1.\" Copyright (c) 2004 David Schultz <das@FreeBSD.org> 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd March 16, 2005 28.Dt FENV 3 29.Os 30.Sh NAME 31.Nm feclearexcept , 32.Nm fegetexceptflag , 33.Nm feraiseexcept , 34.Nm fesetexceptflag , 35.Nm fetestexcept , 36.Nm fegetround , 37.Nm fesetround , 38.Nm fegetenv , 39.Nm feholdexcept , 40.Nm fesetenv , 41.Nm feupdateenv , 42.Nm feenableexcept , 43.Nm fedisableexcept , 44.Nm fegetexcept 45.Nd floating-point environment control 46.Sh LIBRARY 47.Lb libm 48.Sh SYNOPSIS 49.In fenv.h 50.Fd "#pragma STDC FENV_ACCESS ON" 51.Ft int 52.Fn feclearexcept "int excepts" 53.Ft int 54.Fn fegetexceptflag "fexcept_t *flagp" "int excepts" 55.Ft int 56.Fn feraiseexcept "int excepts" 57.Ft int 58.Fn fesetexceptflag "const fexcept_t *flagp" "int excepts" 59.Ft int 60.Fn fetestexcept "int excepts" 61.Ft int 62.Fn fegetround void 63.Ft int 64.Fn fesetround "int round" 65.Ft int 66.Fn fegetenv "fenv_t *envp" 67.Ft int 68.Fn feholdexcept "fenv_t *envp" 69.Ft int 70.Fn fesetenv "const fenv_t *envp" 71.Ft int 72.Fn feupdateenv "const fenv_t *envp" 73.Ft int 74.Fn feenableexcept "int excepts" 75.Ft int 76.Fn fedisableexcept "int excepts" 77.Ft int 78.Fn fegetexcept void 79.Sh DESCRIPTION 80The 81.In fenv.h 82routines manipulate the floating-point environment, 83which includes the exception flags and rounding modes defined in 84.St -ieee754 . 85.Ss Exceptions 86Exception flags are set as side-effects of floating-point arithmetic 87operations and math library routines, and they remain set until 88explicitly cleared. 89The following macros expand to bit flags of type 90.Vt int 91representing the five standard floating-point exceptions. 92.Bl -tag -width ".Dv FE_DIVBYZERO" 93.It Dv FE_DIVBYZERO 94A divide-by-zero exception occurs when the program attempts to 95divide a finite non-zero number by zero. 96.It Dv FE_INEXACT 97An inexact exception is raised whenever there is a loss of precision 98due to rounding. 99.It Dv FE_INVALID 100Invalid operation exceptions occur when a program attempts to 101perform calculations for which there is no reasonable representable 102answer. 103For instance, subtraction of infinities, division of zero by zero, 104ordered comparison involving \*(Nas, and taking the square root of a 105negative number are all invalid operations. 106.It Dv FE_OVERFLOW 107An overflow exception occurs when the magnitude of the result of a 108computation is too large to fit in the destination type. 109.It Dv FE_UNDERFLOW 110Underflow occurs when the result of a computation is too close to zero 111to be represented as a non-zero value in the destination type. 112.El 113.Pp 114Additionally, the 115.Dv FE_ALL_EXCEPT 116macro expands to the bitwise OR of the above flags and any 117architecture-specific flags. 118Combinations of these flags are passed to the 119.Fn feclearexcept , 120.Fn fegetexceptflag , 121.Fn feraiseexcept , 122.Fn fesetexceptflag , 123and 124.Fn fetestexcept 125functions to clear, save, raise, restore, and examine the 126processor's floating-point exception flags, respectively. 127.Pp 128Exceptions may be 129.Em unmasked 130with 131.Fn feenableexcept 132and masked with 133.Fn fedisableexcept . 134Unmasked exceptions cause a trap when they are produced, and 135all exceptions are masked by default. 136The current mask can be tested with 137.Fn fegetexcept . 138.Ss Rounding Modes 139.St -ieee754 140specifies four rounding modes. 141These modes control the direction in which results are rounded 142from their exact values in order to fit them into binary 143floating-point variables. 144The four modes correspond with the following symbolic constants. 145.Bl -tag -width ".Dv FE_TOWARDZERO" 146.It Dv FE_TONEAREST 147Results are rounded to the closest representable value. 