On Sun, Jul 13, 2008 at 06:24:09PM +0200, Michael Kerrisk wrote: > Andries (and others), > > I'd appreciate comments on the following, especially the FIXMES. > > Cheers, > > Michael Hi Michael, The includes <math.h>, <errno.h>, <fenv.h> are not mentioned. The dependence on C99 is not mentioned. > The common convention of returning \-1 on error does not carry over well Often the special return value is NaN, and one can forget about all errno and fetestexcept nonsense. > The > .I math_errhandling > identifier defined by POSIX.1 is not supported. I think math_errhandling is part of C99. > A program that needs to check for an error from a mathematical > function should set errno to zero, and make the following call > feclearexcept(FE_ALL_EXCEPT); > before calling a mathematical function. > Upon return from the mathematical function, if errno > is non-zero, or the following call returns non-zero > fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | > FE_UNDERFLOW); > then an error occurred in the mathematical function. > .\" FIXME My understanding is that you must check *both* errno and > .\" fetestexcep() and an error has occurred if *either* of them > .\" is non-zero. SUSv3 seems a little ambiguous on this point. > .\" I'd appreciate confirmation/contradiction that my understanding > .\" is correct. I think math_errhandling can have the MATH_ERRNO bit set to indicate that errors are signalled via errno, and it can have the MATH_ERREXCEPT bit set to indicate that errors are signalled via floating-point exceptions, but it can also be 0 and then neither errno nor fetestexcept are required to give information. More precisely, the above is required for the float/double functions, but for complex-valued functions setting of errno is never required. (And in practice clog will not give any error for the call clog(-0.0).) Since glibc/gcc do not support math_errhandling, the details of these requirements are not relevant for Linux. gcc has options -ffast-math, -fno-math-errno that influence the error settings. The standard does not follow the SUSv3 math_err setup. In reality it is often advised to check for bad parameters before calling the math functions, avoiding all this nonsense. See, e.g., https://www.securecoding.cert.org/confluence/display/seccode/FLP32-C.+Prevent+or+detect+domain+and+range+errors+in+math+functions > e.g., giving a negative argument to a logarithm function But z = clog(-1.0) is quite ok and returns pi (so instead of "a logarithm function you want perhaps "the function log"). > The error conditions that can occur for mathematical functions > are described below. The text seems taken literally from the standard. Copyright? Andries -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
