Symbol Name Resolution

As explained in Syntax, a symbol identifier present in an expression context evaluates to the value of this symbol. The value typically denotes the virtual address of the storage associated with the symbol in the target's virtual address space. A target can support multiple symbol tables including, but not limited to,

• Primary executable symbol table

• Primary dynamic symbol table

• Runtime link-editor symbol table

• Standard and dynamic symbol tables for each of a number of load objects (such as shared libraries in a user process, or kernel modules in the Solaris kernel)

The target typically searches the primary executable's symbol tables first, then one or more of the other symbol tables. Notice that ELF symbol tables contain only entries for external, global, and static symbols; automatic symbols do not appear in the symbol tables processed by MDB.

Additionally, MDB provides a private user-defined symbol table that is searched prior to any of the target symbol tables. The private symbol table is initially empty, and can be manipulated using the ::nmadd and ::nmdel dcmds.

The ::nm -P option can be used to display the contents of the private symbol table. The private symbol table allows the user to create symbol definitions for program functions or data that were either missing from the original program or stripped out. These definitions are then used whenever MDB converts a symbolic name to an address, or an address to the nearest symbol.

Because targets contain multiple symbol tables, and each symbol table can include symbols from multiple object files, different symbols with the same name can exist. MDB uses the backquote “ ` “ character as a symbol-name scoping operator to allow the programmer to obtain the value of the desired symbol in this situation.

You can specify the scope used to resolve a symbol name as either: object`name, or file`name, or object`file`name. The object identifier refers to the name of a load object. The file identifier refers to the basename of a source file that has a symbol of type STT_FILE in the specified object's symbol table. The object identifier's interpretation depends on the target type.

The MDB kernel target expects object to specify the base name of a loaded kernel module. For example, the symbol name:

specfs`_init

evaluates to the value of the _init symbol in the specfs kernel module.

The mdb process target expects object to specify the name of the executable or of a loaded shared library. It can take any of the following forms:

• Exact match (that is, a full pathname): /usr/lib/libc.so.1

• Exact basename match: libc.so.1

• Initial basename match up to a ``.'' suffix: libc.so or libc

• Literal string a.out which is accepted as an alias for the executable

The process target will also accept any of the four forms described above preceded by an optional link-map id (lmid). The lmid prefix is specified by an initial LM followed by the link-map id in hexadecimal followed by an additional backquote. For example, the symbol name:

LM0`libc.so.1`_init

will evaluate to the value of the _init symbol in the libc.so.1 library that is loaded on link-map 0 (LM_ID_BASE). The link-map specifier may be necessary to resolve symbol naming conflicts in the event that the same library is loaded on more than one link map. For more information on link maps, refer to the Linker and Libraries Guide and the dlopen(3DL) man page. Link-map identifiers will be displayed when symbols are printed according to the setting of the showlmid option, as described under Summary of Command-line Options.

In the case of a naming conflict between symbols and hexadecimal integer values, MDB attempts to evaluate an ambiguous token as a symbol first, before evaluating it as an integer value. For example, the token f can refer either to the decimal integer value 15 specified in hexadecimal (the default base), or to a global variable named f in the target's symbol table. If a symbol with an ambiguous name is present, the integer value can be specified by using an explicit 0x or 0X prefix.