Dear Experts,
I need a function that takes a float, swaps its endianness (htonl) in
place, and returns a char* pointer to its first byte. This is one of a
family of functions that prepare different data types for passing to
another process.
I have got confused by the rules about what won't work, what will work,
and what might work, when casting. Specifically, I have an
implementation that works until I remove my debugging, at which point
the compiler seems to decide that it can optimise away the writes to the
bytes other than the first, or something like that. Here it is:
template <typename T>
inline const char* encode_arg(T& t); // linker error if you try to
// encode a type for which there
// is no implementation
// This one works:
template <>
inline const char* encode_pq_arg<i nt>(int& i) {
i = htonl(i);
return reinterpret_cas t<const char*>(&i);
}
// This one doesn't:
template <>
inline const char* encode_arg<floa t>(float& f) {
uint32_t* ptr = reinterpret_cas t<uint32_t*>(&f );
*ptr = htonl(*ptr);
const char* cptr = reinterpret_cas t<const char*>(ptr);
return cptr;
}
When I dump cptr[0] to cptr[3] before the return, it works. Without the
debug, it fails; it's obviously hard to see what ends up in the result
in that case, but it looks undefined.
So, is there some tweak that will make this work, and be certain to
work? Am I better off using a union, or is that even less defined?
Does anyone know what the rules actually are?
Many thanks,
Phil.
12  2135 
Phil Endecott wrote:
Dear Experts,
I need a function that takes a float, swaps its endianness (htonl) in
place, and returns a char* pointer to its first byte. This is one of a
family of functions that prepare different data types for passing to
another process.
I have got confused by the rules about what won't work, what will work,
and what might work, when casting. Specifically, I have an
implementation that works until I remove my debugging, at which point
the compiler seems to decide that it can optimise away the writes to the
bytes other than the first, or something like that. Here it is:
template <typename T>
inline const char* encode_arg(T& t); // linker error if you try to
// encode a type for which there
// is no implementation
// This one works:
template <>
inline const char* encode_pq_arg<i nt>(int& i) {
Typo? ^^
i = htonl(i);
return reinterpret_cas t<const char*>(&i);
}
Did you intend the bytes of the passed value to be swapped?
// This one doesn't:
template <>
inline const char* encode_arg<floa t>(float& f) {
uint32_t* ptr = reinterpret_cas t<uint32_t*>(&f );
*ptr = htonl(*ptr);
const char* cptr = reinterpret_cas t<const char*>(ptr);
return cptr;
}
When I dump cptr[0] to cptr[3] before the return, it works. Without the
debug, it fails; it's obviously hard to see what ends up in the result
in that case, but it looks undefined.
What doesn't work?
--
Ian Collins.
Ian Collins wrote:
Phil Endecott wrote:
>Dear Experts,
I need a function that takes a float, swaps its endianness (htonl) in
place, and returns a char* pointer to its first byte. This is one of a
family of functions that prepare different data types for passing to
another process.
I have got confused by the rules about what won't work, what will work,
and what might work, when casting. Specifically, I have an
implementati on that works until I remove my debugging, at which point
the compiler seems to decide that it can optimise away the writes to the
bytes other than the first, or something like that. Here it is:
template <typename T>
inline const char* encode_arg(T& t); // linker error if you try to
// encode a type for which there
// is no implementation
// This one works:
template <>
inline const char* encode_pq_arg<i nt>(int& i) {
Typo? ^^
Yes, copy&pastism, sorry!
> i = htonl(i);
return reinterpret_cas t<const char*>(&i);
}
Did you intend the bytes of the passed value to be swapped?
Yes; the caller doesn't need the value any more, so I swap it in-place
rather than making a copy. If I made a copy then I would have to
allocate memory for it and free it later.
>// This one doesn't:
template <>
inline const char* encode_arg<floa t>(float& f) {
uint32_t* ptr = reinterpret_cas t<uint32_t*>(&f );
*ptr = htonl(*ptr);
const char* cptr = reinterpret_cas t<const char*>(ptr);
return cptr;
}
When I dump cptr[0] to cptr[3] before the return, it works. Without the
debug, it fails; it's obviously hard to see what ends up in the result
in that case, but it looks undefined.
What doesn't work?
The four bytes pointed to by the const char* that this function returns
eventually get sent to a socket, and I can observe them at the other
end. They are wrong, i.e. if I pass 2.0 then the value at the other end
might be 8.923461290e-44. There is some non-determinism in the values I
see, which makes me think that I'm looking at uninitialised memory
rather than a systematic corruption. My first thought was to add
somethng like this:
std::cout << "cptr[0] = " << static_cast<int >(cptr[0]) << ....etc for
[1] to [3] ... << "\n";
However, as soon as I add this (just before the return statement), it
works: the correct value is seen in the other process. My feeling is
that the cptr[n] expressions in the debugging tell the compiler that
these bytes are needed; when the debuging is not there, it thinks that
they are not used, and optimises them away. Can you think of any other
explanation?
