Interfacing to C
in [ADA]
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From: Vadim Godunko on 24 Dec 2009 07:35 On Dec 24, 12:46 pm, "RasikaSriniva...(a)gmail.com" <rasikasriniva...(a)gmail.com> wrote: > > type MyStruct is record ..... end record ; > pragma Convention(C,MyStruct); > pragma Convention (C_Pass_by_Copy, MyStruct);
From: Niklas Holsti on 24 Dec 2009 07:34 RasikaSrinivasan(a)gmail.com wrote: > I am building an interface to a C library. This library uses structs > like the following: > > typedef struct { > x : int ; > y : int } MyStruct .... > > While passing variables of this type to functions, the convention in C > is to pass by value (best of my knowledge). Correct, as I understand the C parameter-passing rules. > however when I import a function like: > > type MyStruct is record ..... end record ; > pragma Convention(C,MyStruct); > > procedure process(var : MyStruct); > pragma Import(C,process); > > It appears that Ada passes the var by reference. Yes, by default Ada considers records/struct under convention C as pass-by-reference (LRM B.3 (69/2)). C programmers often choose to pass structs by reference (pointer to struct) and the Ada Convention (C) pragma has been defined accordingly. Or so I guess. > So the question is - how c(an I force Ada (gnat) to pass a record by > value? Use Pragma Convention (C_Pass_By_Copy, MyStruct). See LRM B.3 (60.14/2) and LRM B.3 (68.1/2). -- Niklas Holsti Tidorum Ltd niklas holsti tidorum fi . @ .
From: Keith Thompson on 24 Dec 2009 21:07 "Hibou57 (Yannick Duchêne)" <yannick_duchene(a)yahoo.fr> writes: > On 24 déc, 10:46, "RasikaSriniva...(a)gmail.com" > <rasikasriniva...(a)gmail.com> wrote: >> I am building an interface to a C library. This library uses structs >> like the following: >> >> typedef struct { >> x : int ; >> y : int } MyStruct .... (The correct syntax is: typedef struct { int x; int y; } MyStruct; ) >> While passing variables of this type to functions, the convention in C >> is to pass by value (best of my knowledge). Yes. In fact, C doesn't have pass by reference as a language feature; *all* function arguments are passed by value. For structs, it's very common in C to do the equivalent of pass-by-reference, by explicitly passing the address of the struct object. This is partly because very old versions of C didn't support struct parameters. I find it a bit odd that Ada imposes a pass-by-pointer convention for C structs. (No, C arrays aren't passed by reference. C array expressions, in most contexts, are implicitly converted to pointers; the resulting pointer can be passed by value.) [...] -- Keith Thompson (The_Other_Keith) kst-u(a)mib.org <http://www.ghoti.net/~kst> Nokia "We must do something. This is something. Therefore, we must do this." -- Antony Jay and Jonathan Lynn, "Yes Minister"
From: Hibou57 (Yannick Duchêne) on 28 Dec 2009 02:20 On 24 déc, 13:34, Niklas Holsti <niklas.hol...(a)tidorum.invalid> wrote: > RasikaSriniva...(a)gmail.com wrote: > > I am building an interface to a C library. This library uses structs > > like the following: > > > typedef struct { > > x : int ; > > y : int } MyStruct .... > > > While passing variables of this type to functions, the convention in C > > is to pass by value (best of my knowledge). > > Correct, as I understand the C parameter-passing rules. Shame on me, you are both right. I was confused between standard practice and language standard. Language which allow C interface, like some Pascal dialects, do it using implicit by reference structs, while with C, by reference structs is done explicitly using a point type.
From: Robert A Duff on 31 Dec 2009 18:18
Keith Thompson <kst-u(a)mib.org> writes: > "Hibou57 (Yannick Duch�ne)" <yannick_duchene(a)yahoo.fr> writes: >> On 24 d�c, 10:46, "RasikaSriniva...(a)gmail.com" >> <rasikasriniva...(a)gmail.com> wrote: >>> I am building an interface to a C library. This library uses structs >>> like the following: >>> >>> typedef struct { >>> � � x : int ; >>> � � y : int } �MyStruct .... > > (The correct syntax is: > typedef struct { > int x; > int y; > } MyStruct; > ) Right. >>> While passing variables of this type to functions, the convention in C >>> is to pass by value (best of my knowledge). > > Yes. In fact, C doesn't have pass by reference as a language feature; > *all* function arguments are passed by value. > > For structs, it's very common in C to do the equivalent of > pass-by-reference, by explicitly passing the address of the struct > object. This is partly because very old versions of C didn't > support struct parameters. Which version of C (when?) introduced (by-copy) struct params? > I find it a bit odd that Ada imposes a pass-by-pointer convention for > C structs. It was a mistake. Partly my fault. I was thinking that it's common in C to pass struct params by explicitly passing a pointer-to-struct, so Ada should mimic that. Bad idea. By the time we realized the mistake, it was too late to fix (compatibility!), so we invented C_Pass_By_Copy as a workaround. > (No, C arrays aren't passed by reference. C array expressions, in > most contexts, are implicitly converted to pointers; the resulting > pointer can be passed by value.) Yeah. In other words, C arrays aren't passed, period. That's another bad idea, and it's not my fault. ;-) - Bob |