Mapping Example

The following example is a user-defined mapfile. The numbers on the left are included in the example for tutorial purposes. Only the information to the right of the numbers actually appears in the mapfile.

Example E–1 User-Defined Mapfile

    1.  elephant : .data : peanuts.o *popcorn.o; 
    2.  monkey : $PROGBITS ?AX; 
    3.  monkey : .data; 
    4.  monkey = LOAD V0x80000000 L0x4000; 
    5.  donkey : .data; 
    6.  donkey = ?RX A0x1000; 
    7.  text = V0x80008000;

Four separate segments are manipulated in this example. The implicitly declared segment elephant (line 1) receives all of the .data sections from the files peanuts.o and popcorn.o. Notice that *popcorn.o matches any popcorn.o file that can be supplied to the link-edit. The file need not be in the current directory. On the other hand, if /var/tmp/peanuts.o was supplied to the link-edit, it does not match peanuts.o because it is not preceded by an *.

The implicitly declared segment monkey (line 2) receives all sections that are both $PROGBITS and allocatable-executable (?AX), as well as all sections not already in the segment elephant with the name .data (line 3). The .data sections entering the monkey segment need not be $PROGBITS or allocatable-executable because the section_type and section_flags values are entered on a separate line from the section_name value.

An “and” relationship exists between attributes on the same line as illustrated by $PROGBITS “and” ?AX on line 2. An “or” relationship exists between attributes for the same segment that span more than one line, as illustrated by $PROGBITS ?AX on line 2 “or” .data on line 3.

The monkey segment is implicitly declared in line 2 with segment_type value LOAD, segment_flags value RWX, and no virtual_address, physical_address, length or alignment values specified (defaults are used). In line 4 the segment_type value of monkey is set to LOAD. Because the segment_type attribute value does not change, no warning is issued. The virtual_address value is set to 0x80000000 and the maximum length value to 0x4000.

Line 5 implicitly declares the donkey segment. The entrance criteria are designed to route all .data sections to this segment. Actually, no sections fall into this segment because the entrance criteria for monkey in line 3 capture all of these sections. In line 6, the segment_flags value is set to ?RX and the alignment value is set to 0x1000. Because both of these attribute values changed, a warning is issued.

Line 7 sets the virtual_address value of the text segment to 0x80008000.

The example of a user-defined mapfile is designed to cause warnings for illustration purposes. If you want to change the order of the directives to avoid warnings, use the following example.

    1.  elephant : .data : peanuts.o *popcorn.o; 
    4.  monkey = LOAD V0x80000000 L0x4000; 
    2.  monkey : $PROGBITS ?AX; 
    3.  monkey : .data; 
    6.  donkey = ?RX A0x1000; 
    5.  donkey : .data; 
    7.  text = V0x80008000;

The following mapfile example uses the segment-within-section ordering.

    1.  text = LOAD ?RXN V0xf0004000; 
    2.  text | .text; 
    3.  text | .rodata; 
    4.  text : $PROGBITS ?A!W; 
    5.  data = LOAD ?RWX R0x1000;

The text and data segments are manipulated in this example. Line 1 declares the text segment to have a virtual_address of 0xf0004000 and to not include the ELF header or any program headers as part of this segment's address calculations. Lines 2 and 3 turn on section-within-segment ordering and specify that the .text and .rodata sections are the first two sections in this segment. The result is that the .text section have a virtual address of 0xf0004000, and the .rodata section immediately follows that address.

Any other $PROGBITS section that makes up the text segment follows the .rodata section. Line 5 declares the data segment and specifies that its virtual address must begin on a 0x1000 byte boundary. The first section that constitutes the data segment also resides on a 0x1000 byte boundary within the file image.