HP OpenVMS Systems Documentation

HP OpenVMS Linker Utility Manual

The linker creates image sections by grouping together program sections with similar attributes. Within an image section, the linker organizes program sections alphabetically by name. If more than one object module contributes to the same program section, the linker lays out their contributions in the order it processes them.

Figure 7-3 shows how the linker groups the program sections in the object modules from the sample link into image sections, based on the setting of their significant attributes. In the figure, the settings of these significant attributes are represented by shading. (The figure considers attributes that are significant when creating executable images, not shareable images. Section 7.3.3 provides more information about which program section attributes are significant.)

Note, in the figure, that the overlaid contributions from MYSUB.OBJ and MYTEST.OBJ to the program section, GLOBAL_DATA, both appear in the MYSUB_CLUS cluster, even though the object module MYTEST.OBJ is in the default cluster. The linker puts all contributions to a global program section in the cluster in which it is first defined.

Figure 7-3 Combining Program Sections into Image Sections

When combining program sections into image sections, the linker considers only a subset of program section attributes. The set of significant attributes varies according to the type of image being created. When creating an executable image, the linker considers all combinations of the following attributes when combining program sections into image sections:

• Writability (WRT/NOWRT)
• Executability (EXE/NOEXE)
• Protected vector (VEC/NOVEC)
• Unmodified (NOMOD/MOD) (Alpha linking only)

When creating a shareable image, the linker considers all combinations of the following attributes when combining program sections into image sections:

• Writability (WRT/NOWRT)
• Executability (EXE/NOEXE)
• Shareability (SHR/NOSHR)
• Position independence (PIC/NOPIC)
• Protected vector (VEC/NOVEC)
• Unmodified (NOMOD/MOD) (Alpha linking only)

The linker creates only one large image section for system images, so combining program sections by attributes is not applicable.

Table 7-2 and Table 7-3 list all the possible combinations of program section attributes for executable images and shareable images. Note that the order in which the combinations appear in the table (each row) is the same order in which the linker processes them. For example, the linker first processes all program sections with the NOWRT, NOEXE, and NOVEC attributes, creating an image section of program sections with these attributes. The linker then processes all program sections with the WRT, NOEXE, and NOVEC attributes, creating an image section for these program sections. The linker continues this processing until all the combinations of significant attributes have been processed and all the program sections in the cluster have been placed in an image section.

The tables include only program sections that are relocatable (with the REL attribute). Absolute program sections (with the ABS attribute), by definition, can have no allocation (they contain only constants) and cannot contribute to an image section.

For OpenVMS Alpha images, the tables assume that the images are linked using the /DEMAND_ZERO qualifier, which is the default. (When this qualifier is specified, the linker groups program sections that do not contain any data into demand-zero image sections, allocating memory for the image section but not writing zeros to disk.) If the image is linked with the /NODEMAND_ZERO qualifier, the linker allocates space for the image section in the image file. Note that the /NODEMAND_ZERO qualifier does not affect how the linker sorts program sections; it proceeds exactly as specified by the table. However, when the image is written, the linker allocates disk space for the image section and fills the space with zeros.

The tables also show how a particular combination of program section attributes determines the attributes of the image section in which it is placed. For more information about image section attributes, see Section 7.3.5.

For example, Table 7-4 summarizes the settings of the significant attributes of the program sections in the module MYADD.OBJ. (Because this is an OpenVMS VAX object module, the MOD attribute is not considered.)

The linker puts both the DATA and $CHAR_STRING_CONSTANTS program sections in the same image section because they both have the same settings of significant attributes. Within the image section, the linker organizes the program sections alphabetically, so the $CHAR_STRING_CONSTANTS program section appears before the DATA program section. The linker creates a separate image section for the $CODE program section.

The linker performs similar processing of the program sections in the default cluster. The Image Section Synopsis section of the map file lists the clusters the linker created and lists the image sections it created for each cluster. This section also describes the layout of the image in memory, including the base address of each image section. Example 7-6 illustrates an excerpt of the Image Section Synopsis section from the map file produced with the sample link. The listing includes clusters for contributions for the VAX C Run-Time Library.

