In enumeration type xcb_cap_style_t, XCB_CAP_STYLE_BUTT should be 1, and 
XCB_CAP_STYLE_CAP be 2, according to X.h.
	
In X.h:

/* capStyle */
#define CapNotLast              0
#define CapButt                 1
#define CapRound                2
#define CapProjecting           3

While in xproto.h:

typedef enum xcb_cap_style_t {
    XCB_CAP_STYLE_NOT_LAST = 0,
    XCB_CAP_STYLE_CAP = 1,
    XCB_CAP_STYLE_BUTT = 2,
    XCB_CAP_STYLE_PROJECTING = 3
} xcb_cap_style_t;