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.

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.

Table 8-13 Segment types, p_type

 ---------------------------------------------------------------------
 Name                      Value
 ---------------------------------------------------------------------
 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.