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What (if anything) does the C standard have to say about linking objects? My guess is that, because C only defines language->behavior rules, it completely ignores any potential intermediate form...
#2: Post edited
by
Josh Hyatt
·
2022-01-14T21:48:58Z (almost 3 years ago)
Clarify question intent and provide further examples
- What (if anything) does the C standard have to say about linking objects? My guess is that, because C only defines language->behavior rules, it completely ignores any potential intermediate form the code may take. Obviously, C doesn't guarantee anything about the executable itself because it may well be "compiled" down to something that *isn't an executable for a computer platform* or even something similar. That said, does C make any guarantees about interoperability between compiled output targets on a given platform? For example, if I add this to a source file:
- ```c
- extern int foo(void);
- ```
the resulting C program will expect to be able to jump to such a symbol but doesn't require any knowledge of the definition to successfully compile and output valid code. Does C require that the above *eventually* resolve to an executable entity (i.e. during linking), or is C itself completely indifferent to any linking mechanics that take place after compilation? If the former, what guarantees does C make?
- What (if anything) does the C standard have to say about linking objects? My guess is that, because C only defines language->behavior rules, it completely ignores any potential intermediate form the code may take. Obviously, C doesn't guarantee anything about the executable itself because it may well be "compiled" down to something that *isn't an executable for a computer platform* or even something similar. That said, does C make any guarantees about interoperability between compiled output targets on a given platform? For example, if I add this to a source file:
- ```c
- extern int foo(void);
- ```
- the resulting C program will expect to be able to jump to such a symbol but doesn't require any knowledge of the definition to successfully compile and output valid code. Does C require that the above *eventually* resolve to an executable entity (i.e. during linking), or is C itself completely indifferent to any linking mechanics that take place after compilation? If the former, what guarantees does C make?
- EDIT: Following some commentary, I think the question can be further clarified.
- In a freestanding C program, there is no I/O capability, simply due to the fact that the language imposes no restrictions whatsoever on the capabilities of a compliant C implementation without a hosted environment like an operating system. In practice, to perform a processor I/O call on an x86 machine in a kernel program, a call must be made out of the C code to assembly (or something similar). An assembly module is by no means, of course, required to comply in any way to the C standard. Can such a module be considered a separate translation unit? According to the standard, I'd assume it cannot. The way the C module and the assembly module are linked together is completely dependent on the environment and implementation.
- Having said all this, consider the above function `foo` once more. If this function is defined in assembly, it is not part of any C translation unit, and therefore can only be linked by relying on implementation details. My question is this: is this environment-specific linking behavior implementation-defined? If so, there should be reference to it within the C standard somewhere, and I'd like to know exactly what can be said about linking C translation units with other non-C libraries. However, intuitively I'd assume that, since the definition of `foo` is not contained within any C translation unit, the C program should not be valid. This would mean that every kernel ever written that has any I/O of any kind is not standards-compliant C. That doesn't necessarily matter in practice because a non-compliant program that does its job is just as useful as a compliant one. My question is purely theoretical: is my analysis correct? Is a compliant freestanding C program unable to produce observable results? Or am I missing something in the standard that relates to linking against something that is not a C translation unit?
#1: Initial revision
by
Josh Hyatt
·
2022-01-13T20:51:05Z (almost 3 years ago)
C Language Standard Linking Specifications
What (if anything) does the C standard have to say about linking objects? My guess is that, because C only defines language->behavior rules, it completely ignores any potential intermediate form the code may take. Obviously, C doesn't guarantee anything about the executable itself because it may well be "compiled" down to something that *isn't an executable for a computer platform* or even something similar. That said, does C make any guarantees about interoperability between compiled output targets on a given platform? For example, if I add this to a source file: ```c extern int foo(void); ``` the resulting C program will expect to be able to jump to such a symbol but doesn't require any knowledge of the definition to successfully compile and output valid code. Does C require that the above *eventually* resolve to an executable entity (i.e. during linking), or is C itself completely indifferent to any linking mechanics that take place after compilation? If the former, what guarantees does C make?