Documentation/technical: document ll_merge
[git/git.git] / Documentation / technical / api-merge.txt
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1merge API
2=========
3
4The merge API helps a program to reconcile two competing sets of
5improvements to some files (e.g., unregistered changes from the work
6tree versus changes involved in switching to a new branch), reporting
7conflicts if found. The library called through this API is
8responsible for a few things.
9
10 * determining which trees to merge (recursive ancestor consolidation);
11
12 * lining up corresponding files in the trees to be merged (rename
13 detection, subtree shifting), reporting edge cases like add/add
14 and rename/rename conflicts to the user;
15
16 * performing a three-way merge of corresponding files, taking
17 path-specific merge drivers (specified in `.gitattributes`)
18 into account.
19
20Low-level (single file) merge
21-----------------------------
22
23`ll_merge`::
24
25 Perform a three-way single-file merge in core. This is
26 a thin wrapper around `xdl_merge` that takes the path and
27 any merge backend specified in `.gitattributes` or
28 `.git/info/attributes` into account. Returns 0 for a
29 clean merge.
30
31The caller:
32
331. allocates an mmbuffer_t variable for the result;
342. allocates and fills variables with the file's original content
35 and two modified versions (using `read_mmfile`, for example);
363. calls ll_merge();
374. reads the output from result_buf.ptr and result_buf.size;
385. releases buffers when finished (free(ancestor.ptr); free(ours.ptr);
39 free(theirs.ptr); free(result_buf.ptr);).
40
41If the modifications do not merge cleanly, `ll_merge` will return a
42nonzero value and `result_buf` will generally include a description of
43the conflict bracketed by markers such as the traditional `<<<<<<<`
44and `>>>>>>>`.
45
46The `ancestor_label`, `our_label`, and `their_label` parameters are
47used to label the different sides of a conflict if the merge driver
48supports this.
49
50The `flag` parameter is a bitfield:
51
52 - The least significant bit indicates whether this is an internal
53 merge to consolidate ancestors for a recursive merge.
54
55 - The next two bits allow local conflicts to be automatically
56 resolved in favor of one side or the other (as in 'git merge-file'
57 `--ours`/`--theirs`/`--union` for 01, 10, and 11, respectively).
58
59Everything else
60---------------
61
62Talk about <merge-recursive.h> and merge_file():
63
64 - merge_trees() to merge with rename detection
65 - merge_recursive() for ancestor consolidation
66 - try_merge_command() for other strategies
67 - conflict format
68 - merge options
69
70(Daniel, Miklos, Stephan, JC)