(ld.info.gz) PHDRS

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 3.8 PHDRS Command
 =================
 
 The ELF object file format uses "program headers", also knows as
 "segments".  The program headers describe how the program should be
 loaded into memory.  You can print them out by using the `objdump'
 program with the `-p' option.
 
    When you run an ELF program on a native ELF system, the system loader
 reads the program headers in order to figure out how to load the
 program.  This will only work if the program headers are set correctly.
 This manual does not describe the details of how the system loader
 interprets program headers; for more information, see the ELF ABI.
 
    The linker will create reasonable program headers by default.
 However, in some cases, you may need to specify the program headers more
 precisely.  You may use the `PHDRS' command for this purpose.  When the
 linker sees the `PHDRS' command in the linker script, it will not
 create any program headers other than the ones specified.
 
    The linker only pays attention to the `PHDRS' command when
 generating an ELF output file.  In other cases, the linker will simply
 ignore `PHDRS'.
 
    This is the syntax of the `PHDRS' command.  The words `PHDRS',
 `FILEHDR', `AT', and `FLAGS' are keywords.
 
      PHDRS
      {
        NAME TYPE [ FILEHDR ] [ PHDRS ] [ AT ( ADDRESS ) ]
              [ FLAGS ( FLAGS ) ] ;
      }
 
    The NAME is used only for reference in the `SECTIONS' command of the
 linker script.  It is not put into the output file.  Program header
 names are stored in a separate name space, and will not conflict with
 symbol names, file names, or section names.  Each program header must
 have a distinct name.  The headers are processed in order and it is
 usual for them to map to sections in ascending load address order.
 
    Certain program header types describe segments of memory which the
 system loader will load from the file.  In the linker script, you
 specify the contents of these segments by placing allocatable output
 sections in the segments.  You use the `:PHDR' output section attribute
 to place a section in a particular segment.   Output Section
 Phdr.
 
    It is normal to put certain sections in more than one segment.  This
 merely implies that one segment of memory contains another.  You may
 repeat `:PHDR', using it once for each segment which should contain the
 section.
 
    If you place a section in one or more segments using `:PHDR', then
 the linker will place all subsequent allocatable sections which do not
 specify `:PHDR' in the same segments.  This is for convenience, since
 generally a whole set of contiguous sections will be placed in a single
 segment.  You can use `:NONE' to override the default segment and tell
 the linker to not put the section in any segment at all.
 
    You may use the `FILEHDR' and `PHDRS' keywords after the program
 header type to further describe the contents of the segment.  The
 `FILEHDR' keyword means that the segment should include the ELF file
 header.  The `PHDRS' keyword means that the segment should include the
 ELF program headers themselves.  If applied to a loadable segment
 (`PT_LOAD'), all prior loadable segments must have one of these
 keywords.
 
    The TYPE may be one of the following.  The numbers indicate the
 value of the keyword.
 
 `PT_NULL' (0)
      Indicates an unused program header.
 
 `PT_LOAD' (1)
      Indicates that this program header describes a segment to be
      loaded from the file.
 
 `PT_DYNAMIC' (2)
      Indicates a segment where dynamic linking information can be found.
 
 `PT_INTERP' (3)
      Indicates a segment where the name of the program interpreter may
      be found.
 
 `PT_NOTE' (4)
      Indicates a segment holding note information.
 
 `PT_SHLIB' (5)
      A reserved program header type, defined but not specified by the
      ELF ABI.
 
 `PT_PHDR' (6)
      Indicates a segment where the program headers may be found.
 
 EXPRESSION
      An expression giving the numeric type of the program header.  This
      may be used for types not defined above.
 
    You can specify that a segment should be loaded at a particular
 address in memory by using an `AT' expression.  This is identical to the
 `AT' command used as an output section attribute ( Output Section
 LMA).  The `AT' command for a program header overrides the output
 section attribute.
 
    The linker will normally set the segment flags based on the sections
 which comprise the segment.  You may use the `FLAGS' keyword to
 explicitly specify the segment flags.  The value of FLAGS must be an
 integer.  It is used to set the `p_flags' field of the program header.
 
    Here is an example of `PHDRS'.  This shows a typical set of program
 headers used on a native ELF system.
 
      PHDRS
      {
        headers PT_PHDR PHDRS ;
        interp PT_INTERP ;
        text PT_LOAD FILEHDR PHDRS ;
        data PT_LOAD ;
        dynamic PT_DYNAMIC ;
      }
 
      SECTIONS
      {
        . = SIZEOF_HEADERS;
        .interp : { *(.interp) } :text :interp
        .text : { *(.text) } :text
        .rodata : { *(.rodata) } /* defaults to :text */
        ...
        . = . + 0x1000; /* move to a new page in memory */
        .data : { *(.data) } :data
        .dynamic : { *(.dynamic) } :data :dynamic
        ...
      }
 

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