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specs/sysv-abi-update.html/ch4.symtab.html
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specs/sysv-abi-update.html/ch4.symtab.html
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<html>
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<title>Symbol Table</title><p>
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<h1>Symbol Table</h1><p>
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An object file's symbol table holds information
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needed to locate and relocate a program's symbolic
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definitions and references.
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A symbol table index is a subscript into this array.
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Index 0 both designates the first entry in the table
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and serves as the undefined symbol index. The contents of the
|
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initial entry are specified later in this section.
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<p>
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<table border cellspacing=0>
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<th>Name</th>
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<th>Value</th>
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<tr>
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<td><code>STN_UNDEF</code></td>
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<td align=center><code>0</code></td>
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</tr>
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</table>
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<p>
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A symbol table entry has the following format.
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<hr>
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<b>Figure 4-16: Symbol Table Entry</b>
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<p>
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<pre>
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<code>
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typedef struct {
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Elf32_Word st_name;
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Elf32_Addr st_value;
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Elf32_Word st_size;
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unsigned char st_info;
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unsigned char st_other;
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Elf32_Half st_shndx;
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} Elf32_Sym;
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typedef struct {
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Elf64_Word st_name;
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unsigned char st_info;
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unsigned char st_other;
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Elf64_Half st_shndx;
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Elf64_Addr st_value;
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Elf64_Xword st_size;
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} Elf64_Sym;
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</pre>
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</code>
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<hr>
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<p>
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<DL COMPACT>
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<p><dt><code>st_name</code><dd>
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This member holds an index into the object file's
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symbol string table, which
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holds the character representations of the symbol names.
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If the value is non-zero, it represents a string table
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index that gives the symbol name.
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Otherwise, the symbol table entry has no name.
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</dl>
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<hr>
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<img src=warning.gif alt="NOTE:">
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External C symbols have the same names in C
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and object files' symbol tables.
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<hr><p>
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<DL COMPACT>
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<p><dt><code>st_value</code><dd>
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This member gives the value of the associated symbol.
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Depending on the context, this may be an absolute value,
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an address, and so on; details appear below.
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<p><dt><code>st_size</code><dd>
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Many symbols have associated sizes.
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For example, a data object's size is the number
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of bytes contained in the object.
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This member holds 0 if the symbol has no size or an unknown size.
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<p><dt><code>st_info</code><dd>
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This member specifies the symbol's type and binding attributes.
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A list of the values and meanings appears below.
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The following code shows how to manipulate the values for
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both 32 and 64-bit objects.
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<hr>
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<PRE>
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#define ELF32_ST_BIND(i) ((i)>>4)
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#define ELF32_ST_TYPE(i) ((i)&0xf)
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#define ELF32_ST_INFO(b,t) (((b)<<4)+((t)&0xf))
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#define ELF64_ST_BIND(i) ((i)>>4)
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#define ELF64_ST_TYPE(i) ((i)&0xf)
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#define ELF64_ST_INFO(b,t) (((b)<<4)+((t)&0xf))
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</PRE>
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<hr>
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<a name=st_other></a>
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<p><dt><code>st_other</code><dd>
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This member currently specifies a symbol's visibility.
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A list of the values and meanings appears <a href=#visibility>below</a>.
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The following code shows how to manipulate the values for
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both 32 and 64-bit objects. Other bits contain 0 and have
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no defined meaning.
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<hr>
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<PRE>
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#define ELF32_ST_VISIBILITY(o) ((o)&0x3)
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#define ELF64_ST_VISIBILITY(o) ((o)&0x3)
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</PRE>
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<hr>
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<p><dt><code>st_shndx</code><dd>
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Every symbol table entry is <i>defined</i> in relation
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to some section. This member holds the relevant
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section header table index.
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As the <code>sh_link</code> and <code>sh_info</code> interpretation
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<a href=ch4.sheader.html#sh_link>table</a>
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and the related text describe,
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some section indexes indicate special meanings.
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||||
<p>
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If this member contains <code>SHN_XINDEX</code>,
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then the actual section header index is too large to fit in this field.
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The actual value is contained in the associated
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section of type <code>SHT_SYMTAB_SHNDX</code>.
