I am interfacing the Raspberry Pi with an accelerometer using the code:
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <linux/i2c-dev.h>
#include "LSM9DS0.h"
void readBlock(uint8_t command, uint8_t size, uint8_t *data);
void selectDevice(int file, int addr);
void readACC(int *a);
void writeAccReg(uint8_t reg, uint8_t value);
void enableIMU();
int file;
void readBlock(uint8_t command, uint8_t size, uint8_t *data)
{
int result = i2c_smbus_read_i2c_block_data(file, command, size, data);
if (result != size)
{
printf("Failed to read block from I2C.");
exit(1);
}
}
void selectDevice(int file, int addr)
{
if (ioctl(file, I2C_SLAVE, addr) < 0) {
printf("Failed to select I2C device.");
}
}
void readACC(int *a)
{
printf("entered readACC");
uint8_t block[6];
selectDevice(file,ACC_ADDRESS);
readBlock(0x80 | OUT_X_L_A, sizeof(block), block);
*a = (int16_t)(block[0] | block[1] << 8);
*(a+1) = (int16_t)(block[2] | block[3] << 8);
*(a+2) = (int16_t)(block[4] | block[5] << 8);
printf("X axis: %4.2f, Y axis: %4.2f, Z axis: %4.2f", a, a+1, a+2);
}
void writeAccReg(uint8_t reg, uint8_t value)
{
selectDevice(file,ACC_ADDRESS);
int result = i2c_smbus_write_byte_data(file, reg, value);
if (result == -1)
{
printf ("Failed to write byte to I2C Acc.");
exit(1);
}
}
void enableIMU()
{
__u16 block[I2C_SMBUS_BLOCK_MAX];
int res, bus, size;
char filename[20];
sprintf(filename, "/dev/i2c-%d", 1);
file = open(filename, O_RDWR);
if (file<0) {
printf("Unable to open I2C bus!");
exit(1);
}
// Enable accelerometer.
writeAccReg(CTRL_REG1_XM, 0b10010111); // z,y,x axis enabled, continuos update, 100Hz data rate
writeAccReg(CTRL_REG2_XM, 0b00100000); // +/- 16G full scale
printf("Accelerometer is enabled\n");
}
int main(){
enableIMU();
printf("fine\n");
int *a;
readAcc(a);
return 0;
}
My output looks like this:
Accelerometer is enabled
fine
Segmentation fault
Based on the output, the function enableIMU is working fine, the output also shows "fine" which is right before entering readACC, but I never enter readACC because "entered readACC" doesn't get printed. Instead, I get a segmentation fault.
Do you know what I'm doing wrong here? I would really appreciate your help!
The problem in your code is:
int *a;
readAcc(a);
…
*a = (int16_t)(block[0] | block[1] << 8);
In the first line, you declare a pointer to an int. This pointer points somewhere, since it is not initialized. Then, in the last line, you write to this somewhere in memory.
Instead, you should write this:
int16_t a[3];
readAcc(a);
That way, you define the storage where the function readAcc can write to. The parameter to the readAcc function should not be a pointer to int, but a pointer to int16_t, since you use it as such.
Inside readAcc, you can then write:
a[0] = …;
a[1] = …;
a[2] = …;
Instead of the array, you could also define a struct xyz { int16_t x, y, z; }, which describes more accurately what the code does.
I would write it like this:
struct xyz {
int16_t x, y, z;
};
void readACC(struct xyz *coord)
{
printf("entered readACC\n");
selectDevice(file, ACC_ADDRESS);
uint8_t block[6];
readBlock(0x80 | OUT_X_L_A, sizeof(block), block);
coord->x = (int16_t)(block[0] | block[1] << 8);
coord->y = (int16_t)(block[2] | block[3] << 8);
coord->z = (int16_t)(block[4] | block[5] << 8);
printf("X axis: %4.2f, Y axis: %4.2f, Z axis: %4.2f\n",
coord->x / 256.0, coord->y / 256.0, coord->z / 256.0);
}
struct xyz to make the code self-explaining.selectDevice call above the block declaration, since the block is not needed for that.a parameter with a better name, coord, which has an appropriate data type.printf call to match the %f conversion specifier. (I hope the factor 256.0 is correct.) When you try to print an int using %f, the behavior is undefined (which is the worst thing that can happen in a C program).printf format strings.