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### 9.5 Data file relocation to support cross-profiling

Running the program will cause profile output to be generated. For each source file compiled with `-fprofile-arcs`, an accompanying `.gcda` file will be placed in the object file directory. That implicitly requires running the program on the same system as it was built or having the same absolute directory structure on the target system. The program will try to create the needed directory structure, if it is not already present.

To support cross-profiling, a program compiled with `-fprofile-arcs` can relocate the data files based on two environment variables:

- GCOV_PREFIX contains the prefix to add to the absolute paths in the object file. Prefix must be absolute as well, otherwise its value is ignored. The default is no prefix.
- GCOV_PREFIX_STRIP indicates the how many initial directory names to strip off the hardwired absolute paths. Default value is 0.

  Note: GCOV_PREFIX_STRIP has no effect if GCOV_PREFIX is undefined, empty or non-absolute.

For example, if the object file `/user/build/foo.o` was built with `-fprofile-arcs`, the final executable will try to create the data file `/user/build/foo.gcda` when running on the target system. This will fail if the corresponding directory does not exist and it is unable to create it. This can be overcome by, for example, setting the environment as ‘`GCOV_PREFIX=/target/run`’ and ‘`GCOV_PREFIX_STRIP=1`’. Such a setting will name the data file `/target/run/build/foo.gcda`.

You must move the data files to the expected directory tree in order to use them for profile directed optimizations (`--use-profile`), or to use the `gcov` tool.