This post is for those people who use Exuberant Ctags. If you are using other versions of ctags, this post may not be useful.

When using ctags to generate the tags file for C/C++ projects, usually we use the following command:

ctags -R .

For some users that need more info of the symbols, they may use this command instead:

ctags -R --c++-kinds=+p --fields=+iaS --extra=+q .

No matter which one you use, the generated tags file only contains the symbols in the files in your project source tree, but not any external file, such as standard header files (e.g. stdio.h, stdlib.h), etc. thus editors or IDEs that use tags files, such as Vim, are not able to locate symbols in external header files. There was a solution: generate a tags file for any external header files first, and let the editor or IDE read both the generated tags file and the tags file for the project source tree. For example, the following command will generate a tags file for all your system header files on UNIX/Linux:

ctags -R --c++-kinds=+p --fields=+iaS --extra=+q /usr/include

This command usually takes a very long time to finish, and finally it gives a quite large tags file, which causes the editor or IDE a long time to search this tags file for symbols. To solve this problem, I came up with another idea.

Why must we generate a tags file containing all the symbols in the system header? If we only generate the tags file only for the header files that are related to our projects, would it be faster? That’s the point of this idea. We could first search for the header files that are included in our projects, and then we use ctags to generate a tags file for these files and our source files, in this way, a much smaller tags file that containing all the symbols that maybe useful for the project is generated.

To do this, I wrote a shell script ctags_with_dep.sh :

#!/bin/bash # ./ctags_with_dep.sh file1.c file2.c ... to generate a tags file for these files. gcc -M "$@" | sed -e 's/[\\ ]/

/g' | \ sed -e '/^$/d' -e '/\.o:[ \t]*$/d' | \ ctags -L - --c++-kinds=+p --fields=+iaS --extra=+q

This script is also available on github gist. If you only want to use it, download the script and use the following command to generate the tags file:

./ctags_with_dep.sh file1.c file2.c file3.cpp ...

Read on if you want to know what’s happening here. This script will first use gcc -M to output the list of header files that are included in our C or C++ source files. However, the output could not be directly used by ctags, thus this script uses sed commands to filter the output. Finally, this script uses a pipe to put the file list to the stdin of the ctags program — ctags will read the file list from stdin if -L - is passed to it on the command line.

What if you have other directories besides the standard /usr/include that containing the header files you need? You could do a little modification on this script. For example, you have some header files in ~/include , then you could pass -I ~/include to the gcc command. Just like below:

gcc -M -I ~/include "$@" | sed -e 's/[\\ ]/

/g' | \ sed -e '/^$/d' -e '/\.o:[ \t]*$/d' | \ ctags -L - --c++-kinds=+p --fields=+iaS --extra=+q

If you have any suggestion on this idea, please let me know.