Linux Standard Base Core Specification 3.1
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11.6. Exception Frames

When using languages that support exceptions, such as C++, additional information must be provided to the runtime environment that describes the call frames that must be unwound during the processing of an exception. This information is contained in the special sections .eh_frame and .eh_framehdr.

Note: The format of the .eh_frame section is similar in format and purpose to the .debug_frame section which is specified in DWARF Debugging Information Format, Revision 3.0.0 (Draft). Readers are advised that there are some subtle difference, and care should be taken when comparing the two sections.

11.6.1. The `.eh_frame` section

The .eh_frame section shall contain 1 or more Call Frame Information (CFI) records. The number of records present shall be determined by size of the section as contained in the section header. Each CFI record contains a Common Information Entry (CIE) record followed by 1 or more Frame Description Entry (FDE) records. Both CIEs and FDEs shall be aligned to an addressing unit sized boundary.

Table 11-8. Call Frame Information Format

Common Information Entry Record

Frame Description Entry Record(s)

11.6.1.1. The Common Information Entry Format

Table 11-9. Common Information Entry Format

Length	Required
Extended Length	Optional
CIE ID	Required
Version	Required
Augmentation String	Required
Code Alignment Factor	Required
Data Alignment Factor	Required
Return Address Register	Required
Augmentation Data Length	Optional
Augmentation Data	Optional
Initial Instructions	Required
Padding

Length: A 4 byte unsigned value indicating the length in bytes of the CIE structure, not including the Length field itself. If Length contains the value 0xffffffff, then the length is contained in the Extended Length field. If Length contains the value 0, then this CIE shall be considered a terminator and processing shall end.
Extended Length: A 8 byte unsigned value indicating the length in bytes of the CIE structure, not including the Length and Extended Length fields.
CIE ID: A 4 byte unsigned value that is used to distinguish CIE records from FDE records. This value shall always be 0, which indicates this record is a CIE.
Version: A 1 byte value that identifies the version number of the frame information structure. This value shall be 1.
Augmentation String: This value is a NUL terminated string that identifies the augmentation to the CIE or to the FDEs associated with this CIE. A zero length string indicates that no augmentation data is present. The augmentation string is case sensitive and shall be interpreted as described below.
Code Alignment Factor: An unsigned LEB128 encoded value that is factored out of all advance location instructions that are associated with this CIE or its FDEs. This value shall be multiplied by the delta argument of an adavance location instruction to obtain the new location value.
Data Alignment Factor: A signed LEB128 encoded value that is factored out of all offset instructions that are associated with this CIE or its FDEs. This value shall be multiplied by the register offset argument of an offset instruction to obtain the new offset value.
Augmentation Length: An unsigned LEB128 encoded value indicating the length in bytes of the Augmentation Data. This field is only present if the Augmentation String contains the character 'z'.
Augmentation Data: A block of data whose contents are defined by the contents of the Augmentation String as described below. This field is only present if the Augmentation String contains the character 'z'. The size of this data is given by the Augentation Length.
Initial Instructions: Initial set of Call Frame Instructions. The number of instructions is determined by the remaining space in the CIE record.
Padding: Extra bytes to align the CIE structure to an addressing unit size boundary.

11.6.1.1.1. Augmentation String Format

The Agumentation String indicates the presence of some optional fields, and how those fields should be intepreted. This string is case sensitive. Each character in the augmentation string in the CIE can be interpreted as below:

'z'		A 'z' may be present as the first character of the string. If present, the Augmentation Data field shall be present. The contents of the Augmentation Data shall be intepreted according to other characters in the Augmentation String.
'L'		A 'L' may be present at any position after the first character of the string. This character may only be present if 'z' is the first character of the string. If present, it indicates the presence of one argument in the Augmentation Data of the CIE, and a corresponding argument in the Augmentation Data of the FDE. The argument in the Augmentation Data of the CIE is 1-byte and represents the pointer encoding used for the argument in the Augmentation Data of the FDE, which is the address of a language-specific data area (LSDA). The size of the LSDA pointer is specified by the pointer encoding used.
'P'		A 'P' may be present at any position after the first character of the string. This character may only be present if 'z' is the first character of the string. If present, it indicates the presence of two arguments in the Augmentation Data of the CIE. The first argument is 1-byte and represents the pointer encoding used for the second argument, which is the address of a personality routine handler. The personality routine is used to handle language and vendor-specific tasks. The system unwind library interface accesses the language-specific exception handling semantics via the pointer to the personality routine. The personality routine does not have an ABI-specific name. The size of the personality routine pointer is specified by the pointer encoding used.
'R'		A 'R' may be present at any position after the first character of the string. This character may only be present if 'z' is the first character of the string. If present, The Augmentation Data shall include a 1 byte argument that represents the pointer encoding for the address pointers used in the FDE.

11.6.1.2. The Frame Description Entry Format

Table 11-10. Frame Description Entry Format

Length	Required
Extended Length	Optional
CIE Pointer	Required
PC Begin	Required
PC Range	Required
Augmentation Data Length	Optional
Augmentation Data	Optional
Call Frame Instructions	Required
Padding

Length: A 4 byte unsigned value indicating the length in bytes of the CIE structure, not including the Length field itself. If Length contains the value 0xffffffff, then the length is contained the Extended Length field. If Length contains the value 0, then this CIE shall be considered a terminator and processing shall end.
Extended Length: A 8 byte unsigned value indicating the length in bytes of the CIE structure, not including the Length field itself.
CIE Pointer: A 4 byte unsigned value that when subtracted from the offset of the the CIE Pointer in the current FDE yields the offset of the start of the associated CIE. This value shall never be 0.
PC Begin: An encoded value that indicates the address of the initial location associated with this FDE. The encoding format is specified in the Augmentation Data.
PC Range: An absolute value that indicates the number of bytes of instructions associated with this FDE.
Augmentation Length: An unsigned LEB128 encoded value indicating the length in bytes of the Augmentation Data. This field is only present if the Augmentation String in the associated CIE contains the character 'z'.
Augmentation Data: A block of data whose contents are defined by the contents of the Augmentation String in the associated CIE as described above. This field is only present if the Augmentation String in the associated CIE contains the character 'z'. The size of this data is given by the Augentation Length.
Call Frame Instructions: A set of Call Frame Instructions.
Padding: Extra bytes to align the FDE structure to an addressing unit size boundary.

11.6.2. The `.eh_frame_hdr` section

The .eh_frame_hdr section contains additional information about the .eh_frame section. A pointer to the start of the .eh_frame data, and optionally, a binary search table of pointers to the .eh_frame records are found in this section.

Data in this section is encoded according to Section 11.5.1.

Table 11-11. .eh_frame_hdr Section Format

Encoding	Field
unsigned byte	version
unsigned byte	eh_frame_ptr_enc
unsigned byte	fde_count_enc
unsigned byte	table_enc
encoded	eh_frame_ptr
encoded	fde_count
	binary search table

version		Version of the `.eh_frame_hdr` format. This value shall be 1.
eh_frame_ptr_enc		The encoding format of the eh_frame_ptr field.
fde_count_enc		The encoding format of the fde_count field. A value of DW_EH_PE_omit indicates the binary search table is not present.
table_enc		The encoding format of the entries in the binary search table. A value of DW_EH_PE_omit indicates the binary search table is not present.
eh_frame_ptr		The encoded value of the pointer to the start of the `.eh_frame` section.
fde_count		The encoded value of the count of entries in the binary search table.
binary search table		A binary search table containing fde_count entries. Each entry of the table consist of two encoded values, the initial location, and the address. The entries are sorted in an increasing order by the initial location value.

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