Re: [PATCH] vfprintf: Allocate the user argument buffer on the heap

[Florian Weimer <fw@deneb.enyo.de>]

* Adhemerval Zanella:

>> -	    args_value[cnt].pa_user = alloca (args_size[cnt]);
>> +	    /* Allocate from user_args_buffer.  */
>> +	    size_t allocation_size = args_size[cnt];
>> +	    void *allocation;
>> +	    if (allocation_size == 0)
>> +	      /* Nothing to allocate.  */
>> +	      allocation = NULL;
>> +	    else
>> +	      {
>> +		if (user_args_buffer == NULL)
>> +		  {
>> +		    /* First user argument.  Allocate the complete
>> +		       buffer.  */
>> +		    user_args_buffer = allocate_user_args_buffer
>> +		      (nargs, args_size, args_type);
>> +		    if (user_args_buffer == NULL)
>> +		      {
>> +			done = -1;
>> +			goto all_done;
>> +		      }
>> +		    user_args_buffer_next = user_args_buffer;
>> +		  }
>> +		allocation = user_args_buffer_next;
>> +		user_args_buffer_next
>> +		  += roundup (allocation_size, _Alignof (max_align_t));
>> +	      }
>> +	    /* Install the allocated pointer and use the callback to
>> +	       extract the argument.  */
>> +	    args_value[cnt].pa_user = allocation;
>>  	    (*__printf_va_arg_table[args_type[cnt] - PA_LAST])
>>  	      (args_value[cnt].pa_user, ap_savep);
>
> I am trying to convince myself it is worth to add all this complexity
> to allocate for user defined types, but I am failing to understand why
> can we just simplify it to a malloc using 'args_size[cnt]' (as the alloca
> is already using it).  And why do we need to keep track of the buffer
> allocation after the callback track?  Could we just free it after the
> call?

We need to delay the deallocation until the string has been formatted
because the data is later passed to the formatting function.

We could use separate malloc allocations and a second pass through the
argument array to free the user allocations (if any).  This might be
simpler, but I would have to write it down to be certain.