ld "undefined reference to `clock_gettime'" missing -lrt in CI

By Daniel Zoghalchali·Latchkey

On glibc before 2.17, real-time functions such as clock_gettime and shm_open live in librt. Calling them without -lrt makes the linker report undefined references at link time.

What this error means

Code using clock_gettime or POSIX shared memory compiles but fails to link with undefined references on an older base image, because -lrt is not on the command line.

/usr/bin/ld: timer.o: in function `now':
timer.c:9: undefined reference to `clock_gettime'
collect2: error: ld returned 1 exit status

Common causes

How to fix it

Link -lrt after your objects

1. Append -lrt to the link command, after the objects that call clock_gettime.
2. Relink and confirm the symbols resolve.

gcc timer.o -o app -lrt

Link rt conditionally in CMake

Search for the rt library and link it only where it exists, so newer glibc images (which fold it into libc) are unaffected.

find_library(RT_LIB rt)
if(RT_LIB)
  target_link_libraries(app PRIVATE ${RT_LIB})
endif()

How to prevent it

• Link -lrt after the objects that use real-time functions on older glibc, and probe for the library in CMake so the build works on both old and new images.

Frequently asked questions

Related guides

References

• CMake documentation
• GitHub Actions documentation