Regards,
Phil.
Phil Endecott wrote:
[..]
The four bytes pointed to by the const char* that this function
returns eventually get sent to a socket, and I can observe them at
the other end. They are wrong, i.e. if I pass 2.0 then the value at
the other end might be 8.923461290e-44. There is some
non-determinism in the values I see, which makes me think that I'm
looking at uninitialised memory rather than a systematic corruption. My
first thought was to add somethng like this:
std::cout << "cptr[0] = " << static_cast<int >(cptr[0]) << ....etc for
[1] to [3] ... << "\n";
However, as soon as I add this (just before the return statement), it
works: the correct value is seen in the other process. My feeling is
that the cptr[n] expressions in the debugging tell the compiler that
these bytes are needed; when the debuging is not there, it thinks that
they are not used, and optimises them away. Can you think of any
other explanation?
Compilers are written by humans. Errare humanum est. Hence all
compilers have bugs, known and unknown. If you want to know whether
the bug you're encountering is known, contact the compiler writers.
If you just want a work-around, disable optimization for the small
module in which this function (these functions) is (are), and see if
it makes any difference. I've seen a significant improvement from
seemingly random behaviour with the HP and Sun compilers before, and
even <gasp!with Microsoft's VC++ compiler, if optimizations are
disabled locally.
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
On Sep 15, 5:26 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
Phil Endecott wrote:
[..]
The four bytes pointed to by the const char* that this function
returns eventually get sent to a socket, and I can observe them at
the other end. They are wrong, i.e. if I pass 2.0 then the value at
the other end might be 8.923461290e-44. There is some
non-determinism in the values I see, which makes me think that I'm
looking at uninitialised memory rather than a systematic corruption. My
first thought was to add somethng like this:
std::cout << "cptr[0] = " << static_cast<int >(cptr[0]) << ....etc for
[1] to [3] ... << "\n";
However, as soon as I add this (just before the return statement), it
works: the correct value is seen in the other process. My feeling is
that the cptr[n] expressions in the debugging tell the compiler that
these bytes are needed; when the debuging is not there, it thinks that
they are not used, and optimises them away. Can you think of any
other explanation?
Compilers are written by humans. Errare humanum est. Hence all
compilers have bugs, known and unknown. If you want to know whether
the bug you're encountering is known, contact the compiler writers.
If you just want a work-around, disable optimization for the small
module in which this function (these functions) is (are), and see if
it makes any difference. I've seen a significant improvement from
seemingly random behaviour with the HP and Sun compilers before, and
even <gasp!with Microsoft's VC++ compiler, if optimizations are
disabled locally.
It looks like an auto optimization error.Maybe - as a result of
pointer algebra on small argument type - the enregisterd parameter is
copied to stack and operated upon ,and filled with rubish at
return(you are returning a pointer to the automatic variable).
can you write this:
union exg
{
float f;
char cl[sizeof(float)];
exg& IndianSwap();
};
exg e={value};
e.IndianSwap();
and leave the optimization to the compiler(I think modern compilers
can do fine on this special case)?
regards,
FM.
terminator wrote:
[..]
can you write this:
union exg
{
float f;
char cl[sizeof(float)];
exg& IndianSwap();
};
exg e={value};
e.IndianSwap();
and leave the optimization to the compiler(I think modern compilers
can do fine on this special case)?
Initialising (or assigning to) one part of a union and using another
has undefined behaviour. Or at least it used to. So, I'd stay away
from such code, it is definitely non-portable, although probably just
as reinterpret_cas t to an array of chars.
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
On Sep 15, 7:46 pm, terminator <farid.mehr...@ gmail.comwrote:
On Sep 15, 5:26 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
Phil Endecott wrote:
[..]
The four bytes pointed to by the const char* that this function
returns eventually get sent to a socket, and I can observe them at
the other end. They are wrong, i.e. if I pass 2.0 then the value at
the other end might be 8.923461290e-44. There is some
non-determinism in the values I see, which makes me think that I'm
looking at uninitialised memory rather than a systematic corruption. My
first thought was to add somethng like this:
std::cout << "cptr[0] = " << static_cast<int >(cptr[0]) << ....etc for
[1] to [3] ... << "\n";
However, as soon as I add this (just before the return statement), it
works: the correct value is seen in the other process. My feeling is
that the cptr[n] expressions in the debugging tell the compiler that
these bytes are needed; when the debuging is not there, it thinks that
they are not used, and optimises them away. Can you think of any
other explanation?
Compilers are written by humans. Errare humanum est. Hence all
compilers have bugs, known and unknown. If you want to know whether
the bug you're encountering is known, contact the compiler writers.
