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5.3 Symbol Names

Symbol names begin with a letter or with one of ‘._’. On most machines, you can also use $ in symbol names; exceptions are noted in Machine Dependencies. That character may be followed by any string of digits, letters, dollar signs (unless otherwise noted for a particular target machine), and underscores.

Case of letters is significant: foo is a different symbol name than Foo.

Multibyte characters are supported. To generate a symbol name containing multibyte characters enclose it within double quotes and use escape codes. cf See Strings. Generating a multibyte symbol name from a label is not currently supported.

Each symbol has exactly one name. Each name in an assembly language program refers to exactly one symbol. You may use that symbol name any number of times in a program.

Local Symbol Names

A local symbol is any symbol beginning with certain local label prefixes. By default, the local label prefix is ‘.L’ for ELF systems or ‘L’ for traditional a.out systems, but each target may have its own set of local label prefixes. On the HPPA local symbols begin with ‘L$’.

Local symbols are defined and used within the assembler, but they are normally not saved in object files. Thus, they are not visible when debugging. You may use the ‘-L’ option (see Include Local Symbols: -L) to retain the local symbols in the object files.

Local Labels

Local labels help compilers and programmers use names temporarily. They create symbols which are guaranteed to be unique over the entire scope of the input source code and which can be referred to by a simple notation. To define a local label, write a label of the form ‘N:’ (where N represents any positive integer). To refer to the most recent previous definition of that label write ‘Nb’, using the same number as when you defined the label. To refer to the next definition of a local label, write ‘Nf’—the ‘b’ stands for “backwards” and the ‘f’ stands for “forwards”.

There is no restriction on how you can use these labels, and you can reuse them too. So that it is possible to repeatedly define the same local label (using the same number ‘N’), although you can only refer to the most recently defined local label of that number (for a backwards reference) or the next definition of a specific local label for a forward reference. It is also worth noting that the first 10 local labels (‘0:...9:’) are implemented in a slightly more efficient manner than the others.

Here is an example:

     1:        branch 1f
     2:        branch 1b
     1:        branch 2f
     2:        branch 1b

Which is the equivalent of:

     label_1:  branch label_3
     label_2:  branch label_1
     label_3:  branch label_4
     label_4:  branch label_3

Local label names are only a notational device. They are immediately transformed into more conventional symbol names before the assembler uses them. The symbol names are stored in the symbol table, appear in error messages, and are optionally emitted to the object file. The names are constructed using these parts:

local label prefix
All local symbols begin with the system-specific local label prefix. Normally both as and ld forget symbols that start with the local label prefix. These labels are used for symbols you are never intended to see. If you use the ‘-L’ option then as retains these symbols in the object file. If you also instruct ld to retain these symbols, you may use them in debugging.
This is the number that was used in the local label definition. So if the label is written ‘55:’ then the number is ‘55’.
This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value of ‘\002’ (control-B).
ordinal number
This is a serial number to keep the labels distinct. The first definition of ‘0:’ gets the number ‘1’. The 15th definition of ‘0:’ gets the number ‘15’, and so on. Likewise the first definition of ‘1:’ gets the number ‘1’ and its 15th definition gets ‘15’ as well.

So for example, the first 1: may be named .L1C-B1, and the 44th 3: may be named .L3C-B44.

Dollar Local Labels

as also supports an even more local form of local labels called dollar labels. These labels go out of scope (i.e., they become undefined) as soon as a non-local label is defined. Thus they remain valid for only a small region of the input source code. Normal local labels, by contrast, remain in scope for the entire file, or until they are redefined by another occurrence of the same local label.

Dollar labels are defined in exactly the same way as ordinary local labels, except that they have a dollar sign suffix to their numeric value, e.g., ‘55$:’.

They can also be distinguished from ordinary local labels by their transformed names which use ASCII character ‘\001’ (control-A) as the magic character to distinguish them from ordinary labels. For example, the fifth definition of ‘6$’ may be named ‘.L6C-A5’.