Скачать или смотреть Understanding the -l and -c Options in GCC Compilation

A deep dive into the necessity of the `-l` library option with the `-c` compile-only flag, plus insights on the `-fPIC` flag in GCC.
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Understanding the -l and -c Options in GCC Compilation

When working with GCC (GNU Compiler Collection), developers often face various flags and options that determine how their code is compiled and linked. Two such options that frequently come up are -l for linking libraries and -c for compile-only operations. This guide aims to clarify the relationship between these options and clarify when to use them effectively, especially in the context of makefile optimization.

The Problem: Confusion Around -l and -c

A common question among developers is whether it is necessary to use the -l library option when invoking the -c (compile-only) option in GCC. For instance, if your C program foo.c uses functions from a custom library libbar.so, do you need to specify -lbar during the compilation phase if you are only meant to generate an object file (foo.o)?

This question arises frequently, especially while cleaning up makefiles, leading some to wonder if the presence of -l options is merely the result of poor craftsmanship or if it serves a genuine purpose.

The Solution: When to Use -l with -c

Compile-Only Mode (-c)

The -c option tells GCC to compile the specified source file into an object file without linking it. Here’s what you need to remember:

No -l Needed: When using the -c option, you do not need to include the -lbar library option. This is because linking is not occurring at this stage, and GCC is simply generating an object file (e.g., foo.o).

Include the Header File: You need to ensure that the compiler has access to the appropriate header files (like bar.h) during the compilation to recognize the functions declared within it.

Linking Stage

The actual linking happens later in the build process when you create the final executable or shared object. At that point, you must include the -l option to ensure that the linker can resolve the external symbols provided by libbar.so.

Conclusion on -l Usage

During Compilation: No -l option needed with -c.

During Linking: Yes, include -lbar to link against the library.

The Role of -fPIC in Compilation

When dealing with position-independent code (PIC), especially for shared libraries, you might wonder when to use the -fPIC option.

Understanding -fPIC

The -fPIC option is necessary when you are compiling source files into object files that will eventually be included in a shared library. It allows the code to be loaded at any address in memory, which is a requirement for shared libraries.

When to Use -fPIC

Compilation Phase (with -c): You should include -fPIC during the compilation of foo.c if foo.o will contribute to a shared library later. This means -fPIC is necessary at compile time if you plan to use the compiled object files in a shared library context.

Later Stages: You do not need -fPIC at the linking stage when you are bundling the object files together, but you must ensure that any object files intended for a shared library compilation were compiled with -fPIC.

Summary on -fPIC Usage

During Compilation: Include -fPIC if you intend for the object file to be part of a shared library.

During Linking: Do not include -fPIC at this stage.



By understanding the appropriate contexts for using the -l and -c options along with -fPIC, developers can optimize their makefiles and compilation processes. This clarity not only enhances code quality but also fosters better collaboration on compilation workflows.

Happy coding!