FFI: Vararg function how to tell callee how many arguments i passed?
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FFI: Vararg function how to tell callee how many arguments i passed?
 
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Suppose i want to call printf() using FFI. But it is a variable argument function. What should do to let it know how many arguments i passed? -- Best regards, Igor Stasenko AKA sig. |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On Mon, 27 Sep 2010, Igor Stasenko wrote: > > Suppose i want to call printf() using FFI. > But it is a variable argument function. What should do to let it know > how many arguments i passed? I think you don't tell it, it guesses the number of arguments from the first argument. If you pass invalid arguments, something bad will happen. :) Levente > > -- > Best regards, > Igor Stasenko AKA sig. > |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On 27 September 2010 02:12, Levente Uzonyi <[hidden email]> wrote: > > On Mon, 27 Sep 2010, Igor Stasenko wrote: > >> >> Suppose i want to call printf() using FFI. >> But it is a variable argument function. What should do to let it know >> how many arguments i passed? > > I think you don't tell it, it guesses the number of arguments from the first > argument. If you pass invalid arguments, something bad will happen. :) > Yeah.. i inspected the assembly (gcc -s) for printf call, and there is only pushes of arguments , no extra info. > > Levente > >> >> -- >> Best regards, >> Igor Stasenko AKA sig. >> > -- Best regards, Igor Stasenko AKA sig. |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On 27.09.2010, at 01:23, Igor Stasenko wrote: > > On 27 September 2010 02:12, Levente Uzonyi <[hidden email]> wrote: >> >> On Mon, 27 Sep 2010, Igor Stasenko wrote: >> >>> >>> Suppose i want to call printf() using FFI. >>> But it is a variable argument function. What should do to let it know >>> how many arguments i passed? >> >> I think you don't tell it, it guesses the number of arguments from the first >> argument. If you pass invalid arguments, something bad will happen. :) >> > > Yeah.. i inspected the assembly (gcc -s) for printf call, > and there is only pushes of arguments , no extra info. Vararg functions are specially compiled. They do not use the regular platform calling conventions. That's because the same compiled function needs to be able to take any type of argument. Whereas e.g. floats are normally passed on the float stack, in a vararg function call they might be passed on the regular stack. Also they are converted to doubles at the calling side, just as chars and shorts are promoted to ints. So at the calling site the compiler has to do special magic to construct the argument list. It can only do this if you actually provide a vararg call. That is, you need to literally write a printf() call to make this work. There is no portable way around this - the C FAQ states (*) Q: How can I call a function with an argument list built up at run time? A: There is no guaranteed or portable way to do this. Squeak's FFI does not support vararg functions yet. There are other FFIs that do (CLISP's vacall library, Rubinius FFI), so taking a look there might help if anyone wants to implement this. - Bert - (*) http://c-faq.com/varargs/invvarargs.html |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On Mon, Sep 27, 2010 at 2:28 AM, Bert Freudenberg <[hidden email]> wrote:
While the image-level facilities may not exist the VM provides primitiveCalloutWithArgs which implements ExternalFunction>>invokeWithArguments: and that could be used to do varargs calls. One has to synthesize the ExternalFunction because the primitive still needs a type vector for the arguments. But I think it could do the job.
