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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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PR_SET_FP_MODE(2const) PR_SET_FP_MODE(2const)
PR_SET_FP_MODE - set the floating point mode of the calling
process
Standard C library (libc, -lc)
#include <linux/prctl.h> /* Definition of PR_* constants */
#include <sys/prctl.h>
int prctl(PR_SET_FP_MODE, unsigned long mode);
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)).
On success, 0 is returned. On error, -1 is returned, and errno
is set to indicate the error.
EOPNOTSUPP
mode has an invalid or unsupported value.
Linux. MIPS only.
Linux 4.0 (MIPS).
prctl(2), PR_GET_FP_MODE(2const)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-02 PR_SET_FP_MODE(2const)
Pages that refer to this page: prctl(2), PR_GET_FP_MODE(2const)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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