The runtime linker provides an additional level of flexibility by enabling you to introduce new objects during process initialization by using the environment variable LD_PRELOAD. This environment variable can be initialized to a shared object or relocatable object file name, or a string of file names separated by white space. These objects are loaded after the dynamic executable and before any dependencies. These objects are assigned world search scope, and global symbol visibility.
In the following example, the dynamic executable prog is loaded, followed by the shared object newstuff.so.1. The dependencies defined within prog are then loaded.
$ LD_PRELOAD=./newstuff.so.1 prog
The order in which these objects are processed can be displayed using ldd(1).
$ ldd -e LD_PRELOAD=./newstuff.so.1 prog ./newstuff.so.1 => ./newstuff.so libc.so.1 => /lib/libc.so.1
In the following example, the preloading is a little more complex and time consuming.
$ LD_PRELOAD="./foo.o ./bar.o" prog
The runtime linker first link-edits the relocatable objects foo.o and bar.o to generate a shared object that is maintained in memory. This memory image is then inserted between the dynamic executable and its dependencies in the same manner as the shared object newstuff.so.1 was preloaded in the previous example. Again, the order in which these objects are processed can be displayed with ldd(1).
$ ldd -e LD_PRELOAD="./foo.o ./bar.o" ldd prog ./foo.o => ./foo.o ./bar.o => ./bar.o libc.so.1 => /lib/libc.so.1
These mechanisms of inserting an object after a dynamic executable provide for interposition. You can use these mechanisms to experiment with a new implementation of a function that resides in a standard shared object. If you preload an object containing this function, the object interposes on the original. Thus, the original functionality can be completely hidden with the new preloaded version.
Another use of preloading is to augment a function that resides in a standard shared object. The interposition of the new symbol on the original symbol enables the new function to carry out additional processing. The new function can also call through to the original function. This mechanism typically obtains the original symbol's address using dlsym(3C) with the special handle RTLD_NEXT.