cheers Eliot
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Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On 27.09.2010, at 19:27, Eliot Miranda wrote:
No it can't. Maybe re-read what I wrote when you have a little more time ;) - Bert - |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On 27 September 2010 21:21, Bert Freudenberg <[hidden email]> wrote: > > > On 27.09.2010, at 19:27, Eliot Miranda wrote: > > > On Mon, Sep 27, 2010 at 2:28 AM, Bert Freudenberg <[hidden email]> wrote: >> >> >> On 27.09.2010, at 01:23, Igor Stasenko wrote: >> >> > >> > On 27 September 2010 02:12, Levente Uzonyi <[hidden email]> wrote: >> >> >> >> On Mon, 27 Sep 2010, Igor Stasenko wrote: >> >> >> >>> >> >>> Suppose i want to call printf() using FFI. >> >>> But it is a variable argument function. What should do to let it know >> >>> how many arguments i passed? >> >> >> >> I think you don't tell it, it guesses the number of arguments from the first >> >> argument. If you pass invalid arguments, something bad will happen. :) >> >> >> > >> > Yeah.. i inspected the assembly (gcc -s) for printf call, >> > and there is only pushes of arguments , no extra info. >> >> Vararg functions are specially compiled. They do not use the regular platform calling conventions. That's because the same compiled function needs to be able to take any type of argument. Whereas e.g. floats are normally passed on the float stack, in a vararg function call they might be passed on the regular stack. Also they are converted to doubles at the calling side, just as chars and shorts are promoted to ints. >> >> So at the calling site the compiler has to do special magic to construct the argument list. It can only do this if you actually provide a vararg call. That is, you need to literally write a printf() call to make this work. There is no portable way around this - the C FAQ states (*) >> >> Q: How can I call a function with an argument list built up at run time? >> A: There is no guaranteed or portable way to do this. >> >> Squeak's FFI does not support vararg functions yet. There are other FFIs that do (CLISP's vacall library, Rubinius FFI), so taking a look there might help if anyone wants to implement this. > > While the image-level facilities may not exist the VM provides primitiveCalloutWithArgs which implements ExternalFunction>>invokeWithArguments: and that could be used to do varargs calls. One has to synthesize the ExternalFunction because the primitive still needs a type vector for the arguments. But I think it could do the job. > > No it can't. Maybe re-read what I wrote when you have a little more time ;) > for vararg functions. But from following example, i can conclude that its not. Its up to developer to determine how many arguments (and what they types) passed to function: double average ( int num, ... ) { va_list arguments; // A place to store the list of arguments double sum = 0; va_start ( arguments, num ); // Initializing arguments to store all values after num for ( int x = 0; x < num; x++ ) // Loop until all numbers are added sum += va_arg ( arguments, double ); // Adds the next value in argument list to sum. va_end ( arguments ); // Cleans up the list return sum / num; // Returns some number (typecast prevents truncation) } > cheers > Eliot >> >> - Bert - >> >> (*) http://c-faq.com/varargs/invvarargs.html > > - Bert - > > > -- Best regards, Igor Stasenko AKA sig. |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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In reply to this post by Bert Freudenberg
On 27.09.2010, at 20:21, Bert Freudenberg wrote:
Retract that. A little off-line conversation with Eliot convinced me that the parameter passing isn't actually that much different between regular and varagrs calls (if at all). The FFI already does all the non-portable ABI-dependent trickery needed to set up the call. So this might in fact just work :) Anyone up for trying? - Bert - |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On 28 September 2010 00:22, Bert Freudenberg <[hidden email]> wrote: > > > On 27.09.2010, at 20:21, Bert Freudenberg wrote: > > On 27.09.2010, at 19:27, Eliot Miranda wrote: > > > On Mon, Sep 27, 2010 at 2:28 AM, Bert Freudenberg <[hidden email]> wrote: >> >> >> On 27.09.2010, at 01:23, Igor Stasenko wrote: >> >> > >> > On 27 September 2010 02:12, Levente Uzonyi <[hidden email]> wrote: >> >> >> >> On Mon, 27 Sep 2010, Igor Stasenko wrote: >> >> >> >>> >> >>> Suppose i want to call printf() using FFI. >> >>> But it is a variable argument function. What should do to let it know >> >>> how many arguments i passed? >> >> >> >> I think you don't tell it, it guesses the number of arguments from the first >> >> argument. If you pass invalid arguments, something bad will happen. :) >> >> >> > >> > Yeah.. i inspected the assembly (gcc -s) for printf call, >> > and there is only pushes of arguments , no extra info. >> >> Vararg functions are specially compiled. They do not use the regular platform calling conventions. That's because the same compiled function needs to be able to take any type of argument. Whereas e.g. floats are normally passed on the float stack, in a vararg function call they might be passed on the regular stack. Also they are converted to doubles at the calling side, just as chars and shorts are promoted to ints. >> >> So at the calling site the compiler has to do special magic to construct the argument list. It can only do this if you actually provide a vararg call. That is, you need to literally write a printf() call to make this work. There is no portable way around this - the C FAQ states (*) >> >> Q: How can I call a function with an argument list built up at run time? >> A: There is no guaranteed or portable way to do this. >> >> Squeak's FFI does not support vararg functions yet. There are other FFIs that do (CLISP's vacall library, Rubinius FFI), so taking a look there might help if anyone wants to implement this. > > While the image-level facilities may not exist the VM provides primitiveCalloutWithArgs which implements ExternalFunction>>invokeWithArguments: and that could be used to do varargs calls. One has to synthesize the ExternalFunction because the primitive still needs a type vector for the arguments. But I think it could do the job. > > No it can't. Maybe re-read what I wrote when you have a little more time ;) > > Retract that. > A little off-line conversation with Eliot convinced me that the parameter passing isn't actually that much different between regular and varagrs calls (if at all). The FFI already does all the non-portable ABI-dependent trickery needed to set up the call. So this might in fact just work :) > Anyone up for trying? Well, i made a callout to printf() via NativeBoost in Linux, but was unable to determine if it works, because when i run it, it prints nothing on console. Probably because stdout is closed by default and i need to reopen it first. > - Bert - > -- Best regards, Igor Stasenko AKA sig. |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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On Mon, Sep 27, 2010 at 3:12 PM, Igor Stasenko <[hidden email]> wrote:
Did you try using a console VM (if you're using Cog that's CroquetConsole.exe)? e.
