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OPENSSL_ia32cap(3)                  OpenSSL                 OPENSSL_ia32cap(3)

       OPENSSL_ia32cap - finding the IA-32 processor capabilities

       libcrypto, -lcrypto

        unsigned long *OPENSSL_ia32cap_loc(void);
        #define OPENSSL_ia32cap (*(OPENSSL_ia32cap_loc()))

       Value returned by OPENSSL_ia32cap_loc() is address of a variable
       containing IA-32 processor capabilities bit vector as it appears in EDX
       register after executing CPUID instruction with EAX=1 input value (see
       Intel Application Note #241618). Naturally it's meaningful on IA-32[E]
       platforms only. The variable is normally set up automatically upon
       toolkit initialization, but can be manipulated afterwards to modify
       crypto library behaviour. For the moment of this writing six bits are
       significant, namely:

       1. bit #28 denoting Hyperthreading, which is used to distiguish
          cores with shared cache; 2. bit #26 denoting SSE2 support; 3. bit
       #25 denoting SSE support; 4. bit #23 denoting MMX support; 5. bit #20,
       reserved by Intel, is used to choose between RC4 code
          pathes; 6. bit #4 denoting presence of Time-Stamp Counter.

       For example, clearing bit #26 at run-time disables high-performance
       SSE2 code present in the crypto library. You might have to do this if
       target OpenSSL application is executed on SSE2 capable CPU, but under
       control of OS which does not support SSE2 extentions. Even though you
       can manipulate the value programmatically, you most likely will find it
       more appropriate to set up an environment variable with the same name
       prior starting target application, e.g. on Intel P4 processor 'env
       OPENSSL_ia32cap=0x12900010 apps/openssl', to achieve same effect
       without modifying the application source code. Alternatively you can
       reconfigure the toolkit with no-sse2 option and recompile.

1.0.1u                            2011-06-05                OPENSSL_ia32cap(3)