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.\" $Id: elf_getdata.3 3734 2019-04-22 14:10:49Z jkoshy $
.\"
.Dd April 22, 2019
.Dt ELF_GETDATA 3
.Os
.Sh NAME
.Nm elf_getdata ,
.Nm elf_newdata ,
.Nm elf_rawdata
.Nd iterate through or allocate section data
.Sh LIBRARY
.Lb libelf
.Sh SYNOPSIS
.In libelf.h
.Ft "Elf_Data *"
.Fn elf_getdata "Elf_Scn *scn" "Elf_Data *data"
.Ft "Elf_Data *"
.Fn elf_newdata "Elf_Scn *scn"
.Ft "Elf_Data *"
.Fn elf_rawdata "Elf_Scn *scn" "Elf_Data *data"
.Sh DESCRIPTION
These functions are used to access and manipulate data descriptors
associated with section descriptors.
Data descriptors used by the ELF library are described in
.Xr elf 3 .
.Pp
Function
.Fn elf_getdata
will return the next data descriptor associated with section descriptor
.Ar scn .
The returned data descriptor will be setup to contain translated data.
Argument
.Ar data
may be NULL, in which case the function returns the first data descriptor
associated with section
.Ar scn .
If argument
.Ar data
is not NULL, it must be a pointer to a data descriptor associated with
section descriptor
.Ar scn ,
and function
.Fn elf_getdata
will return a pointer to the next data descriptor for the section,
or NULL when the end of the section's descriptor list is reached.
.Pp
Function
.Fn elf_newdata
will allocate a new data descriptor and append it to the list of data
descriptors associated with section descriptor
.Ar scn .
The new data descriptor will be initialized as follows:
.Bl -tag -width "d_version" -compact -offset indent
.It Va d_align
Set to 1.
.It Va d_buf
Initialized to NULL.
.It Va d_off
Set to (off_t) -1.
This field is under application control if the
.Dv ELF_F_LAYOUT
flag was set on the ELF descriptor.
.It Va d_size
Set to zero.
.It Va d_type
Initialized to
.Dv ELF_T_BYTE .
.It Va d_version
Set to the current working version of the library, as set by
.Xr elf_version 3 .
.El
The application must set these values as appropriate before
calling
.Xr elf_update 3 .
Section
.Ar scn
must be associated with an ELF file opened for writing.
If the application has not requested full control of layout by
setting the
.Dv ELF_F_LAYOUT
flag on descriptor
.Ar elf ,
then the data referenced by the returned descriptor will be positioned
after the existing content of the section, honoring the file alignment
specified in member
.Va d_align .
On successful completion of a call to
.Fn elf_newdata ,
the ELF library will mark the section
.Ar scn
as
.Dq dirty .
.Pp
Function
.Fn elf_rawdata
is used to step through the data descriptors associated with
section
.Ar scn .
In contrast to function
.Fn elf_getdata ,
this function returns untranslated data.
If argument
.Ar data
is NULL, the first data descriptor associated with section
.Ar scn
is returned.
If argument
.Ar data
is not NULL, is must be a data descriptor associated with
section
.Ar scn ,
and function
.Fn elf_rawdata
will return the next data descriptor in the list, or NULL
if no further descriptors are present.
Function
.Fn elf_rawdata
always returns
.Vt Elf_Data
structures of type
.Dv ELF_T_BYTE .
.Ss Special handling of zero-sized and SHT_NOBITS sections
For sections of type
.Dv SHT_NOBITS ,
and for zero-sized sections,
the functions
.Fn elf_getdata
and
.Fn elf_rawdata
return a pointer to a valid
.Vt Elf_Data
structure that has its
.Va d_buf
member set to NULL and its
.Va d_size
member set to the size of the section.
.Pp
If an application wishes to create a section of type
.Dv SHT_NOBITS ,
it should add a data buffer to the section using function
.Fn elf_newdata .
It should then set the
.Va d_buf
and
.Va d_size
members of the returned
.Vt Elf_Data
structure to NULL and the desired size of the section respectively.
.Sh RETURN VALUES
These functions return a valid pointer to a data descriptor if successful, or
NULL if an error occurs.
.Sh ERRORS
These functions may fail with the following errors:
.Bl -tag -width "[ELF_E_RESOURCE]"
.It Bq Er ELF_E_ARGUMENT
Either of the arguments
.Ar scn
or
.Ar data
was NULL.
.It Bq Er ELF_E_ARGUMENT
The data descriptor referenced by argument
.Ar data
is not associated with section descriptor
.Ar scn .
.It Bq Er ELF_E_ARGUMENT
The section denoted by argument
.Ar scn
had no data associated with it.
.It Bq Er ELF_E_DATA
Retrieval of data from the underlying object failed.
.It Bq Er ELF_E_RESOURCE
An out of memory condition was detected.
.It Bq Er ELF_E_SECTION
Section
.Ar scn
had type
.Dv SHT_NULL .
.It Bq Er ELF_E_SECTION
The type of the section
.Ar scn
was not recognized by the library.
.It Bq Er ELF_E_SECTION
The size of the section
.Ar scn
is not a multiple of the file size for its section type.
.It Bq Er ELF_E_SECTION
The file offset for section
.Ar scn
is incorrect.
.It Bq Er ELF_E_UNIMPL
The section type associated with section
.Ar scn
is not supported.
.It Bq Er ELF_E_VERSION
Section
.Ar scn
was associated with an ELF object with an unsupported
version.
.El
.Sh SEE ALSO
.Xr elf 3 ,
.Xr elf_flagdata 3 ,
.Xr elf_flagscn 3 ,
.Xr elf_getscn 3 ,
.Xr elf_getshdr 3 ,
.Xr elf_newscn 3 ,
.Xr elf_rawfile 3 ,
.Xr elf_update 3 ,
.Xr elf_version 3 ,
.Xr gelf 3