scnhdr(5) File Formats Manual scnhdr(5)
Name
scnhdr - section header for a MIPS object file
Syntax
#include < scnhdr.h>
Description
Every MIPS object file has a table of section headers that specify the layout of the data in the file. Each section that is in an object
file has its own header. The C structure appears as follows:
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 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. It cannot have relocation entries or data. Consequently, an uninitialized section does not contain 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 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 is 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; thus, it is ignored when the relocation takes place.
The gp table gives the section size corresponding to each applicable value of the compiler option 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 a small section does not
exist, then the gp table for it is attached to the corresponding large section so the information still gets to the link editor, The C
union for the gp table follows:
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 num option used to produce the object file. An entry must
exist for every num -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 here 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(5), reloc(5)
RISC scnhdr(5)