148If the exact result is exactly half way between two representable 149values, the value whose last binary digit is even (zero) is chosen. 150This is the default mode. 151.It Dv FE_DOWNWARD 152Results are rounded towards negative \*[If]. 153.It Dv FE_UPWARD 154Results are rounded towards positive \*[If]. 155.It Dv FE_TOWARDZERO 156Results are rounded towards zero. 157.El 158.Pp 159The 160.Fn fegetround 161and 162.Fn fesetround 163functions query and set the rounding mode. 164.Ss Environment Control 165The 166.Fn fegetenv 167and 168.Fn fesetenv 169functions save and restore the floating-point environment, 170which includes exception flags, the current exception mask, 171the rounding mode, and possibly other implementation-specific 172state. 173The 174.Fn feholdexcept 175function behaves like 176.Fn fegetenv , 177but with the additional effect of clearing the exception flags and 178installing a 179.Em non-stop 180mode. 181In non-stop mode, floating-point operations will set exception flags 182as usual, but no 183.Dv SIGFPE 184signals will be generated as a result. 185Non-stop mode is the default, but it may be altered by 186non-standard mechanisms. 187.\" XXX Mention fe[gs]etmask() here after the interface is finalized 188.\" XXX and ready to be officially documented. 189The 190.Fn feupdateenv 191function restores a saved environment similarly to 192.Fn fesetenv , 193but it also re-raises any floating-point exceptions from the old 194environment. 195.Pp 196The macro 197.Dv FE_DFL_ENV 198expands to a pointer to the default environment. 199.Sh EXAMPLES 200The following routine computes the square root function. 201It explicitly raises an invalid exception on appropriate inputs using 202.Fn feraiseexcept . 203It also defers inexact exceptions while it computes intermediate 204values, and then it allows an inexact exception to be raised only if 205the final answer is inexact. 206.Bd -literal -offset indent 207#pragma STDC FENV_ACCESS ON 208double sqrt(double n) { 209 double x = 1.0; 210 fenv_t env; 211 212 if (isnan(n) || n < 0.0) { 213 feraiseexcept(FE_INVALID); 214 return (NAN); 215 } 216 if (isinf(n) || n == 0.0) 217 return (n); 218 feholdexcept(&env); 219 while (fabs((x * x) - n) > DBL_EPSILON * 2 * x) 220 x = (x / 2) + (n / (2 * x)); 221 if (x * x == n) 222 feclearexcept(FE_INEXACT); 223 feupdateenv(&env); 224 return (x); 225} 226.Ed 227.Sh SEE ALSO 228.Xr cc 1 , 229.Xr feclearexcept 3 , 230.Xr fedisableexcept 3 , 231.Xr feenableexcept 3 , 232.Xr fegetenv 3 , 233.Xr fegetexcept 3 , 234.Xr fegetexceptflag 3 , 235.Xr fegetround 3 , 236.Xr feholdexcept 3 , 237.Xr feraiseexcept 3 , 238.Xr fesetenv 3 , 239.Xr fesetexceptflag 3 , 240.Xr fesetround 3 , 241.Xr fetestexcept 3 , 242.Xr feupdateenv 3 , 243.Xr fpgetprec 3 , 244.Xr fpsetprec 3 245.Sh STANDARDS 246Except as noted below, 247.In fenv.h 248conforms to 249.St -isoC-99 . 250The 251.Fn feenableexcept , 252.Fn fedisableexcept , 253and 254.Fn fegetexcept 255routines are extensions. 256.Sh HISTORY 257The 258.In fenv.h 259header first appeared in 260.Fx 5.3 . 261It supersedes the non-standard routines defined in 262.In ieeefp.h 263and documented in 264.Xr fpgetround 3 . 265.Sh CAVEATS 266The FENV_ACCESS pragma can be enabled with 267.Dl "#pragma STDC FENV_ACCESS ON" 268and disabled with the 269.Dl "#pragma STDC FENV_ACCESS OFF" 270directive. 271This lexically-scoped annotation tells the compiler that the program 272may access the floating-point environment, so optimizations that would 273violate strict IEEE-754 semantics are disabled. 274If execution reaches a block of code for which 275.Dv FENV_ACCESS 276is off, the floating-point environment will become undefined. 277.Sh BUGS 278The 279.Dv FENV_ACCESS 280pragma is unimplemented in the system compiler. 281However, non-constant expressions generally produce the correct 282side-effects at low optimization levels. 283