CODEWITHSUNDEEP
clinuxlinkerlinkage
user3518295
user3518295

Reputation: 328

Why wrong function was linked by the linker?

I am very confused about the following situation:

I have Linux project (which is written in c) that uses a static library A to get an executable. In this static library A there is a c code which calls sem_init from Linux include. There is include to semaphore.h in the .c file, the file compiled using GCC without warnings. I use ld linker.

Everything was fine until I have added unintentionally a new module(which compiled to the static library B) to the project. New code has a function with the same name (sem_init) but a different signature. Code from A does not include .h file from B that defines the new sem_init().

Now, after the addition of library B to the compilation, I saw that instead of sem_init from semaphore.h, the new sem_init was called from library B.

I know that calling functions with the same name is an awful idea, but within a large project, it can happen unintentionally.

The question: why did this happen??? Why linker has chosen the new sem_init??? Why I did not get a multiple definition linkage error?

Upvotes: 0

Views: 830

Answers (1)

Maxim Egorushkin
Maxim Egorushkin

Reputation: 136237

When the linker is linking static libraries it looks there for undefined symbols only and if it finds a definition, then it links it in and resolves the symbol. sem_init happens to come from B which is linked before -lc (the GNU C standard library which also implements POSIX sem_init) is linked, and hence the linker picks up sem_init from B and then never looks for sem_init again, this is why you don't get multiple symbol definition error.

B library probably doesn't intend to implement POSIX sem_init with a different parameter list. It probably needs to mark its sem_init as static so that it is not visible to other translation units. And/or use a completely different name for it.

Upvotes: 2

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