IS_ERR() macro in Linux

Question

While getting to know how to write device drivers I came across IS_ERR() macro. But I couldn't find how is it working. I have include the code below:

majorNumber = register_chrdev(0, DEVICE_NAME, &fops);

if (majorNumber<0)
{
    printk(KERN_ALERT "Failed to register a major number\n");
    return majorNumber;
}
printk(KERN_INFO "Registered correctly with major number %d\n", majorNumber);

// Register the device class
ebbcharClass = class_create(THIS_MODULE, CLASS_NAME);

if (IS_ERR(ebbcharClass))
{               
  unregister_chrdev(majorNumber, DEVICE_NAME);
  printk(KERN_ALERT "Failed to register device class\n");
  return PTR_ERR(ebbcharClass);          
}

So what does the IS_ERR() macro expand to and how it gets executed.

Answer 1

Tests if the supplied pointer should be considered an error value.

It does not check if the pointer is valid.

In your code IS_ERR is used to check if class_create succeded creating ebbcharClass. If an error occurs unregister the char driver and signal the error.

You can find MACROs and inline funcs in err.h

Answer 2

Be careful for pitfall:

#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
#define MAX_ERRNO       4095

This covers -1 to -4095, which represents error code, not number below 4096, nor NULL (0). Every value from 0 to 4294963201 (0xfffff001) is considered no error. Do not use it to cover NULL checking.