                                             +------------------------+
                                             ! Image Section Synopsis !
                                             +------------------------+

   Cluster      Type Pages   Base Addr  Disk VBN PFC Protection and Paging      ...
   -------      ---- -----   ---------  -------- --- ---------------------

MYSUB_CLUS         0     1    00000200        2   0  READ WRITE   COPY ON REF
                   0     1    00000400        3   0  READ ONLY

VAXCRTL            3     4    00000000-R      0   0  READ ONLY
                   3     1    00000800-R      0   0  READ ONLY
                   4     1    00000A00-R      0   0  READ WRITE   COPY ON REF
                   3    17    00000C00-R      0   0  READ ONLY
                   3   142    00002E00-R      0   0  READ ONLY
                   4    21    00014A00-R      0   0  READ WRITE   COPY ON REF
                   4     1  P-00017400-R      0   0  READ WRITE   COPY ON REF
                   2     3    00017600-R      0   0  READ WRITE   FIXUP VECTORS

LIBRTL             3   193    00000000-R      0   0  READ ONLY
                   4     8    00018200-R      0   0  READ WRITE   DEMAND ZERO

MTHRTL             3   335    00000000-R      0   0  READ ONLY
                   2     1    00029E00-R      0   0  READ WRITE   FIXUP VECTORS

DEFAULT_CLUSTER    0     1    00000600        4   0  READ WRITE   COPY ON REF
                   0     1    00000800        5   0  READ ONLY
                   0     1    00000A00        6   0  READ WRITE   FIXUP VECTORS
                 253    20    7FFFD800        0   0  READ WRITE   DEMAND ZERO

For more information about the image section synopsis section of a map file, see Section 9.2.3.

To find out which program sections the linker placed in each image section, look at the Program Section Synopsis section of the map file. This section lists all the program sections in each cluster and lists the contributions (the number of bytes) to each program section from each object module. By comparing the base-address of the program sections with the base-addresses of the image sections in the Image Section Synopsis section, you can tell in which image section the program sections appear. Example 7-7 is an excerpt from the Program Section Synopsis section of the map file produced by the sample link operation.

                                        +--------------------------+
                                        ! Program Section Synopsis !
                                        +--------------------------+

Psect Name Module Name   Base     End      Length        Align      Attributes
---------- -----------   ----     ---      ------        -----      ----------

$DATA                  00000200 00000203 00000004 (  4.) LONG 2 PIC,USR,CON...
           MYSUB       00000200 00000203 00000004 (  4.) LONG 2

GLOBAL_DATA            00000204 00000207 00000004 (  4.) LONG 2 PIC,USR,OVR...
           MYSUB       00000204 00000207 00000004 (  4.) LONG 2
           MYTEST      00000204 00000207 00000004 (  4.) LONG 2

$CODE                  00000400 0000040B 0000000C ( 12.) LONG 2 PIC,USR,CON...
           MYSUB       00000400 0000040B 0000000C ( 12.) LONG 2

$CHAR_STRING_CONSTANTS 00000600 0000062D 0000002E ( 46.) LONG 2 PIC,USR,CON...
           MYTEST      00000600 00000623 00000024 ( 36.) LONG 2
           MYADD       00000624 0000062D 0000000A ( 10.) LONG 2

$DATA                  00000630 00000637 00000008 (  8.) LONG 2 PIC,USR,CON...
           MYTEST      00000630 00000633 00000004 (  4.) LONG 2
           MYADD       00000634 00000637 00000004 (  4.) LONG 2

$CODE                  00000800 00000858 00000059 ( 89.) LONG 2 PIC,USR,CON...
           MYTEST      00000800 0000083E 0000003F ( 63.) LONG 2
           MYADD       00000840 00000858 00000019 ( 25.) LONG 2
   .
   .
   .

For more information about the program synopsis section of a map file, see Section 9.2.4.