Source Path (Debugging with GDB)

### 9.5 Specifying Source Directories ¶

Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. GDB has a list of directories to search for source files; this is called the source path. Each time GDB wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name.

For example, suppose an executable references the file `/usr/src/foo-1.0/lib/foo.c`, does not record a compilation directory, and the source path is `/mnt/cross`. GDB would look for the source file in the following locations:

1. `/usr/src/foo-1.0/lib/foo.c`
2. `/mnt/cross/usr/src/foo-1.0/lib/foo.c`
3. `/mnt/cross/foo.c`

If the source file is not present at any of the above locations then an error is printed. GDB does not look up the parts of the source file name, such as `/mnt/cross/src/foo-1.0/lib/foo.c`. Likewise, the subdirectories of the source path are not searched: if the source path is `/mnt/cross`, and the binary refers to `foo.c`, GDB would not find it under `/mnt/cross/usr/src/foo-1.0/lib`.

Plain file names, relative file names with leading directories, file names containing dots, etc. are all treated as described above, except that non-absolute file names are not looked up literally. If the source path is `/mnt/cross`, the source file is recorded as `../lib/foo.c`, and no compilation directory is recorded, then GDB will search in the following locations:

1. `/mnt/cross/../lib/foo.c`
2. `/mnt/cross/foo.c`

The source path will always include two special entries ‘`$cdir`’ and ‘`$cwd`’, these refer to the compilation directory (if one is recorded) and the current working directory respectively.

‘`$cdir`’ causes GDB to search within the compilation directory, if one is recorded in the debug information. If no compilation directory is recorded in the debug information then ‘`$cdir`’ is ignored.

‘`$cwd`’ is not the same as ‘`.`’—the former tracks the current working directory as it changes during your GDB session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path.

If a compilation directory is recorded in the debug information, and GDB has not found the source file after the first search using source path, then GDB will combine the compilation directory and the filename, and then search for the source file again using the source path.

For example, if the executable records the source file as `/usr/src/foo-1.0/lib/foo.c`, the compilation directory is recorded as `/project/build`, and the source path is `/mnt/cross:$cdir:$cwd` while the current working directory of the GDB session is `/home/user`, then GDB will search for the source file in the following locations:

1. `/usr/src/foo-1.0/lib/foo.c`
2. `/mnt/cross/usr/src/foo-1.0/lib/foo.c`
3. `/project/build/usr/src/foo-1.0/lib/foo.c`
4. `/home/user/usr/src/foo-1.0/lib/foo.c`
5. `/mnt/cross/project/build/usr/src/foo-1.0/lib/foo.c`
6. `/project/build/project/build/usr/src/foo-1.0/lib/foo.c`
7. `/home/user/project/build/usr/src/foo-1.0/lib/foo.c`
8. `/mnt/cross/foo.c`
9. `/project/build/foo.c`
10. `/home/user/foo.c`

If the file name in the previous example had been recorded in the executable as a relative path rather than an absolute path, then the first look up would not have occurred, but all of the remaining steps would be similar.

When searching for source files on MS-DOS and MS-Windows, where absolute paths start with a drive letter (e.g. `C:/project/foo.c`), GDB will remove the drive letter from the file name before appending it to a search directory from source path; for instance if the executable references the source file `C:/project/foo.c` and source path is set to `D:/mnt/cross`, then GDB will search in the following locations for the source file:

1. `C:/project/foo.c`
2. `D:/mnt/cross/project/foo.c`
3. `D:/mnt/cross/foo.c`

Note that the executable search path is not used to locate the source files.

Whenever you reset or rearrange the source path, GDB clears out any information it has cached about where source files are found and where each line is in the file.

When you start GDB, its source path includes only ‘`$cdir`’ and ‘`$cwd`’, in that order. To add other directories, use the `directory` command.

The search path is used to find both program source files and GDB script files (read using the ‘`-command`’ option and ‘`source`’ command).

In addition to the source path, GDB provides a set of commands that manage a list of source path substitution rules. A substitution rule specifies how to rewrite source directories stored in the program’s debug information in case the sources were moved to a different directory between compilation and debugging. A rule is made of two strings, the first specifying what needs to be rewritten in the path, and the second specifying how it should be rewritten. In set substitute-path, we name these two parts from and to respectively. GDB does a simple string replacement of from with to at the start of the directory part of the source file name, and uses that result instead of the original file name to look up the sources.

