[petsc-dev] [petsc-users] PetscPrintf

[Smith, Barry F.]

   To me the difficulty is knowing how large an array to allocate (in both PetscVSNPrintf() and PetscVFPrintfDefault()). The macro PETSC_MAX_LENGTH_FORMAT() is used in PetscVSNPrintf() but it has nothing to do with reality.  One could walk through the formats (and their string values) to get a much better estimate for total length of buffer needed. 

Maybe a routine something like

   va_list Argp;
    va_start(Argp,format);
    ierr = PetscDetermineOutputLength(format,Argp,size_t *neededlength);CHKERRQ(ierr);
    PetscMalloc(neededlength,&buffer);


But I am not convinced we can't just have an upper bound on the possible format length (so long as we can error out properly if the user requests a something that doesn't fit).


    Barry

Note: there are horrible other buffer size code fragments such as 

    next->size = -1;
    while ((PetscInt)fullLength >= next->size) {
      next->size = fullLength+1;

      ierr = PetscMalloc1(next->size, &next->string);CHKERRQ(ierr);
      va_start(Argp,format);
      ierr = PetscMemzero(next->string,next->size);CHKERRQ(ierr);
      ierr = PetscVSNPrintf(next->string,next->size,format, &fullLength,Argp);CHKERRQ(ierr);
      va_end(Argp);
    }

that seems to try (very badly) to allocate larger and larger buffers (without freeing the previous?) until it has enough room for the result ? Any fix should resolve this as well.

   Maybe two versions of PetscVSNPrintf() one that takes a buffer (and respects it) and one that allocates a buffer large enough.




> On Apr 15, 2018, at 5:41 AM, Patrick Sanan <patrick.sanan at gmail.com> wrote:
> 
> How about the logic of this analysis?
> 
> 1. We are trying to use the same functions (in particular, PetscVFPrintf) for two purposes:
>    a. printing error messages (don't want to malloc)
>    b. use by public API printing functions (don't want length restrictions)
> 
> 2. Right now, PetscVFPrintf works fine for a but not for b. We could make it work for b and not for a by malloc'ing a longer string.
> 
> 3. Printing from error handlers happens through PetscErrorPrintf (default : http://www.mcs.anl.gov/petsc/petsc-dev/src/sys/error/errtrace.c.html#PetscErrorPrintfDefault ), so if there's a special requirement for printing error messages, we can impose it here.
> 
> A solution could then be something which skips the malloc only when printing an error, e.g.
> 
> 1. Add an argument to PetscVFPrintf (say "PetscBool noMalloc") [1]
> 2. API (PetscPrintf(), etc.) functions use noMalloc = PETSC_FALSE
> 3. error functions (PetscErrorPrintf() ) functions use noMalloc = PETSC_TRUE 
> 
> 
> [1] And probably do the same thing in PetscVSNPrintf since, as Dr. Zhang pointed out, this could also call malloc while handling an error, if the string was long enough
> 
> 
> 
> 
> 2018-04-13 15:59 GMT+02:00 Junchao Zhang <jczhang at mcs.anl.gov>:
> On Thu, Apr 12, 2018 at 9:48 AM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
> 
> 
> > On Apr 12, 2018, at 3:59 AM, Patrick Sanan <patrick.sanan at gmail.com> wrote:
> >
> > I also happened to stumble across this yesterday. Is the length restriction for the default printer (l assume from the array of 8*1024 chars in PetscVFPrintfDefault() ) intended behavior to be documented, or a bug to be fixed?
> 
>      You could call it either. My problems are
> 
> 1) that given a format string I don't know in advance how much work space is needed so cannot accurately malloc, for sure, enough space
> 
> 2) since this can be called in an error handler I really don't want it calling malloc().
> PetscVSNPrintf does still contain a malloc "122  ierr      = PetscMalloc1(oldLength, &newformat);CHKERRQ(ierr);"
> Also, vsnprintf returns "the number of characters that would have been written if n had been sufficiently large". I don't know why you void'ed it.
> We can not make the 8K chars a requirement since users don't know how many chars they want to print upfront.
> Anyway, crash is better than silent errors. 
> 
>    Hence it lives in this limbo. I don't even know a way to add a good error checking that detects if the buffer is long enough. All in all it is bad ugly code, any suggestions on improvements would be appreciated.
> 
>    Barry
> 
> >
> > 2018-04-12 2:16 GMT+02:00 Rongliang Chen <rongliang.chan at gmail.com>:
> > Thanks Barry. I found petsc-3.6 and older versions did not have this restriction.
> >
> > Best,
> > Rongliang
> >
> >
> > On 04/12/2018 07:22 AM, Smith, Barry F. wrote:
> >    Yes, PetscPrintf() and related functions have a maximum string length of about 8000 characters.
> >
> >     Barry
> >
> >
> > On Apr 11, 2018, at 6:17 PM, Rongliang Chen <rongliang.chan at gmail.com> wrote:
> >
> > Dear All,
> >
> >
> > When I tried to print a long string using PetscPrintf() I found that it truncated the string. Attached is a simple example for this (run with single processor). I used PetscPrintf() and printf() to print the same string and the printf() seems OK. I am using petsc-3.8.4.
> >
> >
> > Best,
> >
> > Rongliang
> >
> > <ex111.c>
> >
> >
> >
> 
> 
>