Set ALSA master volume from C code

The following works for me. The parameter volume is to be given in the range [0, 100]. Beware, there is no error handling! void SetAlsaMasterVolume(long volume) { long min, max; snd_mixer_t *handle; snd_mixer_selem_id_t *sid; const char *card = “default”; const char *selem_name = “Master”; snd_mixer_open(&handle, 0); snd_mixer_attach(handle, card); snd_mixer_selem_register(handle, NULL, NULL); snd_mixer_load(handle); snd_mixer_selem_id_alloca(&sid); snd_mixer_selem_id_set_index(sid, 0); … Read more

What is a jump table?

A jump table can be either an array of pointers to functions or an array of machine code jump instructions. If you have a relatively static set of functions (such as system calls or virtual functions for a class) then you can create this table once and call the functions using a simple index into … Read more

Static allocation of opaque data types

You can use the _alloca function. I believe that it’s not exactly Standard, but as far as I know, nearly all common compilers implement it. When you use it as a default argument, it allocates off the caller’s stack. // Header typedef struct {} something; int get_size(); something* create_something(void* mem); // Usage handle* ptr = … Read more

Looking for an efficient integer square root algorithm for ARM Thumb2

Integer Square Roots by Jack W. Crenshaw could be useful as another reference. The C Snippets Archive also has an integer square root implementation. This one goes beyond just the integer result, and calculates extra fractional (fixed-point) bits of the answer. (Update: unfortunately, the C snippets archive is now defunct. The link points to the … Read more

Difference between const & const volatile

An object marked as const volatile will not be permitted to be changed by the code (an error will be raised due to the const qualifier) – at least through that particular name/pointer. The volatile part of the qualifier means that the compiler cannot optimize or reorder access to the object. In an embedded system, … Read more