[Fontconfig] Understanding the FcValue type == FcTypeString

[David]

Hi,

I notice that a fc_value_string macro is used to retrieve a string from 
an FcValue object. 

#define FcValueString(v)    FcPointerMember(v, u.s, FcChar8)

Which seems to expand to:

(((intptr_t) (v->u.s) & 1) != 0) ? 
(FcChar8 *) ((intptr_t) v + ((intptr_t) (v->u.s) & ~1))
   :  v->u.s


As I understand it, if the string is at an odd address, then return the 
base FcValue object address + the  31 most significant bits of the 
address of the string, otherwise return the address of the string. 

This happens when using the patelt expression in the xml config files 
and other locations like loading the font map:

#0  0xb744ac49 in FcStrCaseWalkerNext (w=0xbfa94c24) at fcstr.c:160
#1  0xb744ae48 in IA__FcStrCmpIgnoreCase (s1=0xd2c21fe6 <Address 
0xd2c21fe6 out of bounds>, s2=0x200e4d0e <Address 0x200e4d0e out of 
bounds>) at fcstr.c:224
#2  0xb743642e in FcConfigCompareValue (left_o=0x69611ff0, op=FcOpEqual, 
right_o=0xbfa94d18) at fccfg.c:683
#3  0xb743762d in FcConfigMatchValueList (p=0x695a9ff0, t=0x690d6fe8, 
values=0x69611fec) at fccfg.c:1088
#4  0xb7437d16 in IA__FcConfigSubstituteWithPat (config=0x69617fbc, 
p=0x695a9ff0, p_pat=0x0, kind=FcMatchPattern) at fccfg.c:1358
#5  0xb74381f9 in IA__FcConfigSubstitute (config=0x0, p=0x695a9ff0, 
kind=FcMatchPattern) at fccfg.c:1512
#6  0xb7465ebd in pango_cairo_fc_font_map_context_substitute 
(fcfontmap=0x17c1ec60, context=0x29ea9f50, pattern=0x695a9ff0) at 
pangocairo-fcfontmap.c:95
#7  0xb6d39e95 in pango_fc_default_substitute (fontmap=0x17c1ec60, 
context=0x29ea9f50, pattern=0x695a9ff0) at pangofc-fontmap.c:988
#8  0xb6d3a1f8 in pango_fc_font_map_get_patterns (fontmap=0x17c1ec60, 
context=0x29ea9f50, desc=0x110f1f80, language=0x69cf9ffa) at 
pangofc-fontmap.c:1081
#9  0xb6d3a5e3 in pango_fc_font_map_load_fontset (fontmap=0x17c1ec60, 
context=0x29ea9f50, desc=0x110f1f80, language=0x69cf9ffa) at 
pangofc-fontmap.c:1238
#10 0xb73cb232 in pango_font_map_load_fontset (fontmap=0x17c1ec60, 
context=0x29ea9f50, desc=0x110f1f80, language=0x69cf9ffa) at 
pango-fontmap.c:107

In FcConfigCompareValue, left_o and right_o are:

(gdb) print *left_o
$14 = {type = FcTypeString, u = {s = 0x6960fff7 "Sans", i = 1767964663, 
b = 1767964663, d = -1.9593223504712061e-42, m = 0x6960fff7, c = 
0x6960fff7, f = 0x6960fff7, p = 0x6960fff7, l = 0x6960fff7}}
(gdb) print *right_o
$15 = {type = FcTypeString, u = {s = 0x6064fff7 "mono", i = 1617231863, 
b = 1617231863, d = 4.7669276023636118e-270, m = 0x6064fff7, c = 
0x6064fff7, f = 0x6064fff7, p = 0x6064fff7, l = 0x6064fff7}}

And after 'Canonicalize'-ing you get:

(gdb) print left
$16 = {type = FcTypeString, u = {s = 0xd2c21fe6 <Address 0xd2c21fe6 out 
of bounds>, i = -759029786, b = -759029786, d = 3.8689810404977115e-314, 
m = 0xd2c21fe6, c = 0xd2c21fe6, f = 0xd2c21fe6, p = 0xd2c21fe6, l = 
0xd2c21fe6}}
(gdb) print right
$17 = {type = FcTypeString, u = {s = 0x200e4d0e <Address 0x200e4d0e out 
of bounds>, i = 537808142, b = 537808142, d = 2.3877083179811831e-314, m 
= 0x200e4d0e, c = 0x200e4d0e, f = 0x200e4d0e, p = 0x200e4d0e, l = 
0x200e4d0e}}