Overlay attribute in LDScript

For each section within the OVERLAY, the linker automatically defines two symbols. The symbol __load_start_secname is defined as the starting load address of the section. The symbol __load_stop_secname is defined as the final load address of the section. 

OVERLAY 0x1000 : AT (0x4000)
{
   .text0 { o1/*.o(.text) }
   .text1 { o2/*.o(.text) }
}

This will define both `.text0' and `.text1' to start at address 0x1000. `.text0' will be loaded at address 0x4000, and `.text1' will be loaded immediately after `.text0'. The following symbols will be defined: __load_start_text0, __load_stop_text0,__load_start_text1, __load_stop_text1.

C code to copy overlay .text1 into the overlay area might look like the following.

extern char __load_start_text1, __load_stop_text1;
memcpy ((char *) 0x1000, &__load_start_text1,
          &__load_stop_text1 - &__load_start_text1);

Note that the OVERLAY command is just syntactic sugar, since everything it does can be done using the more basic commands. The above example could have been written identically as follows.

.text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
__load_start_text0 = LOADADDR (.text0);
__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0);
.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
__load_start_text1 = LOADADDR (.text1);
__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1);
. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));

Reference from The GNU linker.