Confused by compiler warning

[James H. Newman]

I am playing with some code that has been automatically generated
from ASN.1 data specification found in RFC 3280. One of the structures
generated reads as follows:

typedef struct TBSCertList {
Version_t *version /* OPTIONAL */;
AlgorithmIdenti fier_t signature;
Name_t issuer;
Time_t thisUpdate;
struct Time *nextUpdate /* OPTIONAL */;
struct revokedCertific ates {
A_SEQUENCE_OF(s truct Member {
CertificateSeri alNumber_t userCertificate ;
Time_t revocationDate;
struct Extensions *crlEntryExtens ions /
* OPTIONAL */;

/* Context for parsing across buffer boundaries */
asn_struct_ctx_ t _asn_ctx;
} ) list;

/* Context for parsing across buffer boundaries */
asn_struct_ctx_ t _asn_ctx;
} *revokedCertifi cates;
struct Extensions *crlExtensions /* OPTIONAL */;

/* Context for parsing across buffer boundaries */
asn_struct_ctx_ t _asn_ctx;
} TBSCertList_t;

/* Implementation */
extern asn_TYPE_descri ptor_t asn_DEF_TBSCert List;
}

where A_SEQUENCE_OF() is defined as

#define A_SEQUENCE_OF(t ype) \
struct { \
type **array; \
int count; /* Meaningful size */ \
int size; /* Allocated size */ \
void (*free)(type *); \
}

The preprocessor expands this as

typedef struct TBSCertList {
Version_t *version;
AlgorithmIdenti fier_t signature;
Name_t issuer;
Time_t thisUpdate;
struct Time *nextUpdate;
struct revokedCertific ates {
struct {
struct Member {
CertificateSeri alNumber_t userCertificate ;
Time_t revocationDate;
struct Extensions *crlEntryExtens ions;
asn_struct_ctx_ t _asn_ctx;
} **array;
int count;
int size;
void (*free) (struct Member {
CertificateSeri alNumber_t userCertificate ;
Time_t revocationDate;
struct Extensions * crlEntryExtensi ons;
asn_struct_ctx_ t _asn_ctx;
} *);
} list;

asn_struct_ctx_ t _asn_ctx;
} *revokedCertifi cates;
struct Extensions *crlExtensions;

asn_struct_ctx_ t _asn_ctx;
} TBSCertList_t;

This compiles all right, but the GCC compiler generates the
following warning:

TBSCertList.h:4 7: warning: structure defined inside parms
TBSCertList.h:4 7: warning: `struct Member' declared inside parameter list
TBSCertList.h:4 7: warning: its scope is only this definition or
declaration, which is probably not what you want

Line 47 is the line that reads

} ) list;

in the code above, before the preprocessing.

Now I want to believe that this code is generated the way it is
generated for some good reason. What I do not understand is what the
compiler is complaining about. Anybody care to explain? What is it about
struct Member that the compiler reckons that that's not what we want?

[Peter Nilsson]

"James H. Newman" <NewJa...@exici te.comwrote:
<snip>

* * * * This compiles all right, but the GCC compiler
generates the following warning:

TBSCertList.h:4 7: warning: structure defined inside parms
TBSCertList.h:4 7: warning: `struct Member' declared inside parameter list
TBSCertList.h:4 7: warning: its scope is only this definition or
declaration, which is probably not what you want

This is Question 11.5 in the FAQ...

http://c-faq.com/ansi/structinproto.html

--
Peter

[Army1987]

Peter Nilsson wrote:

"James H. Newman" <NewJa...@exici te.comwrote:
<snip>

>Â* Â* Â* Â* This compiles all right, but the GCC compiler
generates the following warning:

TBSCertList.h: 47: warning: structure defined inside parms
TBSCertList.h: 47: warning: `struct Member' declared inside parameter list
TBSCertList.h: 47: warning: its scope is only this definition or
declaration, which is probably not what you want

This is Question 11.5 in the FAQ...

http://c-faq.com/ansi/structinproto.html

That is about function parameters, not macro parameters.
--
Army1987 (Replace "NOSPAM" with "email")

[Kenneth Brody]

"James H. Newman" wrote:

>
I am playing with some code that has been automatically generated
from ASN.1 data specification found in RFC 3280. One of the structures
generated reads as follows:

[...snip...]

