAC address: %08X%04X\n", __bswap_32(mac_id0_hi), __bswap_16(mac_id0_lo));

    mmio_close(mmio);

    /* Open RTC subsystem */
    if (mmio_open(mmio, 0x44E3E000, 0x1000) < 0) {
        fprintf(stderr, "mmio_open(): %s\n", mmio_errmsg(mmio));
        exit(1);
    }

    regs = mmio_ptr(mmio);

    /* Read current RTC time */
    printf("hours: %02x minutes: %02x seconds %02x\n", regs->hours, regs->minutes, regs->seconds);

    mmio_close(mmio);

    mmio_free(mmio);

    return 0;
}

Go to MMIO documentation.

Serial

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "serial.h"

int main(void) {
    serial_t *serial;
    uint8_t s[] = "Hello World!";
    uint8_t buf[128];
    int ret;

    serial = serial_new();

    /* Open /dev/ttyUSB0 with baudrate 115200, and defaults of 8N1, no flow control */
    if (serial_open(serial, "/dev/ttyUSB0", 115200) < 0) {
        fprintf(stderr, "serial_open(): %s\n", serial_errmsg(serial));
        exit(1);
    }

    /* Write to the serial port */
    if (serial_write(serial, s, sizeof(s)) < 0) {
        fprintf(stderr, "serial_write(): %s\n", serial_errmsg(serial));
        exit(1);
    }

    /* Read up to buf size or 2000ms timeout */
    if ((ret = serial_read(serial, buf, sizeof(buf), 2000)) < 0) {
        fprintf(stderr, "serial_read(): %s\n", serial_errmsg(serial));
        exit(1);
    }

    printf("read %d bytes: _%s_\n", ret, buf);

    serial_close(serial);

    serial_free(serial);

    return 0;
}

Go to Serial documentation.

Building c-periphery with CMake

Static library

Build c-periphery into a static library:

$ mkdir build
$ cd build
$ cmake ..
$ make

Shared Library

Build c-periphery into a shared library:

$ mkdir build
$ cd build
$ cmake -DBUILD_SHARED_LIBS=ON ..
$ make

Install the shared library and headers:

$ sudo make install

Tests

Build c-periphery tests from the build directory:

$ make tests

Cross-compilation

Set the CC environment variable with the cross-compiler prior to build:

$ export CC=arm-linux-gnueabihf-gcc
$ mkdir build
$ cd build
$ cmake ..
$ make

If additional cross-compiler tools are needed, use a CMAKE_TOOLCHAIN_FILE to fully specify the toolchain parameters:

$ mkdir build
$ cd build
$ cmake -DCMAKE_TOOLCHAIN_FILE=/path/to/arm-linux-gnueabihf.cmake ..
$ make

Building c-periphery with vanilla Make

Static library

Build c-periphery into a static library:

$ make

Tests

Build c-periphery tests:

$ make tests

Cross-compilation

Set the CROSS_COMPILE environment variable with the cross-compiler prefix when building:

$ CROSS_COMPILE=arm-linux-gnueabihf- make

Building c-periphery into another project statically

Include the header files from src/ and link in the periphery.a static library:

$ gcc -I/path/to/periphery/src myprog.c /path/to/periphery/periphery.a -o myprog

Building c-periphery into another project dynamically

If the header files and shared library are installed on the system, simply link with -lperiphery:

$ gcc myprog.c -lperiphery -o myprog

Otherwise, additional include (-I) and library (-L) paths may be required.

Documentation

man page style documentation for each interface wrapper is available in docs folder.

Testing

The tests located in the tests folder may be run to test the correctness and functionality of c-periphery. Some tests require interactive probing (e.g. with an oscilloscope), the installation of a physical loopback, or the existence of a particular device on a bus. See the usage of each test for more details on the required test setup.

License

c-periphery is MIT licensed. See the included LICENSE file.