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Program header

An executable or shared object file's program header table is an array of
structures, each describing a segment or other information the system needs
to prepare the program for execution. An object file segment contains one or
more sections.

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NOTE: See ``Segment contents'' for more information.
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Program headers are meaningful only for executable and shared object files.
A file specifies its own program header size with the ELF header's
e_phentsize and e_phnum members. See ``ELF header'' for more information.

   typedef struct {
        Elf32_Word      p_type;
        Elf32_Off       p_offset;
        Elf32_Addr      p_vaddr;
        Elf32_Addr      p_paddr;
        Elf32_Word      p_filesz;
        Elf32_Word      p_memsz;
        Elf32_Word      p_flags;
        Elf32_Word      p_align;
   } Elf32_Phdr;

Figure 8-8 Program header

p_type
     This member tells what kind of segment this array element describes or
     how to interpret the array element's information. Type values and their
     meanings appear in Table 8-4, ``Section types, sh_type''.

p_offset
     This member gives the offset from the beginning of the file at which
     the first byte of the segment resides.

p_vaddr
     This member gives the virtual address at which the first byte of the
     segment resides in memory.
     

p_paddr
     On systems for which physical addressing is relevant, this member is
     reserved for the segment's physical address. Because UNIX System V
     ignores physical addressing for application programs, this member has
     unspecified contents for executable files and shared objects.

p_filesz
     This member gives the number of bytes in the file image of the segment;
     it may be zero.

p_memsz
     This member gives the number of bytes in the memory image of the
     segment; it may be zero.

p_flags
     This member gives flags relevant to the segment. Defined flag values
     appear in Table 8-4, ``Section types, sh_type''.

p_align
     Loadable process segments must have congruent values for p_vaddr and
     p_offset, modulo the page size. See ``Program values'' for more
     information. This member gives the value to which the segments are
     aligned in memory and in the file. Values 0 and 1 mean no alignment is
     required. Otherwise, p_align should be a positive, integral power of 2,
     and p_vaddr should equal p_offset, modulo p_align.

Some entries describe process segments; others give supplementary
information and do not contribute to the process image. Segment entries may
appear in any order, except as explicitly noted in Table 8-13, ``Segment
types, p_type''. Defined type values follow; other values are reserved for
future use. 

Table 8-13 Segment types, p_type

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 Name                      Value
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 PT_NULL                   0
 PT_LOAD                   1
 PT_DYNAMIC                2
 PT_INTERP                 3
 PT_NOTE                   4
 PT_SHLIB                  5
 PT_PHDR                   6
 PT_LOPROC                 0x70000000
 PT_HIPROC                 0x7fffffff

PT_NULL
     The array element is unused; other members' values are undefined. This
     type lets the program header table have ignored entries.

PT_LOAD
     The array element specifies a loadable segment, described by p_filesz
     and p_memsz. The bytes from the file are mapped to the beginning of the
     memory segment. If the segment's memory size (p_memsz) is larger than
     the file size (p_filesz), the ``extra'' bytes are defined to hold the
     value 0 and to follow the segment's initialized area. The file size may
     not be larger than the memory size. Loadable segment entries in the
     program header table appear in ascending order, sorted on the p_vaddr
     member.

PT_DYNAMIC
     The array element specifies dynamic linking information. See ``Dynamic
     section'' for more information.

PT_INTERP
     The array element specifies the location and size of a null-terminated
     path name to invoke as an interpreter. This segment type is meaningful
     only for executable files (though it may occur for shared objects); it
     may not occur more than once in a file. If it is present, it must
     precede any loadable segment entry. See ``Program interpreter'' for
     more information.

PT_NOTE
     The array element specifies the location and size of auxiliary
     information. See ``Note section'' for more information.

PT_SHLIB
     This segment type is reserved but has unspecified semantics.

PT_PHDR
     The array element, if present, specifies the location and size of the
     program header table itself, both in the file and in the memory image
     of the program. This segment type may not occur more than once in a
     file. Moreover, it may occur only if the program header table is part
     of the memory image of the program. If it is present, it must precede
     any loadable segment entry. See ``Program interpreter'' for more
     information.

PT_LOPROC through PT_HIPROC
     Values in this inclusive range are reserved for processor-specific
     semantics.

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NOTE: Unless specifically required elsewhere, all program header segment
types are optional. A file's program header table may contain only those
elements relevant to its contents.
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