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Re: FFI: Vararg function how to tell callee how many arguments i passed?
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In reply to this post by Igor Stasenko
On Tue, 28 Sep 2010, Igor Stasenko wrote: > > On 28 September 2010 00:22, Bert Freudenberg <[hidden email]> wrote: >> >> >> On 27.09.2010, at 20:21, Bert Freudenberg wrote: >> >> On 27.09.2010, at 19:27, Eliot Miranda wrote: >> >> >> On Mon, Sep 27, 2010 at 2:28 AM, Bert Freudenberg <[hidden email]> wrote: >>> >>> >>> On 27.09.2010, at 01:23, Igor Stasenko wrote: >>> >>>> >>>> On 27 September 2010 02:12, Levente Uzonyi <[hidden email]> wrote: >>>>> >>>>> On Mon, 27 Sep 2010, Igor Stasenko wrote: >>>>> >>>>>> >>>>>> Suppose i want to call printf() using FFI. >>>>>> But it is a variable argument function. What should do to let it know >>>>>> how many arguments i passed? >>>>> >>>>> I think you don't tell it, it guesses the number of arguments from the first >>>>> argument. If you pass invalid arguments, something bad will happen. :) >>>>> >>>> >>>> Yeah.. i inspected the assembly (gcc -s) for printf call, >>>> and there is only pushes of arguments , no extra info. >>> >>> Vararg functions are specially compiled. They do not use the regular platform calling conventions. That's because the same compiled function needs to be able to take any type of argument. Whereas e.g. floats are normally passed on the float stack, in a vararg function call they might be passed on the regular stack. Also they are converted to doubles at the calling side, just as chars and shorts are promoted to ints. >>> >>> So at the calling site the compiler has to do special magic to construct the argument list. It can only do this if you actually provide a vararg call. That is, you need to literally write a printf() call to make this work. There is no portable way around this - the C FAQ states (*) >>> >>> Q: How can I call a function with an argument list built up at run time? >>> A: There is no guaranteed or portable way to do this. >>> >>> Squeak's FFI does not support vararg functions yet. There are other FFIs that do (CLISP's vacall library, Rubinius FFI), so taking a look there might help if anyone wants to implement this. >> >> While the image-level facilities may not exist the VM provides primitiveCalloutWithArgs which implements ExternalFunction>>invokeWithArguments: and that could be used to do varargs calls. One has to synthesize the ExternalFunction because the primitive still needs a type vector for the arguments. But I think it could do the job. >> >> No it can't. Maybe re-read what I wrote when you have a little more time ;) >> >> Retract that. >> A little off-line conversation with Eliot convinced me that the parameter passing isn't actually that much different between regular and varagrs calls (if at all). The FFI already does all the non-portable ABI-dependent trickery needed to set up the call. So this might in fact just work :) >> Anyone up for trying? > > Well, i made a callout to printf() via NativeBoost in Linux, > but was unable to determine if it works, because when i run it, it > prints nothing on console. > Probably because stdout is closed by default and i need to reopen it first. for stdio on windows which worked like this: fprintf := ExternalLibraryFunction name: 'fprintf' module: 'msvcrt.dll' callType: ExternalFunction callTypeCDecl returnType: ExternalType signedLong argumentTypes: { (ExternalType structTypeNamed: #FILE) asPointerType. ExternalType string. ExternalType signedLong }. file := Stdio default fopenWith: 'test.txt' with: 'w'. fprintf invokeWith: file with: 'Your number is %d.' with: 42. Stdio default fcloseWith: file. I didn't bother writing a parser which extracts the type information from the format string, but if that's done it's pretty easy to do the rest. Caching the functions probably improves the performance a lot. I can imagine two caches (for *printf functions): - a larger cache which has typeInfo -> externalFunction mapping - a smaller cache which maps formatString -> typeInfo The problem with this method is that FILE structure is platform specific. Levente > >> - Bert - >> > > -- > Best regards, > Igor Stasenko AKA sig. > |
Re: FFI: Vararg function how to tell callee how many arguments i passed?
