The merge API helps a program to reconcile two competing sets of improvements to some files (e.g., unregistered changes from the work tree versus changes involved in switching to a new branch), reporting conflicts if found. The library called through this API is responsible for a few things.

  • determining which trees to merge (recursive ancestor consolidation);

  • lining up corresponding files in the trees to be merged (rename detection, subtree shifting), reporting edge cases like add/add and rename/rename conflicts to the user;

  • performing a three-way merge of corresponding files, taking path-specific merge drivers (specified in .gitattributes) into account.

Data structures

  • mmbuffer_t, mmfile_t

These store data usable for use by the xdiff backend, for writing and for reading, respectively. See xdiff/xdiff.h for the definitions and diff.c for examples.

  • struct ll_merge_options

This describes the set of options the calling program wants to affect the operation of a low-level (single file) merge. Some options:


Behave as though this were part of a merge between common ancestors in a recursive merge. If a helper program is specified by the [merge "<driver>"] recursive configuration, it will be used (see gitattributes(5)).


Resolve local conflicts automatically in favor of one side or the other (as in git merge-file --ours/--theirs/--union). Can be 0, XDL_MERGE_FAVOR_OURS, XDL_MERGE_FAVOR_THEIRS, or XDL_MERGE_FAVOR_UNION.


Resmudge and clean the "base", "theirs" and "ours" files before merging. Use this when the merge is likely to have overlapped with a change in smudge/clean or end-of-line normalization rules.

Low-level (single file) merge


Perform a three-way single-file merge in core. This is a thin wrapper around xdl_merge that takes the path and any merge backend specified in .gitattributes or .git/info/attributes into account. Returns 0 for a clean merge.

Calling sequence:

  • Prepare a struct ll_merge_options to record options. If you have no special requests, skip this and pass NULL as the opts parameter to use the default options.

  • Allocate an mmbuffer_t variable for the result.

  • Allocate and fill variables with the file’s original content and two modified versions (using read_mmfile, for example).

  • Call ll_merge().

  • Read the merged content from result_buf.ptr and result_buf.size.

  • Release buffers when finished. A simple free(ancestor.ptr); free(ours.ptr); free(theirs.ptr); free(result_buf.ptr); will do.

If the modifications do not merge cleanly, ll_merge will return a nonzero value and result_buf will generally include a description of the conflict bracketed by markers such as the traditional <<<<<<< and >>>>>>>.

The ancestor_label, our_label, and their_label parameters are used to label the different sides of a conflict if the merge driver supports this.

Everything else

Talk about <merge-recursive.h> and merge_file():

  • merge_trees() to merge with rename detection

  • merge_recursive() for ancestor consolidation

  • try_merge_command() for other strategies

  • conflict format

  • merge options

(Daniel, Miklos, Stephan, JC)