I understand %#x give the same effect of 0x%x and it meets POSIX standard. But people mention that some compilers do not support it. Is that true, any example?
Aside from perhaps some broken embedded-systems C libraries, the # modifier should be universally supported. However %#x and 0x%x are not the same. They yield different results for the value 0, and the # modifier will always print the x in the same case as the hex digits (e.g. %#x gives 0xa and %#X gives 0XA) while using 0x%X would allow you to have a lowercase x and capital hex digits (much more visually pleasing, at least to me). As such, I find the # modifier is rarely useful in practice.