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. See 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 (see 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
       ...
     }