On Fri, Nov 06, 2015 at 09:24:40PM +0100, Alexander Huemer wrote:
Hi,
while fixing some trivial compiler warnings in openbsc I stumbled over this:
Making all in abis CC abis_test.o abis_test.c: In function ‘test_simple_sw_config’: abis_test.c:68:9: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘long int’ [-Wformat=] printf("Start: %u len: %zu\n", descr[0].start - simple_config, descr[0].len); ^ abis_test.c: In function ‘test_dual_sw_config’: abis_test.c:111:9: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘long int’ [-Wformat=] printf("Start: %u len: %zu\n", descr[0].start - dual_config, descr[0].len); ^ abis_test.c:115:9: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘long int’ [-Wformat=] printf("Start: %u len: %zu\n", descr[1].start - dual_config, descr[1].len); ^ abis_test.c: In function ‘test_sw_selection’: abis_test.c:132:9: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘long int’ [-Wformat=] printf("Start: %u len: %zu\n", descr[0].start - load_config, descr[0].len); ^ abis_test.c:136:9: warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘long int’ [-Wformat=] printf("Start: %u len: %zu\n", descr[1].start - load_config, descr[1].len);
The relevant expression is:
descr[1].start - load_configand variations thereof. descr[1].start is declared include/openbsc/abis_nm.h as:
const uint8_t *start;simple_config is declared as:
static const uint8_t simple_config[]So an address is taken and the first element (implicitly) of an uint8 array is substracted from it.
No, not at all :)
simple_config, a uint8_t*, is passed to abis_nm_parse_sw_config. After that, uint8_t *start is actually pointing at a byte within simple_config. It's a subtraction of two uint8_t* from each other to get a byte offset:
Subtract simple_config's address (the start of the data) from the "start" pointer's address value, and the result is a byte offset to get from simple_config to where "start" is pointing.
The declaration
static const uint8_t simple_config[]
yields a uint8_t*, but declaring it with [] allows initializing a static array. (Also, simple_config == &simple_config == &simple_config[0], all uint8_t*, which spaces me out every time I come across it.)
Also note the slight counter intuitiveness of subtracting pointers to get byte offsets: (T*)a - (T*)b == ((uint)a - (uint)b))/sizeof(T), so this case gives byte offsets precisely because sizeof(uint8_t) == 1...
~Neels