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Symbol (Computer Systems)

  • Undefined Symbol Vtable

There are three types of Symbols: Global Symbols, External Symbols, and Local Symbols.

Global Symbols are symbols that are not static. They are usually function names and global variables.

External Global Symbols are also not static symbols, but they are undefined. These can include global variables with extern (so we don't know the value) or function prototypes (so we don't see the function)

Local Symbols are static symbols — static global variables and functions. Note that these are not local variables! Those are in the stack. Duplicate local symbol causes an error.

When there are multiple global variables, we classify them into strong and weak symbols. Weak variables mean uninitialized variables. When there are multiple "strong" symbols, it errors. When there are strong and weak symbols combined, the linker chooses the strong one. The linker will arbitrarily select the symbol when there are multiple weak symbols, depending on the implementation.

When dealing with global variables, it is good to

  • use static if possible
  • initialize global variables (take advantage of the linker for dx)
  • use extern keyword when referencing outside static global variables