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</dl>
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<p>
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A symbol's binding determines the linkage visibility
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and behavior.
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<hr>
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<b>Figure 4-17: Symbol Binding</b>
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<p>
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<table border cellspacing=0>
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<th>Name</th>
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<th>Value</th>
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<tr>
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<td><code>STB_LOCAL</code></td>
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<td align=right><code>0</code></td>
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</tr>
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<tr>
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<td><code>STB_GLOBAL</code></td>
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<td align=right><code>1</code></td>
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</tr>
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<tr>
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<td><code>STB_WEAK</code></td>
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<td align=right><code>2</code></td>
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</tr>
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<tr>
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<td><code>STB_LOOS</code></td>
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<td align=right><code>10</code></td>
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</tr>
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<tr>
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<td><code>STB_HIOS</code></td>
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<td align=right><code>12</code></td>
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</tr>
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<tr>
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<td><code>STB_LOPROC</code></td>
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<td align=right><code>13</code></td>
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</tr>
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<tr>
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<td><code>STB_HIPROC</code></td>
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<td align=right><code>15</code></td>
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</tr>
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</table>
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<hr>
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<dl compact>
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<p><dt><code>STB_LOCAL</code><dd>
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Local symbols are not visible outside the object file
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containing their definition.
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Local symbols of the same name may exist in
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multiple files without interfering with each other.
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||||
<p><dt><code>STB_GLOBAL</code><dd>
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Global symbols are visible to all object files being combined.
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One file's definition of a global symbol will satisfy
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another file's undefined reference to the same global symbol.
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<p><dt><code>STB_WEAK</code><dd>
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Weak symbols resemble global symbols, but their
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definitions have lower precedence.
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<p><dt><code>STB_LOOS</code> through <code>STB_HIOS</code><dd>
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||||
Values in this inclusive range
|
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are reserved for operating system-specific semantics.
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||||
<p><dt><code>STB_LOPROC</code> through <code>STB_HIPROC</code><dd>
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||||
Values in this inclusive range
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are reserved for processor-specific semantics. If meanings are
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||||
specified, the processor supplement explains them.
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</dl>
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<p>
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Global and weak symbols differ in two major ways.
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<ul>
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<p><li>
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When the link editor combines several relocatable object files,
|
||||
it does not allow multiple definitions of <code>STB_GLOBAL</code>
|
||||
symbols with the same name.
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On the other hand, if a defined global symbol exists,
|
||||
the appearance of a weak symbol with the same name
|
||||
will not cause an error.
|
||||
The link editor honors the global definition and ignores
|
||||
the weak ones.
|
||||
Similarly, if a common symbol exists
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||||
(that is, a symbol whose <code>st_shndx</code>
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||||
field holds <code>SHN_COMMON</code>),
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||||
the appearance of a weak symbol with the same name will
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||||
not cause an error.
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||||
The link editor honors the common definition and
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||||
ignores the weak ones.
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||||
<p><li>
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||||
When the link editor searches archive libraries [see ``Archive File''
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||||
in Chapter 7],
|
||||
it extracts archive members that contain definitions of
|
||||
undefined global symbols.
|
||||
The member's definition may be either a global or a weak symbol.
|
||||
The link editor does not
|
||||
extract archive members to resolve undefined weak symbols.
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||||
Unresolved weak symbols have a zero value.
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||||
</ul>
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||||
<a name="weak_note"></a>
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<hr>
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<img src=warning.gif alt="NOTE:">
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||||
The behavior of weak symbols in areas not specified by this document is
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||||
implementation defined.
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Weak symbols are intended primarily for use in system software.
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Applications using weak symbols are unreliable
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||||
since changes in the runtime environment
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||||
might cause the execution to fail.
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<hr><p>
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||||
In each symbol table, all symbols with <code>STB_LOCAL</code>
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binding precede the weak and global symbols.
