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Linker and Libraries Guide

x86: Initial Executable (IE)

There are two code-sequences for the IE model. One sequence is for position independent code which uses a GOT-pointer. The other sequence is for position dependent code which does not use a GOT-pointer. Both of these code sequences can only be used in a dynamic executable. These code sequences can reference a TLS variable defined in either the executable or any of the shared libraries loaded at process startup. This model can not reference TLS variables from shared libraries loaded after process startup.

Table 8–10 x86: Initial Executable, Position Independent, Thread-Local Variable Access Codes

Code Sequence 

Initial Relocations 

Symbol 

 
0x00 movl  %gs:0, %eax
0x06 addl  x@gotntpoff(%ebx), %eax

# %eax - contains address of TLS variable
 
<none>
R_386_TLS_GOTIE
 
 
x
 

Outstanding Relocations 

Symbol 

 
GOT[n]
 
R_386_TLS_TPOFF
 
x

The addl instruction generates a R_386_TLS_GOTIE relocation that instructs the link–editor to create space in the global offset table to store the static TLS offset for symbol x. A R_386_TLS_TPOFF relocation is left outstanding against the GOT table for the runtime linker to fill in with the static TLS offset for symbol x.

Table 8–11 x86: Initial Executable, Position Dependent, Thread-Local Variable Access Codes

Code Sequence 

Initial Relocations 

Symbol 

 
0x00 movl  %gs:0, %eax
0x06 addl  x@indntpoff, %eax

# %eax - contains address of TLS variable
 
<none>
R_386_TLS_IE
 
 
x
 

Outstanding Relocations 

Symbol 

 
GOT[n]
 
R_386_TLS_TPOFF
 
x

The addl instruction generates a R_386_TLS_IE relocation, that instructs the link-editor to create space in the global offset table to store the static TLS offset for symbol x. The main difference between this sequence and the position independent form, is that the instruction is bound directly to the GOT entry created, instead of via an offset off of the GOT-pointer register. A R_386_TLS_TPOFF relocation is left outstanding against the GOT for the runtime linker to fill in with the static TLS offset for symbol x.

The contents of variable x, rather then the address, can be loaded by embedding the offset directly into the memory reference as shown in the next two sequences.

Table 8–12 x86: Initial Executable, Position Independent, Dynamic Thread-Local Variable Access Codes

Code Sequence 

Initial Relocations 

Symbol 

 
0x00 movl  x@gotntpoff(%ebx), %eax
0x06 movl  %gs:(%eax), %eax

# %eax - contains address of TLS variable
 
R_386_TLS_GOTIE
<none>
 
x
 

Outstanding Relocations 

Symbol 

 
GOT[n]
 
R_386_TLS_TPOFF
 
x

Table 8–13 x86: Initial Executable, Position Independent, Thread-Local Variable Access Codes

Code Sequence 

Initial Relocations 

Symbol 

 
0x00 movl  x@indntpoff, %ecx
0x06 movl  %gs:(%ecx), %eax

# %eax - contains address of TLS variable
 
R_386_TLS_IE
<none>
 
x
 

Outstanding Relocations 

Symbol 

 
GOT[n]
 
R_386_TLS_TPOFF
 
x

In the last sequence, if the %eax register is used instead of the %ecx above, the first instruction may be either 5 or 6 bytes long.