Sections

Symbol Values

If an executable file contains a reference to a function defined in one of its associated shared objects, the symbol table section for the file will contain an entry for that symbol. The st_shndx field of that symbol table entry contains SHN_UNDEF. This informs the dynamic linker that the symbol definition for that function is not contained in the executable file itself. If that symbol has been allocated a Procedure Linkage Table entry in the executable file, and the st_value field for that symbol table entry is nonzero, the value is the virtual address of the first instruction of that PLT entry. Otherwise the st_value field contains zero. This PLT entry address is used by the dynamic linker in resolving references to the address of the function. See the Section called Function Addresses in the Chapter called Program loading and dynamic linking for details.

Relocation Types

Relocation entries describe how to alter the instruction and data relocation fields shown in Figure 1 (bit numbers appear in the lower box corners; byte numbers appear in the upper left box corners).

Calculations in Table 4 assume the actions are transforming a relocatable file into either an executable or a shared object file. Conceptually, the linkage editor merges one or more relocatable files to form the output. It first determines how to combine and locate the input files, next it updates the symbol values, and then it performs relocations.

Relocations applied to executable or shared object files are similar and accomplish the same result. The following notations are used in Table 4:

Relocation entries apply to bytes, halfwords or words. In either case, the r_offset value designates the offset or virtual address of the first byte of the affected storage unit. The relocation type specifies which bits to change and how to calculate their values. The zSeries family uses only the Elf64_Rela relocation entries with explicit addends. For the relocation entries, the r_addend field serves as the relocation addend. In all cases, the offset, addend, and the computed result use the byte order specified in the ELF header.

The following general rules apply to the interpretation of the relocation types in Table 4:

• "+" and "-" denote 64-bit modulus addition and subtraction, respectively. ">>" denotes arithmetic right-shifting (shifting with sign copying) of the value of the left operand by the number of bits given by the right operand.

• For relocation type half16, the upper 48 bits of the value computed must be all ones or all zeroes. For relocation type pc16, the upper 47 bits of the value computed must be all ones or all zeroes and the lowest bit must be zero. For relocation type pc32, the upper 31 bits of the value computed must be all ones or all zeroes and the lowest bit must be zero. For relocation type low12, the upper 52 bits of the value computed must all be zero and for relocation type byte8, the upper 56 bits of the value computed must all be zero.

• Reference in a calculation to the value G or O implicitly creates a GOT entry for the indicated symbol and a reference to L implicitly creates a PLT entry.