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As
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<a href=ch4.sheader.html>``Sections''</a>,
|
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above describes,
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a symbol table section's <code>sh_info</code>
|
||||
section header member holds the symbol table index
|
||||
for the first non-local symbol.
|
||||
<p>
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||||
A symbol's type provides a general classification for
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||||
the associated entity.
|
||||
<hr>
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<b>Figure 4-18: Symbol Types</b>
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||||
<p>
|
||||
<table border cellspacing=0>
|
||||
<th>Name</th>
|
||||
<th>Value</th>
|
||||
<tr>
|
||||
<td><code>STT_NOTYPE</code></td>
|
||||
<td align=right><code>0</code></td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>STT_OBJECT</code></td>
|
||||
<td align=right><code>1</code></td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>STT_FUNC</code></td>
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||||
<td align=right><code>2</code></td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>STT_SECTION</code></td>
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||||
<td align=right><code>3</code></td>
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||||
</tr>
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||||
<tr>
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||||
<td><code>STT_FILE</code></td>
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||||
<td align=right><code>4</code></td>
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||||
</tr>
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||||
<tr>
|
||||
<td><code>STT_COMMON</code></td>
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||||
<td align=right><code>5</code></td>
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||||
</tr>
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||||
<tr>
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||||
<td><code>STT_TLS</code></td>
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||||
<td align=right><code>6</code></td>
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||||
</tr>
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||||
<tr>
|
||||
<td><code>STT_LOOS</code></td>
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||||
<td align=right><code>10</code></td>
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||||
</tr>
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||||
<tr>
|
||||
<td><code>STT_HIOS</code></td>
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||||
<td align=right><code>12</code></td>
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||||
</tr>
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||||
<tr>
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||||
<td><code>STT_LOPROC</code></td>
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||||
<td align=right><code>13</code></td>
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||||
</tr>
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||||
<tr>
|
||||
<td><code>STT_HIPROC</code></td>
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||||
<td align=right><code>15</code></td>
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||||
</tr>
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||||
</table>
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||||
<hr>
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||||
<p>
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<DL COMPACT>
|
||||
<p><dt><code>STT_NOTYPE</code><dd>
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The symbol's type is not specified.
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<p><dt><code>STT_OBJECT</code><dd>
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The symbol is associated with a data object,
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such as a variable, an array, and so on.
|
||||
<p><dt><code>STT_FUNC</code><dd>
|
||||
The symbol is associated with a function or other executable code.
|
||||
<p><dt><code>STT_SECTION</code><dd>
|
||||
The symbol is associated with a section.
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||||
Symbol table entries of this type exist primarily for relocation
|
||||
and normally have <code>STB_LOCAL</code> binding.
|
||||
<p><dt><code>STT_FILE</code><dd>
|
||||
Conventionally, the symbol's name gives the name of
|
||||
the source file associated with the object file.
|
||||
A file symbol has <code>STB_LOCAL</code>
|
||||
binding, its section index is <code>SHN_ABS</code>,
|
||||
and it precedes the other <code>STB_LOCAL</code>
|
||||
symbols for the file, if it is present.
|
||||
<p><dt><code>STT_COMMON</code><dd>
|
||||
The symbol labels an uninitialized common block.
|
||||
See <a href=#stt_common>below</a> for details.
|
||||
<a name=stt_tls></a>
|
||||
<p><dt><code>STT_TLS</code><dd>
|
||||
The symbol specifies a <i>Thread-Local Storage</i> entity.
|
||||
When defined, it gives the assigned offset for the symbol,
|
||||
not the actual address.
|
||||
Symbols of type <code>STT_TLS</code> can be referenced
|
||||
by only special thread-local storage relocations
|
||||
and thread-local storage relocations can only reference
|
||||
symbols with type <code>STT_TLS</code>.
|
||||
Implementation need not support thread-local storage.
|
||||
<p><dt><code>STT_LOOS</code> through <code>STT_HIOS</code><dd>
|
||||
Values in this inclusive range
|
||||
are reserved for operating system-specific semantics.
|
||||
<p><dt><code>STT_LOPROC</code> through <code>STT_HIPROC</code><dd>
|
||||
Values in this inclusive range
|
||||
are reserved for processor-specific semantics.
|
||||
If meanings are specified, the processor supplement explains
|
||||
them.
|
||||
</dl>
|
||||
<p>
|
||||
Function symbols (those with type
|
||||
<code>STT_FUNC</code>) in shared object files have special significance.
|
||||
When another object file references a function from
|
||||
a shared object, the link editor automatically creates a procedure
|
||||
linkage table entry for the referenced symbol.
|
||||
Shared object symbols with types other than
|
||||
<code>STT_FUNC</code> will not
|
||||
be referenced automatically through the procedure linkage table.
|
||||
<a name=stt_common></a>
|
||||
<p>
|
||||
Symbols with type <code>STT_COMMON</code> label uninitialized
|
||||
common blocks. In relocatable objects, these symbols are
|
||||
not allocated and must have the special section index
|
||||
<code>SHN_COMMON</code> (see <a href=#shn_common>below</a>).
|
||||
In shared objects and executables these symbols must be
|
||||
allocated to some section in the defining object.
|
||||
<p>
|
||||
In relocatable objects, symbols with type <code>STT_COMMON</code>
|
||||
are treated just as other symbols with index <code>SHN_COMMON</code>.
|
||||
If the link-editor allocates space for the <code>SHN_COMMON</code>
|
||||
symbol in an output section of the object it is producing, it
|
||||
must preserve the type of the output symbol as <code>STT_COMMON</code>.
|
||||
<p>
|
||||
When the dynamic linker encounters a reference to a symbol
|
||||
that resolves to a definition of type <code>STT_COMMON</code>,
|
||||
it may (but is not required to) change its symbol resolution
|
||||
rules as follows: instead of binding the reference to
|
||||
the first symbol found with the given name, the dynamic linker searches
|
||||
for the first symbol with that name with type other
|
||||
than <code>STT_COMMON</code>. If no such symbol is found,
|
||||
it looks for the <code>STT_COMMON</code> definition of that
|
||||
name that has the largest size.
|
||||
<a name=visibility></a>
|
||||
<p>
|
||||
A symbol's visibility, although it may be specified in a relocatable
|
||||
object, defines how that symbol may be accessed once it has
|
||||
become part of an executable or shared object.
|
||||
<hr>
|
||||
<b>Figure 4-19: Symbol Visibility</b>
|
||||
<p>
|
||||
<table border cellspacing=0>
|
||||
<th>Name</th>
|
||||
<th>Value</th>
|
||||
<tr>
|
||||
<td><code>STV_DEFAULT</code></td>
|
||||
<td align=right><code>0</code></td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>STV_INTERNAL</code></td>
|
||||
<td align=right><code>1</code></td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>STV_HIDDEN</code></td>
|
||||
<td align=right><code>2</code></td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>STV_PROTECTED</code></td>
|
||||
<td align=right><code>3</code></td>
|
||||
</tr>
|
||||
</table>
|
||||
<hr>
|
||||
<p>
|
||||
<DL COMPACT>
|
||||
<p><dt><code>STV_DEFAULT</code><dd>
|
||||
The visibility of symbols with the <code>STV_DEFAULT</code>
|
||||
attribute is as specified by the symbol's binding type.
|
||||
That is, global and weak symbols are visible
|
||||
outside of their defining <i>component</i>
|
||||
(executable file or shared object).
|
||||
Local symbols are <i>hidden</i>, as described below.
|
||||
Global and weak symbols are also <i>preemptable</i>,
|
||||
that is, they may by preempted by definitions of the same
|
||||
name in another component.
|
||||
<hr>
|
||||
<img src=warning.gif alt="NOTE:">
|
||||
An implementation may restrict the set of global and weak
|
||||
symbols that are externally visible.
|
||||
<hr><p>
|
||||
<p><dt><code>STV_PROTECTED</code><dd>
|
||||
A symbol defined in the current component is <i>protected</i>
|
||||
if it is visible in other components but not preemptable,
|
||||
meaning that any reference to such a symbol from within the
|
||||
defining component must be resolved to the definition in
|
||||
that component, even if there is a definition in another
|
||||
component that would preempt by the default rules.
|
||||
A symbol with <code>STB_LOCAL</code> binding may not have
|
||||
<code>STV_PROTECTED</code> visibility.
|
||||
<a name=protected_resolution></a>
|
||||
If a symbol definition with <code>STV_PROTECTED</code> visibility
|
||||
from a shared object is taken as resolving a reference
|
||||
from an executable or another shared object,
|
||||
the <code>SHN_UNDEF</code> symbol table entry created
|
||||
has <code>STV_DEFAULT</code> visibility.
|
||||
<hr>
|
||||
<img src=warning.gif alt="NOTE:">
|
||||
<a name=protected_note></a>
|
||||
The presence of the <code>STV_PROTECTED</code> flag on a symbol
|
||||
in a given load module does not affect the symbol resolution
|
||||
rules for references to that symbol from outside the containing
|
||||
load module.
|
||||
<hr><p>
|
||||
<p><dt><code>STV_HIDDEN</code><dd>
|
||||
A symbol defined in the current component is <i>hidden</i>
|
||||
if its name is not visible to other components. Such a symbol
|
||||
is necessarily protected. This attribute may be used to
|
||||
control the external interface of a component. Note that
|
||||
an object named by such a symbol may still be referenced
|
||||
from another component if its address is passed outside.
|
||||
<p>
|
||||
A hidden symbol contained in a relocatable object must be
|
||||
either removed or converted to <code>STB_LOCAL</code> binding
|
||||
by the link-editor when the relocatable object is included in an
|
||||
executable file or shared object.
|
||||
<p><dt><code>STV_INTERNAL</code><dd>
|
||||
The meaning of this visibility attribute may be defined by processor
|
||||
supplements to further constrain hidden symbols. A processor
|
||||
supplement's definition should be such that generic tools
|
||||
can safely treat internal symbols as hidden.
|
||||
<p>
|
||||
An internal symbol contained in a relocatable object must be
|
||||
either removed or converted to <code>STB_LOCAL</code> binding
|
||||
by the link-editor when the relocatable object is included in an
|
||||
executable file or shared object.
|
||||
</dl>
|
||||
<p>
|
||||
None of the visibility attributes affects resolution of symbols
|
||||
within an executable or shared object during link-editing -- such
|
||||
resolution is controlled by the binding type. Once the link-editor
|
||||
has chosen its resolution, these attributes impose two requirements,
|
||||
both based on the fact that references in the code being linked may
|
||||
have been optimized to take advantage of the attributes.
|
||||
<ul>
|
||||
<li>
|
||||
First, all of the non-default visibility attributes, when applied
|
||||
to a symbol reference, imply that a definition to satisfy that
|
||||
reference must be provided within the current executable or
|
||||
shared object. If such a symbol reference has no definition within the
|
||||
component being linked, then the reference must have
|
||||
<code>STB_WEAK</code> binding and is resolved to zero.
|
||||
<li>
|
||||
Second, if any reference to or definition of a name is a symbol with
|
||||
a non-default visibility attribute, the visibility attribute
|
||||
must be propagated to the resolving symbol in the linked object.
|
||||
If different visibility attributes are specified for distinct
|
||||
references to or definitions of a symbol, the most constraining
|
||||
visibility attribute must be propagated to the resolving symbol
|
||||
in the linked object. The attributes, ordered from least
|
||||
to most constraining, are: <code>STV_PROTECTED</code>,
|
||||
<code>STV_HIDDEN</code> and <code>STV_INTERNAL</code>.
|
||||
</ul>
|
||||
<p>
|
||||
If a symbol's value refers to a
|
||||
specific location within a section,
|
||||
its section index member, <code>st_shndx</code>,
|
||||
holds an index into the section header table.
|
||||
As the section moves during relocation, the symbol's value
|
||||
changes as well, and references to the symbol
|
||||
continue to ``point'' to the same location in the program.
|
||||
Some special section index values give other semantics.
|
||||
<DL COMPACT>
|
||||
<p><dt><code>SHN_ABS</code><dd>
|
||||
The symbol has an absolute value that will not change
|
||||
because of relocation.
|
||||
<a name=shn_common></a>
|
||||
<p><dt><code>SHN_COMMON</code><dd>
|
||||
The symbol labels a common block that has not yet been allocated.
|
||||
The symbol's value gives alignment constraints,
|
||||
similar to a section's
|
||||
<code>sh_addralign</code> member.
|
||||
The link editor will allocate the storage for the symbol
|
||||
at an address that is a multiple of
|
||||
<code>st_value</code>.
|
||||
The symbol's size tells how many bytes are required.
|
||||
Symbols with section index <code>SHN_COMMON</code> may
|
||||
appear only in relocatable objects.
