undefined reference to 'function'

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Include source file in compilation
g++ main.cpp utils.cpp -o program
# Check for missing library linkage
g++ main.cpp -lmylib -L/path/to/lib
# In CMake, add source files
add_executable(app main.cpp utils.cpp)
target_link_libraries(app mylib)

How to diagnose C++ errors

C++ errors fall into two very different worlds. Compile and link errors (undefined reference, multiple definition, template substitution failure) are deterministic and are almost always about declarations, the One Definition Rule, or link order. Runtime memory errors (segfaults, double free, use-after-free) are non-deterministic and should never be debugged by reading code alone; a sanitizer will find in seconds what code review misses for days.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Rebuild with sanitizers before anything else: -fsanitize=address,undefined -fno-omit-frame-pointer -g. AddressSanitizer reports the exact allocation and free sites for use-after-free and double-free.
2. For undefined-reference errors, check link order, remembering that with GNU ld libraries must come after the objects that use them, and check for a C/C++ linkage mismatch that needs extern "C".
3. Use nm -C libfoo.a | grep symbol to confirm the symbol is actually present and to see the demangled signature. A signature that differs by a const is a different symbol.
4. Enable core dumps (ulimit -c unlimited) and open them in gdb with gdb ./binary core, then bt full. A stack trace beats speculation.
5. For template errors, read the message from the bottom up. The final line is usually the real constraint that failed; everything above is instantiation context.

Tools worth reaching for

• -fsanitize=address,undefined
• valgrind
• gdb / lldb
• nm -C
• ldd

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 gcc.gnu.org

Related C++ errors

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• C++: runtime error: signed integer overflow (UBSan)Signed integer overflow is undefined behaviour in C and C++. The program may appear to work…
• C++: Segmentation fault (core dumped)Program attempted to access memory it doesn't own (e.g., dereferencing null pointer, buffer…
• C++: undefined reference to `vtable for X'This is not a missing call to the named function. The compiler emits a class's vtable…
• CMake Error: Could not find a package configuration file provided by Xfind_package looked for a config file the library installs, XConfig.cmake or x-config.cmake…
• double free or corruptionHeap memory was freed twice, corrupting the memory allocator. Often caused by manual delete…
• heap-use-after-freeAccessing memory after it has been freed. Detected by AddressSanitizer. Causes undefined…
• multiple definition of 'symbol'The same symbol is defined in multiple translation units. Usually caused by defining…

See all 11 C++ errors →

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This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.