Is there a way to limit the use of a function to its library in C?

So I'm working on a static C library (like a library.a file) for a school project. There are multiple functions, some of which are placed in different files, so I can't use the static keyword. Is there a way that those functions could be limited to the library itself, an equivalent to static for libraries?

The C language does not have any formal sense of a unit of program organization larger than a single translation unit but smaller than a whole program. Libraries are a foreign concept to the language specification, provided by substantially all toolchains, but not part of the language itself. Thus, no, the C language does not define a mechanism other than static to declare that a function identifier can be referenced only by a proper subset of all translation units contributing to a program.

Such limitations are supported by some shared library formats, such as ELF, and it is common for C implementations targeting such shared libraries to provide extensions that enable those facilities to be engaged, but the same generally is not true for static libraries.

Note also that in all these cases, we're talking about the linkage of function identifiers, not about genuinely controlling access to the functions. In principle, any function in the program can be called from anywhere in the program via a function pointer pointing to it.

Frame challenge: why do you care?

The usual accommodation for having functions with external linkage that you don't want library clients to call directly would be to omit those functions from the library's public header files. What does it matter if some intrepid person can analyze the library to discover and possibly call those functions? Your public headers and documentation tell people how the library should be used. If people use it in other ways then that's on them.

It may not be possible to completely "hide" the existence of a set of restricted functions from other source files if they are defined with external linkage, since their identifiers will be visible when linking (as noted in the other answer).

However, if you are only looking to prevent someone from inadvertently calling restricted functions, one of these approaches may be useful:

1. In some of my projects, I have used #define and #ifdef statements to block restricted functions from being used throughout the program. For example, in my Hardware Abstraction Layer (HAL) library C source files, I typically place #define HAL__ prior to any #include statements. Then I place an #ifdef HAL__ ... #endif block around any restricted function definitions in my header files. Someone with intention could easily easily bypass this by adding #define HAL__ to their source code or by modifying the header file, but it provides some protection against unintentional use of restricted functions and other definitions.
2. Place the restricted function definitions in a separate header file used to build the library itself (for example library.a) and provide only header files containing non-restricted function declarations with the library. The identifiers for any functions defined will still be visible to the linker, but without the prototypes, it will be difficult for anyone to call them.

Again, if having the identifiers for any restricted functions visible throughout the program would be a problem (for example, duplicating other identifiers), then these options will not work. Also, if the goal is to prevent developers from intentionally calling restricted functions, then these options will not work, although option 2 would make this more difficult. If the intention is only to prevent unintentional calls to the restricted functions and there is no concern with having duplicate identifiers in the program, then these options may help.