PR_SET_FP_MODE(2const) - Linux manual page (original) (raw)

PRSETFPMODE(2const) PRSETFPMODE(2const)

NAME top

   PR_SET_FP_MODE - set the floating point mode of the calling
   process

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <linux/prctl.h>** /* Definition of **PR_*** constants */
   **#include <sys/prctl.h>**

   **int prctl(PR_SET_FP_MODE, unsigned long** _mode_**);**

DESCRIPTION top

   On the MIPS architecture, user-space code can be built using an
   ABI which permits linking with code that has more restrictive
   floating-point (FP) requirements.  For example, user-space code
   may be built to target the O32 FPXX ABI and linked with code built
   for either one of the more restrictive FP32 or FP64 ABIs.  When
   more restrictive code is linked in, the overall requirement for
   the process is to use the more restrictive floating-point mode.

   Because the kernel has no means of knowing in advance which mode
   the process should be executed in, and because these restrictions
   can change over the lifetime of the process, the **PR_SET_FP_MODE**
   operation is provided to allow control of the floating-point mode
   from user space.

   The _mode_ argument is a bit mask describing the floating-point mode
   used:

   **PR_FP_MODE_FR**
          When this bit is _unset_ (so called **FR=0** or **FR0** mode), the 32
          floating-point registers are 32 bits wide, and 64-bit
          registers are represented as a pair of registers (even- and
          odd- numbered, with the even-numbered register containing
          the lower 32 bits, and the odd-numbered register containing
          the higher 32 bits).

          When this bit is _set_ (on supported hardware), the 32
          floating-point registers are 64 bits wide (so called **FR=1**
          or **FR1** mode).  Note that modern MIPS implementations (MIPS
          R6 and newer) support **FR=1** mode only.

          Applications that use the O32 FP32 ABI can operate only
          when this bit is _unset_ (**FR=0**; or they can be used with FRE
          enabled, see below).  Applications that use the O32 FP64
          ABI (and the O32 FP64A ABI, which exists to provide the
          ability to operate with existing FP32 code; see below) can
          operate only when this bit is _set_ (**FR=1**).  Applications
          that use the O32 FPXX ABI can operate with either **FR=0** or
          **FR=1**.

   **PR_FP_MODE_FRE**
          Enable emulation of 32-bit floating-point mode.  When this
          mode is enabled, it emulates 32-bit floating-point
          operations by raising a reserved-instruction exception on
          every instruction that uses 32-bit formats and the kernel
          then handles the instruction in software.  (The problem
          lies in the discrepancy of handling odd-numbered registers
          which are the high 32 bits of 64-bit registers with even
          numbers in **FR=0** mode and the lower 32-bit parts of odd-
          numbered 64-bit registers in **FR=1** mode.)  Enabling this bit
          is necessary when code with the O32 FP32 ABI should operate
          with code with compatible the O32 FPXX or O32 FP64A ABIs
          (which require **FR=1** FPU mode) or when it is executed on
          newer hardware (MIPS R6 onwards) which lacks **FR=0** mode
          support when a binary with the FP32 ABI is used.

          Note that this mode makes sense only when the FPU is in
          64-bit mode (**FR=1**).

          Note that the use of emulation inherently has a significant
          performance hit and should be avoided if possible.

   In the N32/N64 ABI, 64-bit floating-point mode is always used, so
   FPU emulation is not required and the FPU always operates in **FR=1**
   mode.

   This operation is mainly intended for use by the dynamic linker
   ([ld.so(8)](../man8/ld.so.8.html)).

RETURN VALUE top

   On success, 0 is returned.  On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is
   set to indicate the error.

ERRORS top

   **EOPNOTSUPP**
          _mode_ has an invalid or unsupported value.

STANDARDS top

   Linux.  MIPS only.

HISTORY top

   Linux 4.0 (MIPS).

SEE ALSO top

   [prctl(2)](../man2/prctl.2.html), [PR_GET_FP_MODE(2const)](../man2/PR%5FGET%5FFP%5FMODE.2const.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 PRSETFPMODE(2const)

Pages that refer to this page:prctl(2), PR_GET_FP_MODE(2const)