The preprocessor expands this as

typedef struct TBSCertList {

[...]

struct revokedCertific ates {
struct {
struct Member {
CertificateSeri alNumber_t userCertificate ;
Time_t revocationDate;
struct Extensions *crlEntryExtens ions;
asn_struct_ctx_ t _asn_ctx;
} **array;
int count;
int size;
void (*free) (struct Member {
CertificateSeri alNumber_t userCertificate ;
Time_t revocationDate;
struct Extensions * crlEntryExtensi ons;
asn_struct_ctx_ t _asn_ctx;
} *);
} list;

[...]

} TBSCertList_t;

This compiles all right, but the GCC compiler generates the
following warning:

TBSCertList.h:4 7: warning: structure defined inside parms
TBSCertList.h:4 7: warning: `struct Member' declared inside parameter list
TBSCertList.h:4 7: warning: its scope is only this definition or
declaration, which is probably not what you want

Line 47 is the line that reads

} ) list;

in the code above, before the preprocessing.

[...]

Look at this part in the middle of the macro expansion:

void (*free) (struct Member {
CertificateSeri alNumber_t userCertificate ;
Time_t revocationDate;
struct Extensions * crlEntryExtensi ons;
asn_struct_ctx_ t _asn_ctx;
} *);

You are declaring a member of the struct called "free", which is
a function pointer, and you are prototyping that function. Within
that prototype, you are declaring a struct called "Member". This
"struct Member" is not the same "struct Member" that you define a
few lines earlier, because it is an actual definition, not just a
reference. (Yes, it is the same name and an identical layout, but
that doesn't matter as far as the warning is concerned.)

Consider this valid, though probably useless, snippet:

struct foo { int a; int b; };
extern void func1( struct foo *foopt );
extern void func2( struct foo { float a; float b; } *foopt );

Just as the third warning states:

its scope is only this definition or declaration, which is
probably not what you want

Change it so that the expansion is:

void (*free)(struct Member *);

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>

[Keith Thompson]

Army1987 <ar******@NOSPA M.itwrites:

Peter Nilsson wrote:

>"James H. Newman" <NewJa...@exici te.comwrote:
<snip>

>>Â* Â* Â* Â* This compiles all right, but the GCC compiler
generates the following warning:

TBSCertList.h :47: warning: structure defined inside parms
TBSCertList.h :47: warning: `struct Member' declared inside parameter list
TBSCertList.h :47: warning: its scope is only this definition or
declaration , which is probably not what you want

This is Question 11.5 in the FAQ...

http://c-faq.com/ansi/structinproto.html

That is about function parameters, not macro parameters.

The warning messages are about function parameters, not macro
parameters. The function parameter declaration happens to be within a
macro expansion, but that's irrelevant. (By the time the compiler
gets to the phase where it produces the warning, all macros have been
processed and are no longer visible.)

--
Keith Thompson (The_Other_Keit h) <ks***@mib.or g>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Army1987]

Keith Thompson wrote:

Army1987 <ar******@NOSPA M.itwrites:

>That is about function parameters, not macro parameters.

The warning messages are about function parameters, not macro
parameters. The function parameter declaration happens to be within a
macro expansion, but that's irrelevant. (By the time the compiler
gets to the phase where it produces the warning, all macros have been
processed and are no longer visible.)

Oh, I had missed the declaration of free.

--
Army1987 (Replace "NOSPAM" with "email")

[Stephen Sprunk]

"Jack Klein" <ja*******@spam cop.netwrote in message
news:o4******** *************** *********@4ax.c om...

[snip: excellent explanation of what the warning means]

If somebody changes the definition of the struct type in one place or
the other, but not all places, then suddenly you have problems,
undefined behavior, "mysterious " defects, etc.

It would be much better to put the definition of struct Member in a
header, and include that header everywhere it is used.

While I'd agree with all of the above if the code were maintained by a
human, it should be pointed out that the OP stated this code is
automatically generated. As long as one trusts that tool to keep the
definitions consistent (and in this example, it obviously does so), the code
is safe and the warning can be ignored or disabled. If the tool breaks,
you'll likely have far, far bigger problems than a simple compiler warning.

I'd, however, look into whether it's possible to modify the tool to generate
the code a bit more like a human would, i.e. in a way that doesn't trigger
this warning.

S

--
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking