4.1 Introduction to Assembler Output

The main output produced by assembling an input assembly language source file is the translation of that file into an object file in Extensible and Linking Format (ELF). ELF files thus produced by the assembler are relocatable files that hold code and/or data. They are input files for the linker, which combines these relocatable files with other ELF object files to create an executable file or a shared object file in the next stage of program building, after translation from source files into object files.

The three main kinds of ELF files are relocatable, executable and shared object files. The assembler may also produce ancillary output incidental to the translation process. For example, if the assembler is invoked with the -V option, it may write information to standard output and to standard error.

The assembler also creates a default output file when standard input or multiple input files are used. Ancillary output has little direct connection to the translation process, so it is not properly a subject for this manual. Information about such output appears in as(1) manual page.

Certain assembly language statements are directives to the assembler regarding the organization or content of the object file to be generated. Therefore, they have a direct effect on the translation performed by the assembler. To understand these directives, which are presented in Chapter 3, "Instruction-Set Mapping," it is helpful to have some working knowledge of ELF, at least for relocatable files.

Hence, this chapter presents an overview of ELF for the relocatable object files produced by the assembler. The fully detailed definition of ELF appears in the System V Application Binary Interface and the Intel 386 Processor Supplement.

4.2 Object Files in Extensible and Linking Format (ELF)

Relocatable ELF files produced by the assembler consist of:

• an ELF header
• a section header table
• sections

The ELF header is always the first part of an ELF file. It is essentally a structure of fixed size and format. The fields, or members, of this structure describe the nature, organization and contents of the rest of the file. In particular, the ELF header has a field which specifies the location within the file at which the section header table begins.

The section header table is an array of section headers, which are structures of fixed size and format. The section headers are thus the elements of the array, or the entries in the table. The section header table has exactly one entry for each section in the ELF file. However, the table may also have entries (section headers) that do not correspond to any section in the file. Such entries and their array indices are reserved. The members of each section header constitute information useful to the linker about the contents of the corresponding section, if any.

All of a relocatable file's information that does not lie within its ELF header or its section header table lies within its sections. Sections contain most of the information needed to combine relocatable files with other ELF files to produce shared object files or executable files. Sections also contain the material to be combined. For example, sections may hold:

• Relocation tables
• Symbol tables
• String tables

Each section in an ELF file fills a contiguous (possibly empty) sequence of that file's bytes. Sections never overlap. However, the (set theoretic) union of a relocatable file's ELF header, the file's section header table, and all the file's

sections may omit some of the file's bytes. Bytes of a relocatable file that are not in the file's ELF header, or in the file's section header table, or in any of the file's sections constitute the file's inactive space. The contents of a file's inactive space, if any, are unspecified.

ELF Header

The ELF header is always located at the beginning of the ELF file. It describes the ELF file organization and contains the actual sizes of the object file control structures.

The ELF header consists of the following fields, or members, some of which have the value 0 for relocatable files:

e_ident - This is a byte array consisting of the EI_NIDENT initial bytes of the ELF header, where EI_NIDENT is a name for 16. The elements of this array mark the file as an ELF object file and provide machine-independent data which may be used to decode and interpret the file's contents.

e_type - Identifies the object file type. A value of 1, which has the name ET_REL, specifies a relocatable file. Table 4-1 describes all the object file types.

e_machine - Specifies the required architecture for an individual file. A value of 3, which has the name EM_386, specifies Intel 80386. EM_486, specifies Intel 80486.

e_version - Identifies the version of this object file's format. This field should have the current version number, named EV_CURRENT.

e_entry - Virtual address at which the process is to start. A value of 0 indicates no associated entry point.

e_phoff - Program header table's file offset, in bytes. The value of 0 indicates no program header. (Relocatable files do not need a program header table.)

e_shoff - Section header table's file offset, in bytes. The value of 0 indicates no section header table. (Relocatable files must have a section header table.)

e_flag - Processor-specific flags associated with the file. For the Intel 80386, this field has value 0.

e_ehsize - ELF header's size, in bytes.

e_phentsize - Size, in bytes, of entries in the program header table. All entries are the same size. (Relocatable files do not need a program header table.)

e_phnum - Number of entries in program header table. A value of 0 indicates the file has no program header table. (Relocatable files do not need a program header table.)

e_shentsize - Size, in bytes, of the section header structure. A section header is one entry in the section header table; all entries are the same size.

e_shnum - Number of entries in section header table. A value of 0 indicates the file has no section header table. (Relocatable files must have a section header table.)

e_shstrndx - Section header table index of the entry associated with the section name string table. A value of SHN_UNDEF indicates the file does not have a section name string table.

Table 4-1 Object File Types

Type	Value	Description
none	0	No file type
rel	1	Relocatable file
exec	2	Executable file
dyn	3	Shared object file
core	4	Core file
loproc	0xff00	Processor-specific
hiproc	0xffff	Processor-specific

Section Header

The section header table has all of the information necessary to locate and isolate each of the file's sections. A section header entry in a section header table contains information characterizing the contents of the corresponding section, if the file has such a section.

Each entry in the section header table is a section header, which is a structure of fixed size and format, consisting of the following fields, or members:

sh_name - Specifies the section name. The value of this field is an index into the section header string table section, wherein it indicates the beginning of a null-terminated string that names the section.

sh_type - Categorizes the section's contents and semantics. Table 4-3 describes the section types.

sh_flags - One-bit descriptions of section attributes. Table 4-2 describes the section attribute flags.

sh_addr - Address at which the first byte resides if the section appears in the memory image of a process; a value of 0 indicates the section will not appear in the memory image of a process.

sh_offset - Specifies the byte offset from the beginning of the file to the first byte in the section.

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Note - If the section type is SHT_NOBITS, the corresponding section occupies no space in the file. In this case, sh_offset specifies the location at which the section would have begun if it did occupy space within the file.

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sh_size - Specifies the size, in byte units, of the section.

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Note - Even if the section type is SHT_NOBITS, sh_size may be non-zero; however, the corresponding section still occupies no space in the file.

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sh_link - Section header table index link. The interpretation of this information depends on the section type, as described in Table 4-3.

sh_info - Extra information. The interpretation of this information depends on the section type, as described in Table 4-3.

sh_addralign - If a section has an address alignment constraint, the value in this field is the modulus, in byte units, by which the value of sh_addr must be congruent to 0; i.e., sh_addr = 0 (mod sh_addralign).

For example, if a section contains a long (32 bits), the entire section must be ensured long alignment, so sh_addralign would have the value 4. Only 0 and positive integral powers of 2 are currently allowed as values for this field. A value of 0 or 1 indicates no address alignment constraints.

sh_entsize - Size, in byte units, for entries in a section which is a table of fixed-size entries, such as a symbol table. Has the value 0 if the section is not a table of fixed-size entries.

Table 4-2 Section Attribute Flags

Flag	Default Value	Description
SHF_WRITE	0x1	Contains data that is writable during process execution.
SHF_ALLOC	0x2	Occupies memory during process execution. This attribute is off if a control section does not reside in the memory image of the object file.
SHF_EXECINSTR	0x4	Contains executable machine instructions.
SHF_MASKPROC	0xf0000000	Reserved for processor-specific semantics.

