ELF_UPDATE(3ELF) ELF Library Functions ELF_UPDATE(3ELF)


NAME


elf_update - update an ELF descriptor

SYNOPSIS


cc [ flag ... ] file ... -lelf [ library ... ]
#include <libelf.h>

off_t elf_update(Elf *elf, Elf_Cmd cmd);


DESCRIPTION


The elf_update() function causes the library to examine the information
associated with an ELF descriptor, elf, and to recalculate the structural
data needed to generate the file's image.


The cmd argument can have the following values:

ELF_C_NULL
This value tells elf_update() to recalculate various
values, updating only the ELF descriptor's memory
structures. Any modified structures are flagged with the
ELF_F_DIRTY bit. A program thus can update the structural
information and then reexamine them without changing the
file associated with the ELF descriptor. Because this does
not change the file, the ELF descriptor may allow reading,
writing, or both reading and writing (see
elf_begin(3ELF)).


ELF_C_WRITE
If cmd has this value, elf_update() duplicates its
ELF_C_NULL actions and also writes any ``dirty''
information associated with the ELF descriptor to the
file. That is, when a program has used elf_getdata(3ELF)
or the elf_flagdata(3ELF) facilities to supply new (or
update existing) information for an ELF descriptor, those
data will be examined, coordinated, translated if
necessary (see elf32_xlatetof(3ELF)), and written to the
file. When portions of the file are written, any
ELF_F_DIRTY bits are reset, indicating those items no
longer need to be written to the file (see
elf_flagdata(3ELF)). The sections' data are written in the
order of their section header entries, and the section
header table is written to the end of the file. When the
ELF descriptor was created with elf_begin(), it must have
allowed writing the file. That is, the elf_begin() command
must have been either ELF_C_RDWR or ELF_C_WRITE.


If elf_update() succeeds, it returns the total size of the file image
(not the memory image), in bytes. Otherwise an error occurred, and the
function returns -1.


When updating the internal structures, elf_update() sets some members
itself. Members listed below are the application's responsibility and
retain the values given by the program.


The following table shows ELF Header members:


Member Notes

e_ident[EI_DATA] Library controls other e_ident values
e_type
e_machine
e_version
e_entry
e_phoff Only when ELF_F_LAYOUT asserted
e_shoff Only when ELF_F_LAYOUT asserted
e_flags
e_shstrndx


The following table shows the Program Header members:


Member Notes

p_type The application controls all
p_offset program header entries
p_vaddr
p_paddr
p_filesz
p_memsz
p_flags
p_align


The following table shows the Section Header members:


Member Notes

sh_name
sh_type
sh_flags
sh_addr
sh_offset Only when ELF_F_LAYOUT asserted
sh_size Only when ELF_F_LAYOUT asserted
sh_link
sh_info
sh_addralign Only when ELF_F_LAYOUT asserted
sh_entsize


The following table shows the Data Descriptor members:


Member Notes

d_buf
d_type
d_size
d_off Only when ELF_F_LAYOUT asserted
d_align
d_version


Note that the program is responsible for two particularly important
members (among others) in the ELF header. The e_version member controls
the version of data structures written to the file. If the version is
EV_NONE, the library uses its own internal version. The e_ident[EI_DATA]
entry controls the data encoding used in the file. As a special case, the
value may be ELFDATANONE to request the native data encoding for the host
machine. An error occurs in this case if the native encoding doesn't
match a file encoding known by the library.


Further note that the program is responsible for the sh_entsize section
header member. Although the library sets it for sections with known
types, it cannot reliably know the correct value for all sections.
Consequently, the library relies on the program to provide the values for
unknown section types. If the entry size is unknown or not applicable,
the value should be set to 0.


When deciding how to build the output file, elf_update() obeys the
alignments of individual data buffers to create output sections. A
section's most strictly aligned data buffer controls the section's
alignment. The library also inserts padding between buffers, as
necessary, to ensure the proper alignment of each buffer.

ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Stable |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO


elf(3ELF), elf32_fsize(3ELF), elf32_getehdr(3ELF), elf32_getshdr(3ELF),
elf32_xlatetof(3ELF), elf_begin(3ELF), elf_flagdata(3ELF),
elf_getdata(3ELF), libelf(3LIB), attributes(5)

NOTES


As mentioned above, the ELF_C_WRITE command translates data as necessary,
before writing them to the file. This translation is not always
transparent to the application program. If a program has obtained
pointers to data associated with a file (for example, see
elf32_getehdr(3ELF) and elf_getdata(3ELF)), the program should
reestablish the pointers after calling elf_update().


July 11, 2001 ELF_UPDATE(3ELF)