Is sizeof() legal within a #if preprocessor line?
Specifically, I would like to use the following "sanity check" at compile
time, but I'm getting an error on the compile about the #if line.
struct foo {
int a;
int b;
};
#if sizeof(struct foo) != 8
#pragma error "sizeof foo is not 8!"
#endif
(Assuming the "#pragma error" generates a compile-time error.)
Yes, I know all about the dangers and non-portability of things like this,
but it's in a low-level TCP/IP communications module, and I'd just like to
add a sanity check at compile time that I'm not on a platform where the
structs don't match the communications protocol.
--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer .h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com> 18  21817 
On Fri, 14 Jan 2005 12:57:56 -0500, Kenneth Brody wrote: Is sizeof() legal within a #if preprocessor line?
No.
Specifically, I would like to use the following "sanity check" at compile
time, but I'm getting an error on the compile about the #if line.
struct foo {
int a;
int b;
};
#if sizeof(struct foo) != 8
Another problem would be that struct foo doesn't exist during
preprocessing phases.
#pragma error "sizeof foo is not 8!"
#endif
(Assuming the "#pragma error" generates a compile-time error.)
Yes, I know all about the dangers and non-portability of things like
this, but it's in a low-level TCP/IP communications module, and I'd just
like to add a sanity check at compile time that I'm not on a platform
where the structs don't match the communications protocol.
If you're expecting representation to match there are more issues than
just size.
Lawrence
In article <41************ ***@spamcop.net >,
Kenneth Brody <ke******@spamc op.net> wrote: Is sizeof() legal within a #if preprocessor line?
No. The preprocessor doesn't know about such things.
I'd just like to
add a sanity check at compile time that I'm not on a platform where the
structs don't match the communications protocol.
There are various constructs that you could use to produce
compile-time errors if the size is wrong. For example:
char zzz[1 - 2*(sizeof(struc t foo) != 8)];
But the error message will not be very enlightening.
-- Richard
Richard Tobin <ri*****@cogsci .ed.ac.uk> wrote: char zzz[1 - 2*(sizeof(struc t foo) != 8)];
A very cute idea! I've stolen it, thank you!
#define CCASSERT(predic ate) _x_CCASSERT_LIN E(predicate, __LINE__)
[...]
#define _x_CCASSERT_LIN E_CAT(predicate , line) \
typedef char constraint_viol ated_on_line_## line[2*((predicate)! =0)-1];
Due to typedef declaration no object is created, and the declaration
requires `predicate' to be a constant (scalar) expression. Another
equivalent possibility would be to replace "typedef" with "extern".
But the error message will not be very enlightening.
The message from gcc is enough informative:
constr.c:30: size of array `constraint_vio lated_on_line_3 0' is negative
--
Stan Tobias
mailx `echo si***@FamOuS.Be dBuG.pAlS.INVALID | sed s/[[:upper:]]//g`
"S.Tobias" wrote:
Richard Tobin <ri*****@cogsci .ed.ac.uk> wrote:
char zzz[1 - 2*(sizeof(struc t foo) != 8)];
A very cute idea! I've stolen it, thank you!
ITYM "snarf". ;-)
#define CCASSERT(predic ate) _x_CCASSERT_LIN E(predicate, __LINE__)
[...]
#define _x_CCASSERT_LIN E_CAT(predicate , line) \
typedef char constraint_viol ated_on_line_## line[2*((predicate)! =0)-1];
Due to typedef declaration no object is created, and the declaration
requires `predicate' to be a constant (scalar) expression. Another
equivalent possibility would be to replace "typedef" with "extern".
But the error message will not be very enlightening.
The message from gcc is enough informative:
constr.c:30: size of array `constraint_vio lated_on_line_3 0' is negative
Nice enhancement.
However, why did you switch "1 - 2*x" to "2*x - 1"?
--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer .h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>
Lawrence Kirby wrote:
On Fri, 14 Jan 2005 12:57:56 -0500, Kenneth Brody wrote:
Is sizeof() legal within a #if preprocessor line?
No.
That's the conclusion I was coming to. Thanks.
[...] Yes, I know all about the dangers and non-portability of things like
this, but it's in a low-level TCP/IP communications module, and I'd just
like to add a sanity check at compile time that I'm not on a platform
where the structs don't match the communications protocol.
If you're expecting representation to match there are more issues than
just size.
