Patches and Rejects in Software Configuration Management

Patch Options

The patch program has a number of options. (You can refer to the patch man page for more details.) However, in practice, the only option that matters much is the -p argument, which is used to align the absolute paths used in the patch file with the local directory structure containing the sources that the patch is being applied to. When you run cvs diff to create a patch file, it is best to do it from within the source tree, at the highest level in the directory hierarchy necessary to include all the files that have changes. The resulting patch file will, for each file that has changes, identify the file with a relative path, and patch uses this relative path to figure out what file in the target directory to apply changes to. For example:

Index: layout/layout.cpp =================================================================== RCS file: /usr/cvsroot/crossplatform/layout/layout.cpp,v retrieving revision 1.33
diff -u -3 -r1.33 layout.cpp
--- layout/layout.cpp 27 May 2006 09:31:47 -0000 1.33
+++ layout/layout.cpp 7 Jun 2006 10:43:22 -0000
@@ -327,7 +327,7 @@
return document;
}

-int main(int argc, char *argv[])
+int LayoutMain(int argc, char *argv[])
{
int run, parse;
char *src = NULL;

The first line in the preceding patch (the line prefixed with Index:) specifies the pathname of the file to be patched. Assuming that the patch is contained in a file named patch.txt, then, if the preceding patch file were copied to the same relative location in the target tree, then issuing the following command is sufficient for patch to locate the files that are specified in the patch file:

$ patch -p0 < patch.txt

The -p argument will remove the smallest prefix containing the specified number of leading slashes from each filename in the patch file, using the result to locate the file in the local source tree. Because the patch file was copied to the same relative location of the target tree that was used to generate the patch file in the source tree, we must use -p0 because we do not want patch to remove any portion of the path names when searching for files. If -p1 were used (with the same patch file, located in the same place in the target tree), the pathname layout/layout.cpp would be reduced to layout.cpp, and as a result, patch would not be able to locate the file, because the file would not be located in the current directory. Copying the patch file down into the layout directory would fix this, but this could only be done if, and only if, the patch file affected only sources that were located in the layout directory, because the -p0 is applied by patch to all sources represented in the patch file.