Table 4-3 Section Types

Name	Value	Description		Interpretation by
			sh_info		sh_link
SHT_NULL	0	Marks section header as inactive; file has no corresponding section.	0		SHN_UNDEF
SHT_PROGBITS	1	Contains information defined by the program, and in a format and with a meaning determined solely by the program.	0		SHN_UNDEF

Table 4-3 Section Types (Continued)

Name	Value	Description	Interpretation by
			sh_info	sh_link
SHT_SYMTAB	2	Is a complete symbol table, usually for link editing. This table may also be used for dynamic linking; however, it may contain many unnecessary symbols. Note: Only one section of this type is allowed in a file	One greater than the symbol table index of the last local symbol.	The section header index of the associated string table.
SHT_STRTAB	3	Is a string table. A file may have multiple string table sections.	0	SHN_UNDEF
SHT_RELA	4	Contains relocation entries with explicit addends. A file may have multiple relocation sections.	The section header index of the section to which the relocation applies.	The section header index of the associated symbol table.
SHT_HASH	5	Is a symbol rehash table.	0	The section
		Note: Only one section of this type is allowed in a file		header index of the symbol table to which the hash table applies.
SHT_DYNAMIC	6	Contains dynamic linking information.	0	The section header index of
		Note: Only one section of this type is allowed in a file		the string table used by entries in the section.
SHT_NOTE	7	Contains information that marks the file.	0	SHN_UNDEF

Table 4-3 Section Types (Continued)

Name	Value	Description	Interpretation by
			sh_info	sh_link
SHT_NOBITS	8	Contains information defined by the program, and in a format and with a meaning determined solely by the program. However, a section of this type occupies no space in the file, but the section header's offset field specifies the location at which the section would have begun if it did occupy space within the file.	0	SHN_UNDEF
SHT_REL	9	Contains relocation entries without explicit addends. A file may have multiple relocation sections.	The section header index of the section to which the relocation applies.	The section header index of the associated symbol table.
SHT_SHLIB	10	Reserved.	0	SHN_UNDEF
SHT_DYNSYM	11	Is a symbol table with a minimal set of symbols for dynamic linking.	One greater than the symbol table index of	The section header index of
		Note: Only one section of this type is allowed in a file	the last local symbol.	the associated string table.
SHT_LOPROC	0x70000000	Lower and upper bounds of range of	0	SHN_UNDEF
SHT_HIPROC	0x7fffffff	section types reserved for processor-specific semantics.
SHT_LOUSER	0x80000000	Lower and upper bounds of range of	0	SHN_UNDEF
SHT_HIUSER	0xffffffff	section types reserved for application programs. Note: Section types in this range may be used by an application without conflicting with system-defined section types.

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Note - Some section header table indices are reserved, and the object file will not contain sections for these special indices.

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Sections

A section is the smallest unit of an object file that can be relocated. Sections containing the following material usually appear in relocatable ELF files:

• Executable text
• Read-only data
• Read-write data
• Read-write uninitialized data (only section header appears)

Sections do not need to occur in any particular order within the object file. The sections of a relocatable ELF file contain all of that file's information which is not contained in the ELF header or in the section header table. The sections in any ELF file must satisfy several conditions:

1. Every section in the file must have exactly one section header entry in the section header table to describe the section. However, the section header table may have section header entries which correspond to no section in the file.

2. Each section occupies one contiguous sequence of bytes within a file. The section may be empty (even so, its section header entry in the section header table may have a non-zero value for the field sh_size).

3. A byte in a file can reside in at most one section. Sections in a file cannot overlap.

4. An object file may have inactive space, which is the set of all bytes in the file which are not part of the ELF header, or of the section header table, or of the program header table (for executable files), or of any section in the file. The contents of the inactive space are unspecified.

Sections can be added for multiple text or data segments, shared data, user-defined sections, or information in the object file for debugging.

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Note - Not all of the sections for which there are entries in the file's section header table need to be present.

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Predefined Sections

Sections having certain names beginning with "." (dot) are predefined, with their types and attributes already assigned. These special sections are of two kinds: predefined user sections and predefined non-user sections.

Predefined User Sections

Sections that an assembly language programmer can manipulate by issuing section control directives in the source file are user sections. The predefined user sections are those predefined sections that are also user sections. Table 4-4 lists the names of the predefined user sections and briefly describes each.