If you just want a work-around, disable optimization for the small
module in which this function (these functions) is (are), and see if
it makes any difference. I've seen a significant improvement from
seemingly random behaviour with the HP and Sun compilers before, and
even <gasp!with Microsoft's VC++ compiler, if optimizations are
disabled locally.
It looks like an auto optimization error.Maybe - as a result of
pointer algebra on small argument type - the enregisterd parameter is
copied to stack and operated upon ,and filled with rubish at
return(you are returning a pointer to the automatic variable).
It's not a bug. The compiler is allowed to assume that a
variable is only modified/read through expressions of its type,
or through expressions of char or unsigned char type. The fact
that something is modified through an uint32_t* is irrelevant
when the compiler is optimizing accesses to a float, for
example.
can you write this:
union exg
{
float f;
char cl[sizeof(float)];
exg& IndianSwap();
};
exg e={value};
e.IndianSwap();
and leave the optimization to the compiler(I think modern
compilers can do fine on this special case)?
If "IndianSwap " accesses cl, then it's undefined behavior.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Sep 15, 8:23 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
terminator wrote:
[..]
can you write this:
union exg
{
float f;
char cl[sizeof(float)];
exg& IndianSwap();
};
exg e={value};
e.IndianSwap();
and leave the optimization to the compiler(I think modern compilers
can do fine on this special case)?
Initialising (or assigning to) one part of a union and using another
has undefined behaviour. Or at least it used to.
It still does, although some compilers may guarantee this as an
extension. (I think g++ does.)
So, I'd stay away
from such code, it is definitely non-portable, although probably just
as reinterpret_cas t to an array of chars.
reinterpret_cas t to an array of char's is probably OK. The
problem in the original code was assigning through the
uint32_t*; that's undefined behavior if, as was the case, the
pointer was the result of a reinterpret_cas t from a float*.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
On Sep 14, 12:02 pm, Phil Endecott
<spam_from_usen et_0...@chezphi l.orgwrote:
Dear Experts,
I need a function that takes a float, swaps its endianness (htonl) in
place, and returns a char* pointer to its first byte. This is one of a
family of functions that prepare different data types for passing to
another process.
I have got confused by the rules about what won't work, what will work,
and what might work, when casting. Specifically, I have an
implementation that works until I remove my debugging, at which point
the compiler seems to decide that it can optimise away the writes to the
bytes other than the first, or something like that. Here it is:
template <typename T>
inline const char* encode_arg(T& t); // linker error if you try to
// encode a type for which there
// is no implementation
// This one works:
template <>
inline const char* encode_pq_arg<i nt>(int& i) {
i = htonl(i);
return reinterpret_cas t<const char*>(&i);
}
// This one doesn't:
template <>
inline const char* encode_arg<floa t>(float& f) {
uint32_t* ptr = reinterpret_cas t<uint32_t*>(&f );
*ptr = htonl(*ptr);
const char* cptr = reinterpret_cas t<const char*>(ptr);
return cptr;
}
And it shouldn't. The encode_arg() function is effectively returning a
pointer to a local variable (the parameter f). So the caller of
encode_arg() receives a pointer to an object that no longer exists;
and therefore the value of the bytes obtained by dereferencing the
returned pointer - could be anything.
Greg
Greg Herlihy wrote:
On Sep 14, 12:02 pm, Phil Endecott
<spam_from_usen et_0...@chezphi l.orgwrote:
>Dear Experts,
I need a function that takes a float, swaps its endianness (htonl) in
place, and returns a char* pointer to its first byte. This is one
of a family of functions that prepare different data types for
passing to another process.
I have got confused by the rules about what won't work, what will
work, and what might work, when casting. Specifically, I have an
implementati on that works until I remove my debugging, at which point
the compiler seems to decide that it can optimise away the writes to
the bytes other than the first, or something like that. Here it is:
template <typename T>
inline const char* encode_arg(T& t); // linker error if you try to
// encode a type for which
there // is no implementation
// This one works:
template <>
inline const char* encode_pq_arg<i nt>(int& i) {
i = htonl(i);
return reinterpret_cas t<const char*>(&i);
}
// This one doesn't:
template <>
inline const char* encode_arg<floa t>(float& f) {
uint32_t* ptr = reinterpret_cas t<uint32_t*>(&f );
*ptr = htonl(*ptr);
const char* cptr = reinterpret_cas t<const char*>(ptr);
return cptr;
}
And it shouldn't. The encode_arg() function is effectively returning a
pointer to a local variable (the parameter f).
A pointer to the argument 'f'? You mean the address of it? The arg
is a reference. The address of a reference is the address of the
referenced object. Nothing is destroyed. Please revise your analysis.
So the caller of
encode_arg() receives a pointer to an object that no longer exists;
and therefore the value of the bytes obtained by dereferencing the
returned pointer - could be anything.
Greg
V
--
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