scnhdr(4) FILE FORMATS scnhdr(4)
NAME
scnhdr - section header for a MIPS object file
SYNOPSIS
#include <scnhdr.h>
DESCRIPTION
Every MIPS object file has a table of section headers to
specify the layout of the data within the file. Each sec-
tion within an object file has its own header. The C struc-
ture appears below:
struct scnhdr
{
char s_name[8]; /* section name */
long s_paddr; /* physical address, aliased s_nlib */
long s_vaddr; /* virtual address */
long s_size; /* section size */
long s_scnptr; /* file ptr to raw data for section */
long s_relptr; /* file ptr to relocation */
long s_lnnoptr; /* file ptr to gp table */
unsigned short s_nreloc; /* number of relocation entries */
unsigned short s_nlnno; /* number of gp table entries */
long s_flags; /* flags */
};
File pointers are byte offsets into the file; they can be
used as the offset in a call to FSEEK [see ldfcn(4)]. If a
section is initialized, the file contains the actual bytes.
An uninitialized section is somewhat different. It has a
size, symbols defined in it, and symbols that refer to it.
But it can have no relocation entries or data. Conse-
quently, an uninitialized section has no raw data in the
object file, and the values for s_scnptr, s_relptr, and
s_nreloc are zero.
The entries that refer to line numbers (s_lnnoptr, and
s_nlnno) are not used for line numbers on MIPS machines.
See the header file sym.h for the entries to get to the line
number table. The entries that were for line numbers in the
section header are used for gp tables on MIPS machines.
The number of relocation entries for a section is found in
the s_nreloc field of the section header. This field being
a `C' language short and can overflow with large objects.
If this field overflows the section header s_flags field has
the S_NRELOC_OVFL bit set. In this case the true number of
relocation entries is found in the r_vaddr field of the
first relocation entry for that section. That relocation
entry has a type of R_ABS so it is ignored when the reloca-
tion takes place. This is a kluge.
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scnhdr(4) FILE FORMATS scnhdr(4)
The gp table gives the section size corresponding to each
applicable value of the compiler option -G num (always
including 0), sorted by smallest size first. It is pointed
to by the s_lnnoptr field in the section header and its
number of entries (including the header) is in the s_nlnno
field in the section header. This table only needs to exist
for the .sdata and .sbss sections. If there is no ``small''
section then the gp table for it is attached to the
coresponding ``large'' section so the information still gets
to the link editor, ld(1). The C union for the gp table
appears below.
union gp_table
{
struct {
long current_g_value;/* actual value */
long unused;
} header;
struct {
long g_value; /* hypothetical value */
long bytes; /* section size corresponding to hypothetical value */
} entry;
};
Each gp table has one header structure that contains the
actual value of the -G num option used to produce the object
file. An entry must exist for every applicable value of the
-G num option. The applicable values are all the sizes of
the data items in that section.
For .lib sections the number of shared libraries is in the
s_nlib field (an alias to s_paddr). The .lib section is
made up of s_nlib descriptions of shared libraries. Each
description of a shared library is a libscn structure fol-
lowed by the path name to the shared library. The C struc-
ture appears below and is defined in scnhdr.h .
struct libscn
{
long size; /* size of this entry (including target name) */
long offset; /* offset from start of entry to target name */
long tsize; /* text size in bytes, padded to DW boundary */
long dsize; /* initialized data size */
long bsize; /* uninitialized data */
long text_start; /* base of text used for this library */
long data_start; /* base of data used for this library */
long bss_start; /* base of bss used for this library */
/* pathname of target shared library */
};
SEE ALSO
ld(1), fseek(3S), a.out(4), reloc(4).
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