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 section within an object file has its own header.  The C structure appears below:

struct scnhdr
{
chars_name[8];/∗ section name ∗/
longs_paddr;/∗ physical address, aliased s_nlib ∗/
longs_vaddr;/∗ virtual address ∗/
longs_size;/∗ section size ∗/
longs_scnptr;/∗ file ptr to raw data for section ∗/
longs_relptr;/∗ file ptr to relocation ∗/
longs_lnnoptr;/∗ file ptr to gp table ∗/
unsigned shorts_nreloc;/∗ number of relocation entries ∗/
unsigned shorts_nlnno;/∗ number of gp table entries ∗/
longs_flags;/∗ flags ∗/
};

File pointers are byte offsets into the file; they can be used as the offset in a call to FSEEK [see ldfcn(3X)]. 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.  Consequently, 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 RISC NEWS 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 RISC NEWS 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 relocation takes place.  This is a kluge. 

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 {
longcurrent_g_value;/∗ actual value ∗/
longunused;
} header;
struct {
longg_value;/∗ hypothetical value ∗/
longbytes;/∗ 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 followed by the path name to the shared library.  The C structure appears below and is defined in scnhdr.h.

struct libscn
{
longsize;/∗ size of this entry (including target name) ∗/
longoffset;/∗ offset from start of entry to target name ∗/
longtsize;/∗ text size in bytes, padded to DW boundary ∗/
longdsize;/∗ initialized data size ∗/
longbsize;/∗ uninitialized data ∗/
longtext_start;/∗ base of text used for this library ∗/
longdata_start;/∗ base of data used for this library ∗/
longbss_start;/∗ base of bss used for this library ∗/
/∗ pathname of target shared library ∗/
};

SEE ALSO

fseek(3S), a.out(5), reloc(5), ld(1).

NEWS-OSRelease 4.2.1R