Identifying Hardware Capabilities

The hardware capabilities of an object identify the hardware requirements of a platform necessary for the object to execute correctly. An example of this requirement might be the identification of code that requires the MMX or SSE features that are available on some x86 architectures.

Hardware capability requirements can be identified using the following mapfile syntax.

hwcap_1 = TOKEN | Vval [ OVERRIDE ];

The hwcap_1 declaration is qualified with one or more tokens, which are symbolic representations of hardware capabilities. In addition, or alternatively, a numeric value representing one of more capabilities can be supplied by prefixing the value with a V. For SPARC platforms, hardware capabilities are defined as AV_ values in sys/auxv_SPARC.h. For x86 platforms, hardware capabilities are defined as AV_ values in sys/auxv_386.h.

The following x86 example shows the declaration of MMX and SSE as hardware capabilities required by the object foo.so.1.

$ egrep "MMX|SSE" /usr/include/sys/auxv_386.h
#define AV_386_MMX    0x0040
#define AV_386_SSE    0x0800
$ cat mapfile
hwcap_1 = SSE MMX;
$ cc -o foo.so.1 -G -K pic -Mmapfile foo.c -lc
$ elfdump -H foo.so.1

Hardware/Software Capabilities Section:  .SUNW_cap
     index  tag               value
       [0]  CA_SUNW_HW_1     0x840  [ SSE  MMX ]

Relocatable objects can contain hardware capabilities values. The link-editor combines any hardware capabilities values from multiple input relocatable objects. The resulting CA_SUNW_HW_1 value is a bitwise-inclusive OR of the associated input values. By default, these values are combined with the hardware capabilities specified by a mapfile.

The hardware capability requirements of the output file can be controlled explicitly from a mapfile by using the OVERRIDE keyword. An OVERRIDE keyword, together with a hardware capability value of 0, effectively removes any hardware capabilities requirement from the object being built.

$ elfdump -H foo.o

Hardware/Software Capabilities Section:  .SUNW_cap
     index  tag               value
       [0]  CA_SUNW_HW_1     0x840  [ SSE  MMX ]
$ cat mapfile
hwcap_1 = V0x0 OVERRIDE;
$ cc -o bar.o -r -Mmapfile foo.o
$ elfdump -H bar.o
$ 

Any hardware capability requirements defined by an object are validated by the runtime linker against the hardware capabilities that are available to the process. If any of the hardware capability requirements can not be satisfied, the object is not loaded at runtime. For example, if the SSE feature is not available to a process, ldd(1) indicates the following error.

$ ldd prog
        foo.so.1 =>      ./foo.so.1  - hardware capability unsupported: \
                         0x800 [ SSE ]
        ....

Dynamic objects that exploit different hardware capabilities can provide a flexible runtime environment using filters. See Hardware Capability Specific Shared Objects.