Table 4-4 Predefined User Sections

Section Name	Description
".bss"	Uninitialized read-write data.
".comment"	Version control information.
".data" & ".data1"	Initialized read-write data.
".debug"	Debugging information.
".fini"	Runtime finalization instructions.
".init"	Runtime initialization instructions.
".rodata" & ".rodata1"	Read-only data.
".text"	Executable instructions.
".line"	Line # info for symbolic debugging.
".note"	Special information from vendors or system builders.

Predefined Non-User Sections

Table 4-5 lists and briefly describes the predefined sections that are not user sections, because assembly language programmers can not manipulate them by issuing section control directives in the source file.

Table 4-5 Predefined Non-User Sections

Section Name	Description
".dynamic"	Dynamic linking information.
".dynstr"	Strings needed for dynamic linking.
".dynsym"	Dynamic linking symbol table.
".got"	Global offset table.
".hash"	A symbol hash table.
".interp"	The path name of a program interpreter.
".plt"	The procedure linking table.
"relname" & ".relaname"	Relocation information. name is the section to which the relocations apply. e.g. ".rel.text", ".rela.text".
".shstrtab"	String table for the section header table names.
".strtab"	The string table.
".symtab"	The symbol table.

Relocation Tables

Locations represent addresses in memory if a section is allocatable; that is, its contents are to be placed in memory at program runtime. Symbolic references to these locations must be changed to addresses by the link editor.

The assembler produces a companion relocation table for each relocatable section. The table contains a list of relocations (that is, adjustments to locations in the section) to be performed by the link editor.

Symbol Tables

The symbol table contains information to locate and relocate symbolic definitions and references. The assembler creates the symbol table section for the object file. It makes an entry in the symbol table for each symbol that is defined or referenced in the input file and is needed during linking.

The symbol table is then used by the link editor during relocation. The symbol table's section header contains the symbol table index for the first non-local symbol.

The symbol table contains the following information:

st_name - Index into the object file's symbol string table. A value of zero indicates the corresponding entry in the symbol table has no name; otherwise, the value represents the string table index that gives the symbol name.

st_value - Value of the associated symbol. This value is dependent on the context; for example, it may be an address, or it may be an absolute value.

st_size - Size of symbol. A value of 0 indicates that the symbol has either no size or an unknown size.

st_info - Specifies the symbol type and binding attributes. Table 4-6 and Table 4-7 describe the symbol types and binding attributes.

st_other - Undefined meaning. Current value is 0.

st_shndx - Contains the section header table index to another relevant section, if specified. As a section moves during relocation, references to the symbol will continue to point to the same location because the value of the symbol will change as well.

Table 4-6 Symbol Types

Value	Type	Description
0	notype	Type not specified.
1	object	Symbol is associated with a data object; for example, a variable or an array.
2	func	Symbol is associated with a function or other executable code. 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.
3	section	Symbol is associated with a section. These types of symbols are primarily used for relocation.
4	file	Gives the name of the source file associated with the object file.
13	loproc	Values reserved for processor-specific semantics.
15	hiproc

Table 4-7 Symbol Bindings

Value	Binding	Description
0	local	Symbol is defined in the object file and not accessible in other files. Local symbols of the same name may exist in multiple files.
1	global	Symbol is either defined externally or defined in the object file and accessible in other files.
2	weak	Symbol is either defined externally or defined in the object file and accessible in other files; however, these definitions have a lower precedence than globally defined symbols.
13	loproc	Values reserved for processor-specific semantics.
15	hiproc

String Tables

A string table is a section which contains null-terminated variable-length character sequences, or strings. The object file uses these strings to represent symbol names and file names. The strings are referenced by indices into the string table section. The first and last bytes of a string table must be the null character.

• A string table index may refer to any byte in the section.
• Empty string table sections are permitted if zero is the value of sh_size in the section header entry for the string table in the section header table.

A string may appear multiple times and may also be referenced multiple times. References to substrings may exist, and unreferenced strings are allowed.