2.6.3 Weak Symbols

Weak symbols are symbols that may or may not be defined.

The linker processes symbols that are defined with a "weak" binding differently from symbols that are defined with global binding. Instead of including a weak symbol in the object file's symbol table (as it would for a global symbol), the linker only includes a weak symbol in the output of a "final" link if the symbol is required to resolve an otherwise unresolved reference.

This allows the linker to minimize the number of symbols it includes in the output file's symbol table by omitting those that are not needed to resolve references. Reducing the size of the output file's symbol table reduces the time required to link, especially if there are a large number of pre-loaded symbols to link against. This feature is particularly helpful for OpenCL applications.

You can define a weak symbol using either the .weak assembly directive or the weak operator in the linker command file.

• Using Assembly: To define a weak symbol in an input object file, the source file can be written in assembly. Use the .weak and .set directives in combination as shown in the following example, which defines a weak symbol "ext_addr_sym":
.weak ext_addr_sym ext_addr_sym .set 0x12345678

Assemble the source file that defines weak symbols, and include the resulting object file in the link. The "ext_addr_sym" in this example is available as a weak symbol in a final link. It is a candidate for removal if the symbol is not referenced elsewhere in the application. See .weak directive.

• Using the Linker Command File: To define a weak symbol in a linker command file, use the "weak" operator in an assignment expression to designate that the symbol as eligible for removal from the output file's symbol table if it is not referenced. In a linker command file, an assignment expression outside a MEMORY or SECTIONS directive can be used to define a weak linker-defined symbol. For example, you can define "ext_addr_sym" as follows:
weak(ext_addr_sym) = 0x12345678;

If the linker command file is used to perform the final link, then "ext_addr_sym" is presented to the linker as a weak symbol; it will not be included in the resulting output file if the symbol is not referenced. See Section 8.6.2.

• Using C/C++ code: See information about the WEAK pragma and weak GCC-style variable attribute in the ARM Optimizing C/C++ Compiler User's Guide.

If there are multiple definitions of the same symbol, the linker uses certain rules to determine which definition takes precedence. Some definitions may have weak binding and others may have strong binding. "Strong" in this context means that the symbol has not been given a weak binding by either of the two methods described above. Some definitions may come from an input object file (that is, using assembly directives) and others may come from an assignment statement in a linker command file.

The linker uses the following guidelines to determine which definition is used when resolving references to a symbol:

• A strongly bound symbol always takes precedence over a weakly bound symbol.
• If two symbols are both strongly bound or both weakly bound, a symbol defined in a linker command file takes precedence over a symbol defined in an input object file.
• If two symbols are both strongly bound and both are defined in an input object file, the linker provides a symbol redefinition error and halts the link process.