CMake "Syntax error in cmake code ... Invalid character escape" in CI

By Kaveh Alemi·Latchkey

CMake treats backslash as an escape character inside quoted arguments. A literal Windows path like "C:\src" makes it report "Invalid character escape" and stop parsing the listfile.

What this error means

Configuration on a Windows runner fails with a syntax error pointing at a quoted path or string that contains a backslash CMake cannot interpret.

CMake Error at CMakeLists.txt:7:
  Syntax error in cmake code at

    C:/proj/CMakeLists.txt:7

  when parsing string

    C:\build\out

  Invalid character escape '\b'.

Common causes

How to fix it

Use forward slashes in CMake paths

1. Replace backslashes with forward slashes, which CMake accepts on Windows.
2. Reconfigure and confirm the listfile parses.

# instead of "C:\build\out"
set(OUT_DIR "C:/build/out")

Normalize incoming paths

When a path arrives from the environment, normalize it with file(TO_CMAKE_PATH) so backslashes become slashes before use.

file(TO_CMAKE_PATH "$ENV{BUILD_DIR}" OUT_DIR)

How to prevent it

• Always write paths with forward slashes in CMake code and run external paths through file(TO_CMAKE_PATH) so backslash escapes never reach the parser.

Frequently asked questions

Related guides

References

• CMake documentation
• GitHub Actions documentation