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In reply to this post by Eliot Miranda-2
On 28 September 2010 01:57, Eliot Miranda <[hidden email]> wrote: > > > > On Mon, Sep 27, 2010 at 3:12 PM, Igor Stasenko <[hidden email]> wrote: >> >> On 28 September 2010 00:22, Bert Freudenberg <[hidden email]> wrote: >> > >> > >> > On 27.09.2010, at 20:21, Bert Freudenberg wrote: >> > >> > On 27.09.2010, at 19:27, Eliot Miranda wrote: >> > >> > >> > On Mon, Sep 27, 2010 at 2:28 AM, Bert Freudenberg <[hidden email]> wrote: >> >> >> >> >> >> On 27.09.2010, at 01:23, Igor Stasenko wrote: >> >> >> >> > >> >> > On 27 September 2010 02:12, Levente Uzonyi <[hidden email]> wrote: >> >> >> >> >> >> On Mon, 27 Sep 2010, Igor Stasenko wrote: >> >> >> >> >> >>> >> >> >>> Suppose i want to call printf() using FFI. >> >> >>> But it is a variable argument function. What should do to let it know >> >> >>> how many arguments i passed? >> >> >> >> >> >> I think you don't tell it, it guesses the number of arguments from the first >> >> >> argument. If you pass invalid arguments, something bad will happen. :) >> >> >> >> >> > >> >> > Yeah.. i inspected the assembly (gcc -s) for printf call, >> >> > and there is only pushes of arguments , no extra info. >> >> >> >> Vararg functions are specially compiled. They do not use the regular platform calling conventions. That's because the same compiled function needs to be able to take any type of argument. Whereas e.g. floats are normally passed on the float stack, in a vararg function call they might be passed on the regular stack. Also they are converted to doubles at the calling side, just as chars and shorts are promoted to ints. >> >> >> >> So at the calling site the compiler has to do special magic to construct the argument list. It can only do this if you actually provide a vararg call. That is, you need to literally write a printf() call to make this work. There is no portable way around this - the C FAQ states (*) >> >> >> >> Q: How can I call a function with an argument list built up at run time? >> >> A: There is no guaranteed or portable way to do this. >> >> >> >> Squeak's FFI does not support vararg functions yet. There are other FFIs that do (CLISP's vacall library, Rubinius FFI), so taking a look there might help if anyone wants to implement this. >> > >> > While the image-level facilities may not exist the VM provides primitiveCalloutWithArgs which implements ExternalFunction>>invokeWithArguments: and that could be used to do varargs calls. One has to synthesize the ExternalFunction because the primitive still needs a type vector for the arguments. But I think it could do the job. >> > >> > No it can't. Maybe re-read what I wrote when you have a little more time ;) >> > >> > Retract that. >> > A little off-line conversation with Eliot convinced me that the parameter passing isn't actually that much different between regular and varagrs calls (if at all). The FFI already does all the non-portable ABI-dependent trickery needed to set up the call. So this might in fact just work :) >> > Anyone up for trying? >> >> Well, i made a callout to printf() via NativeBoost in Linux, >> but was unable to determine if it works, because when i run it, it >> prints nothing on console. >> Probably because stdout is closed by default and i need to reopen it first. > > Did you try using a console VM (if you're using Cog that's CroquetConsole.exe)? Oh. I didn't not yet started adopting NB with Cog. But im about to do it in few days. > e. >> >> > - Bert - >> > >> >> -- >> Best regards, >> Igor Stasenko AKA sig. > > > -- Best regards, Igor Stasenko AKA sig. |
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