Ruby bcrypt Gem Build Fails - Missing Compiler in CI

The bcrypt gem compiles a small self-contained C extension with no external library dependency. When it fails to build, the cause is almost always a missing C compiler or the Ruby headers - not a missing system library.

What this error means

bundle install fails building bcrypt’s native extension. Because bcrypt bundles its own C source, the error points at a missing compiler (gcc/clang) or missing Ruby headers rather than an absent -lXXX library.

Installing bcrypt 3.1.20 with native extensions
ERROR: Failed to build gem native extension.

make: gcc: No such file or directory
make: *** [Makefile:245: bcrypt_ext.o] Error 127

Common causes

No C compiler on the runner

bcrypt needs gcc/clang to compile its bundled C source. A slim or Alpine image without a compiler fails the build immediately, since there is no precompiled gem to fall back to in many setups.

Ruby development headers missing

Even with a compiler present, the extension needs the Ruby headers (ruby-dev) to compile against the interpreter. Without them mkmf cannot configure the build.

How to fix it

Install the compiler and Ruby headers

# Debian/Ubuntu
apt-get update && apt-get install -y build-essential ruby-dev
# Alpine
apk add --no-cache build-base ruby-dev

Confirm the toolchain works

A quick check that gcc and the headers are present catches the problem before bundle install.

gcc --version
ruby -e 'puts RbConfig::CONFIG["rubyhdrdir"]'

How to prevent it

• Bake build-essential and ruby-dev into images that build bcrypt.
• Use a full base image rather than slim when native gems are required.
• Cache vendor/bundle so the compile only happens when the gem changes.

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