elf_update(3E) — ELF LIBRARY FUNCTIONS
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
elf_update 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.
cmd may 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(3E)].
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 or the elf_flag facilities to supply new (or update existing) information for an ELF descriptor, those data will be examined, coordinated, translated if necessary [see elf_xlate(3E)], 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_flag(3E)]. 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.
| Member | Notes | |
| ELF Header | 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 |
| Member | Notes | |
| Program Header | p_type | The application controls all |
| p_offset | program header entries | |
| p_vaddr | ||
| p_paddr | ||
| p_filesz | ||
| p_memsz | ||
| p_flags | ||
| p_align |
| Member | Notes | |
| Section Header | 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 |
| Member | Notes | |
| Data Descriptor | d_buf | |
| d_type | ||
| d_size | ||
| d_off | Only when ELF_F_LAYOUT asserted | |
| d_align | ||
| d_version |
Note 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 type. If the entry size is unknown or not applicable, the value should be set to zero.
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.
SEE ALSO
elf(3E), elf_begin(3E), elf_flag(3E), elf_fsize(3E), elf_getdata(3E), elf_getehdr(3E), elf_getshdr(3E), elf_xlate(3E)
NOTE
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 elf_getehdr(3E) and elf_getdata(3E)], the program should reestablish the pointers after calling elf_update.
As elf_begin(3E) describes, a program may “update” a COFF file to make the image consistent for ELF . The ELF_C_NULL command updates only the memory image; one can use the ELF_C_WRITE command to modify the file as well. Absolute executable files (a.out files) require special alignment, which cannot normally be preserved between COFF and ELF . Consequently, one may not update an executable COFF file with the ELF_C_WRITE command (though ELF_C_NULL is allowed).
— ELF Library