True. But, as I said, this was just a "sanity check", and "the odds
are" that if the size of the struct matches, the component pieces of
the struct align as expected. Obviously, further testing on a new
platform would be necessary to assure that this was the case.
--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer .h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>
Kenneth Brody wrote: Is sizeof() legal within a #if preprocessor line?
Specifically, I would like to use the following "sanity check" at compile
time, but I'm getting an error on the compile about the #if line.
struct foo {
int a;
int b;
};
#if sizeof(struct foo) != 8
#pragma error "sizeof foo is not 8!"
#endif
(Assuming the "#pragma error" generates a compile-time error.)
Yes, I know all about the dangers and non-portability of things like this,
but it's in a low-level TCP/IP communications module, and I'd just like to
add a sanity check at compile time that I'm not on a platform where the
structs don't match the communications protocol.
--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer .h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>
limits.h has the size of all the variables. You should be able to use something
in there to get the size of int.
Neil Kurzman <ns*@mail.asb.c om> writes: Kenneth Brody wrote: Is sizeof() legal within a #if preprocessor line?
Specifically, I would like to use the following "sanity check" at compile
time, but I'm getting an error on the compile about the #if line.
struct foo {
int a;
int b;
};
#if sizeof(struct foo) != 8
#pragma error "sizeof foo is not 8!"
#endif
(Assuming the "#pragma error" generates a compile-time error.)
[...]
limits.h has the size of all the variables. You should be able to
use something in there to get the size of int.
<limits.h> has the bounds of the predefined integer types, not their
sizes. If there are padding bits, you can't reliably determine their
sizes from their bounds.
Also, that doesn't necessarily give you sizeof(struct foo). The
compiler could insert padding between a and b or after b. That's not
likely for a struct with just two int members (sizeof(struct foo) is
almost certainly equal to 2*sizeof(int)), but presumably the OP wants
to do this for more elaborate types.
Note that you can use assert() to force a run-time error. If the
program aborts immediately, that's almost as good as bombing at
compile time.
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Kenneth Brody <ke******@spamc op.net> writes:
[...] #if sizeof(struct foo) != 8
#pragma error "sizeof foo is not 8!"
#endif
(Assuming the "#pragma error" generates a compile-time error.)
Just use "#error" rather than "#pragma error".
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
Kenneth Brody wrote on 14/01/05 : Is sizeof() legal within a #if preprocessor line?
Not portably. A question of stage of translation. At this stage
(preprocessor), the standard says that sizeof is not yet evaluated and
returns 0.
--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html
"Clearly your code does not meet the original spec."
"You are sentenced to 30 lashes with a wet noodle."
-- Jerry Coffin in a.l.c.c++ This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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last post by:
Dear All,
A class having no member variables and only a method sizeof(object)
will return 1byte in ANSI and two bytes in Unicode.
I have the answer for this of how in works in ANSI. But I don't
know it returns two bytes in UniCode.
Please help...
For ANSI:
In ISO/ANSI C++ Standard, 5.3.3 § 1, it stays: "The sizeof operator
yields the number of bytes in the object representation of its
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Hi, all.
What will we get from sizeof(a class without data members and virtual
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For example:
class abnormity {
public:
string name() { return "abnormity"; }
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Hi,
I have a funtion which shall compute the amount for a later malloc. In
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instance or pointer of the struct.
As "sizeof(int)" is legal I assumed "sizeof(struct x.y)" to be legal
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#include <dirent.h>
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This doesn't work with any C compiler that I can find. They all report a
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printf ("%d\n", (int)sizeof (char)(char)2);
Now the question is "why?" "sizeof" and "(char)" have identical precedence
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#include<stdio.h>
int main(void)
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unsigned int a=20,b=50, c = sizeof b+a;
printf("%d\n",c);
return 0;
}
out put:
24
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Some people say sizeof(type) and other say sizeof(variable).
Why?
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Consider the following piece of code:
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int main (void)
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int i, j=1;
char c;
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Hi,
How can we write a function, which functionality is similar to sizeof
function
any one send me source code
Reddy
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Hi,
Somebody recently asked me to implement the sizeof operator, i.e. to
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Abhishek Srivastava
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Does this reliably cause a compile-time error
when int is not 4 bytes ?
enum { int_size_checked = 1/(sizeof(int)==4) };
Any better way to check the value of an expression
involving sizeof before runtime ? I also have:
{
void check_foo_size(void);
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