Using the previous example, suppose the `foo-1.0` tree has been moved from `/usr/src` to `/mnt/cross`, then you can tell GDB to replace `/usr/src` in all source path names with `/mnt/cross`. The first lookup will then be `/mnt/cross/foo-1.0/lib/foo.c` in place of the original location of `/usr/src/foo-1.0/lib/foo.c`. To define a source path substitution rule, use the `set substitute-path` command (see set substitute-path).

To avoid unexpected substitution results, a rule is applied only if the from part of the directory name ends at a directory separator. For instance, a rule substituting `/usr/source` into `/mnt/cross` will be applied to `/usr/source/foo-1.0` but not to `/usr/sourceware/foo-2.0`. And because the substitution is applied only at the beginning of the directory name, this rule will not be applied to `/root/usr/source/baz.c` either.

In many cases, you can achieve the same result using the `directory` command. However, `set substitute-path` can be more efficient in the case where the sources are organized in a complex tree with multiple subdirectories. With the `directory` command, you need to add each subdirectory of your project. If you moved the entire tree while preserving its internal organization, then `set substitute-path` allows you to direct the debugger to all the sources with one single command.

`set substitute-path` is also more than just a shortcut command. The source path is only used if the file at the original location no longer exists. On the other hand, `set substitute-path` modifies the debugger behavior to look at the rewritten location instead. So, if for any reason a source file that is not relevant to your executable is located at the original location, a substitution rule is the only method available to point GDB at the new location.

You can configure a default source path substitution rule by configuring GDB with the ‘`--with-relocated-sources=dir`’ option. The dir should be the name of a directory under GDB’s configured prefix (set with ‘`--prefix`’ or ‘`--exec-prefix`’), and directory names in debug information under dir will be adjusted automatically if the installed GDB is moved to a new location. This is useful if GDB, libraries or executables with debug information and corresponding source code are being moved together.

`directory dirname …`
`dir dirname …`

  Add directory dirname to the front of the source path. Several directory names may be given to this command, separated by ‘`:`’ (‘`;`’ on MS-DOS and MS-Windows, where ‘`:`’ usually appears as part of absolute file names) or whitespace. You may specify a directory that is already in the source path; this moves it forward, so GDB searches it sooner.

  The special strings ‘`$cdir`’ (to refer to the compilation directory, if one is recorded), and ‘`$cwd`’ (to refer to the current working directory) can also be included in the list of directories dirname. Though these will already be in the source path they will be moved forward in the list so GDB searches them sooner.

`directory`

  Reset the source path to its default value (‘`$cdir:$cwd`’ on Unix systems). This requires confirmation.

`set directories path-list` ¶

  Set the source path to path-list. ‘`$cdir:$cwd`’ are added if missing.

`show directories` ¶

  Print the source path: show which directories it contains.

`set substitute-path from to` ¶

  Define a source path substitution rule, and add it at the end of the current list of existing substitution rules. If a rule with the same from was already defined, then the old rule is also deleted.

  For example, if the file `/foo/bar/baz.c` was moved to `/mnt/cross/baz.c`, then the command

      (gdb) set substitute-path /foo/bar /mnt/cross

  will tell GDB to replace ‘`/foo/bar`’ with ‘`/mnt/cross`’, which will allow GDB to find the file `baz.c` even though it was moved.

  In the case when more than one substitution rule have been defined, the rules are evaluated one by one in the order where they have been defined. The first one matching, if any, is selected to perform the substitution.

  For instance, if we had entered the following commands:

      (gdb) set substitute-path /usr/src/include /mnt/include
      (gdb) set substitute-path /usr/src /mnt/src

  GDB would then rewrite `/usr/src/include/defs.h` into `/mnt/include/defs.h` by using the first rule. However, it would use the second rule to rewrite `/usr/src/lib/foo.c` into `/mnt/src/lib/foo.c`.

`unset substitute-path [path]` ¶

  If a path is specified, search the current list of substitution rules for a rule that would rewrite that path. Delete that rule if found. A warning is emitted by the debugger if no rule could be found.

  If no path is specified, then all substitution rules are deleted.

`show substitute-path [path]` ¶

  If a path is specified, then print the source path substitution rule which would rewrite that path, if any.

  If no path is specified, then print all existing source path substitution rules.

If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows:

1. Use `directory` with no argument to reset the source path to its default value.
2. Use `directory` with suitable arguments to reinstall the directories you want in the source path. You can add all the directories in one command.