scnhdr.5

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 5 代码 · 共 139 行

.TH scnhdr 5 RISC
.SH Name
scnhdr \- section header for a MIPS object file
.SH Syntax
.B "#include < scnhdr.h>"
.SH 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:
.EX
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 */
};
.EE
.PP
File pointers are byte offsets into the file; they can be used as the 
offset in a call to FSEEK (see
.MS ldfcn 5 ).
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
.I "s_scnptr, s_relptr,"
and
.I s_nreloc
are zero.
.PP
The entries that refer to line numbers 
.RI ( s_lnnoptr " and " s_nlnno )
are not used for line numbers on MIPS machines.  See the header file 
.PN 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.
.PP
The number of relocation entries for a section is found in the
.I 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
.I s_flags
field has the
.SM S_NRELOC_OVFL
bit set.  In this case, the true number of relocation entries is found in the
.I r_vaddr
field of the first relocation entry for that section.  That relocation entry
has a type of
.SM R_ABS;
thus, it is ignored when the relocation takes place. 
.PP
The gp table gives the section size corresponding to each applicable value
of the compiler option 
.PN -G
.I num
(always including 0), sorted by smallest size first. It is pointed to
by the
.I s_lnnoptr
field in the section header and its number of entries
(including the header) is in the
.I s_nlnno
field in the section header.
This table only needs to exist for the
.I .sdata
and
.I .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, 
.PN ld .
The C union for the gp table follows:
.EX
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;
}; 
.EE
.PP
Each gp table has one header structure that contains the actual value of the
.PN -G
.I num
option used to produce the object file.  An entry must exist for every
.PN -G
.I num
.BI \-G " num"
option.  The applicable values are all the sizes of the data items
in that section.
.PP
For
.I .lib
sections, the number of shared libraries is in the
.I s_nlib
field (an alias to
.IR s_paddr ).
The
.I .lib
section is made up of
.I s_nlib
descriptions of shared libraries.
Each description of a shared library is a
.I libscn
structure followed by the path name to the shared library.
The C structure appears here and is defined in
.I scnhdr.h :
.EX
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 */
};
.EE
.SH See Also
ld(1), fseek(3s), a.out(5), reloc(5)