Unclear pointer and array Post: 302884281

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Top Forums Programming Unclear pointer and array Post 302884281 by yifangt on Sunday 19th of January 2014 05:41:07 PM

01-19-2014

Registered User

unmatching format argument for printf

Thanks Corona!
What I meant panic is to match the format arguments of printf(), especially when pointers are involved. My bad part is the not-enough-understanding of the pointer, as I did not realize my wrong catches until I saw the warnings/errors.
Go back to the original post of this thread, First, I was wondering why array worked but pointer not. After the discussion I seemed to have some ideas that when array is used, the memory is automatically allocated, but memory was only allocated to the variable address if a pointer is used, such that enough memory needs to be allocated to the pointers of the string/substring.
The second question is what the correct way is if pointer is used. (Maybe using pointer is not the correct way, but that is another story.) But this topic was distracted by other problems with sprintf() and printf() format arguments like Don's tricks.
The most difficult parts with C to me are: 1) dynamic memory allocation of pointers, 2) match the format arguments of printf() when pointers are involved. That's what I am always panic with. The critical point is still the pointer manipulation. Need more read and study. Could not find a good course around that digs C deep as I want. Neither could I find a good instruction/book on this topic yet, or too many diverse information about this for me to catch the critical points, I should say. Thanks you lots any way. Any suggestion?

yifangt

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LEARN ABOUT DEBIAN

bup-margin

bup-margin(1)						      General Commands Manual						     bup-margin(1)

NAME

       bup-margin - figure out your deduplication safety margin

SYNOPSIS

       bup margin [options...]

DESCRIPTION

       bup margin  iterates  through  all  objects  in	your  bup repository, calculating the largest number of prefix bits shared between any two
       entries.  This number, n, identifies the longest subset of SHA-1 you could use and still encounter a collision between your object ids.

       For example, one system that was tested had a collection of 11 million objects (70 GB), and bup margin returned 45.  That  means  a  46-bit
       hash  would be sufficient to avoid all collisions among that set of objects; each object in that repository could be uniquely identified by
       its first 46 bits.

       The number of bits needed seems to increase by about 1 or 2 for every doubling of the number of objects.  Since SHA-1 hashes have 160 bits,
       that  leaves 115 bits of margin.  Of course, because SHA-1 hashes are essentially random, it's theoretically possible to use many more bits
       with far fewer objects.

       If you're paranoid about the possibility of SHA-1 collisions, you can monitor your repository by running bup margin occasionally to see	if
       you're getting dangerously close to 160 bits.

OPTIONS

       --predict
	      Guess  the offset into each index file where a particular object will appear, and report the maximum deviation of the correct answer
	      from the guess.  This is potentially useful for tuning an interpolation search algorithm.

       --ignore-midx
	      don't use .midx files, use only .idx files.  This is only really useful when used with --predict.

EXAMPLE

	      $ bup margin
	      Reading indexes: 100.00% (1612581/1612581), done.
	      40
	      40 matching prefix bits
	      1.94 bits per doubling
	      120 bits (61.86 doublings) remaining
	      4.19338e+18 times larger is possible

	      Everyone on earth could have 625878182 data sets
	      like yours, all in one repository, and we would
	      expect 1 object collision.

	      $ bup margin --predict
	      PackIdxList: using 1 index.
	      Reading indexes: 100.00% (1612581/1612581), done.
	      915 of 1612581 (0.057%)

SEE ALSO

       bup-midx(1), bup-save(1)

BUP

       Part of the bup(1) suite.

AUTHORS

       Avery Pennarun <apenwarr@gmail.com>.

Bup unknown-															     bup-margin(1)