Thank you for the reply. After 3 hours trials and reading the source code of Postgres backend, now I can figure out a way to transfer float data from sever to client and then write back to server *without lose any precision*.
At server part, it uses strtod to convert received string to float value, while it uses sprintf(str, "%.*g", DBL_DIG + extra_float_digits, val) to convert float value to string. Here the extra_float_digits == 0 in default and DBL_DIG ==15 which is defined in "float.h". Therefore, at client part, we should use the *EXACT* same way to do conversion. I mean we should use sprintf(str, "%.15g", val) and strtod. I have tested about 10000 random float values, it works fine. Interestingly, if I use "%.16g", "%.17g"...etc, precision lost will occur. -- ShenLei 2010/1/29 Vincenzo Romano <vincenzo.rom...@notorand.it> > 2010/1/29 沈雷 <drshen...@gmail.com>: > > Hi! > > > > In C Language, there is a way to format float numbers into a hex string > by > > using "%a" in printf. > > eg: > > the value: 1.2345 can be expressed as '0x1.3c083126e978dp+0' which is the > > hex representation of a float number. > > > > I have tried this in Postgres: > > SELECT '0x1.3c083126e978dp+0'::float; > > float8 > > -------- > > 1.2345 > > (1 row) > > This means that Postgres can accepty hex float as *input*. However I > cannot > > find any format function for *output*. > > to_char(..., ...) does not have "%a"-like format. Is there any way to do > > that? > > > > Thank you in advance. > > > > -- > > ShenLei > > Well, your question sould be about a scanf, not printf, as this is > input, not output. > While being an "interesting" feature, I don't see and use for it. > And, in my opinion, this is not the first and only feature that works > and is not documented. > So, please, don't rely on it until it gets documented. >