|
||||
<p><dt><code>SHN_UNDEF</code><dd>
|
||||
This section table index means the symbol is undefined.
|
||||
When the link editor combines this object file with
|
||||
another that defines the indicated symbol,
|
||||
this file's references to the symbol will be linked
|
||||
to the actual definition.
|
||||
<p><dt><code>SHN_XINDEX</code><dd>
|
||||
<a name=many_sections></a>
|
||||
This value is an escape value.
|
||||
It indicates that the symbol refers to a specific location within a section,
|
||||
but that the section header index for that section is too large to be
|
||||
represented directly in the symbol table entry.
|
||||
The actual section header index is found in the associated
|
||||
<code>SHT_SYMTAB_SHNDX</code> section.
|
||||
The entries in that section correspond one to one
|
||||
with the entries in the symbol table.
|
||||
Only those entries in <code>SHT_SYMTAB_SHNDX</code>
|
||||
that correspond to symbol table entries with <code>SHN_XINDEX</code>
|
||||
will hold valid section header indexes;
|
||||
all other entries will have value <code>0</code>.
|
||||
</dl>
|
||||
<p>
|
||||
The symbol table entry for index 0 (<code>STN_UNDEF</code>)
|
||||
is reserved; it holds the following.
|
||||
<hr>
|
||||
<b>Figure 4-20: Symbol Table Entry:Index 0</b>
|
||||
<p>
|
||||
<table border cellspacing=0>
|
||||
<th><b>Name</b></th>
|
||||
<th><b>Value</b></th>
|
||||
<th><b>Note</b></th>
|
||||
<tr>
|
||||
<td><code>st_name</code></td>
|
||||
<td align=center><code>0</code></td>
|
||||
<td>No name</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>st_value</code></td>
|
||||
<td align=center><code>0</code></td>
|
||||
<td>Zero value</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>st_size</code></td>
|
||||
<td align=center><code>0</code></td>
|
||||
<td>No size</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>st_info</code></td>
|
||||
<td align=center><code>0</code></td>
|
||||
<td>No type, local binding</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>st_other</code></td>
|
||||
<td align=center><code>0</code></td>
|
||||
<td>Default visibility</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><code>st_shndx</code></td>
|
||||
<td align=center><code>SHN_UNDEF</code></td>
|
||||
<td>No section</td>
|
||||
</tr>
|
||||
</table>
|
||||
<hr>
|
||||
<a name=symbol_values></a>
|
||||
<h2>Symbol Values</h2>
|
||||
Symbol table entries for different object file types have
|
||||
slightly different interpretations for the <code>st_value</code> member.
|
||||
<ul>
|
||||
<p><li>
|
||||
In relocatable files, <code>st_value</code> holds alignment constraints for a symbol
|
||||
whose section index is <code>SHN_COMMON</code>.
|
||||
<p><li>
|
||||
In relocatable files, <code>st_value</code> holds
|
||||
a section offset for a defined symbol.
|
||||
<code>st_value</code> is an offset from the beginning of the section that
|
||||
<code>st_shndx</code> identifies.
|
||||
<p><li>
|
||||
In executable and shared object files,
|
||||
<code>st_value</code> holds a virtual address.
|
||||
To make these files' symbols more useful
|
||||
for the dynamic linker, the section offset (file interpretation)
|
||||
gives way to a virtual address (memory interpretation)
|
||||
for which the section number is irrelevant.
|
||||
</ul>
|
||||
Although the symbol table values have similar meanings
|
||||
for different object files, the data allows
|
||||
efficient access by the appropriate programs.
|
||||
<hr>
|
||||
<a href=ch4.strtab.html><img src=previous.gif alt="Previous"></a>
|
||||
<a href=contents.html><img src=contents.gif alt="Contents"></a>
|
||||
<a href=ch4.reloc.html><img src=next.gif alt="Next"></a>
|
||||
<hr>
|
||||
<i>
|
||||
<small>
|
||||
© 1997, 1998, 1999, 2000, 2001 The Santa Cruz Operation, Inc. All rights reserved.
|
||||
</small>
|
||||
</i>
|
||||
</html>
|
||||
Reference in New Issue
Block a user