IBM developerWorks: Override the GNU C Library–Painlessly

[ Thanks to Maria
for this link. ]

“What do you do if you don’t have the source for your
application and it’s failing because a GNU Library for C (glibc)
function is returning something bad to the application? Because
glibc is open-source, you can of course get the source code, make
your changes, rebuild, and install. This is not for the faint of
heart, however, because although the API is well documented, the
internal organization of the GNU C library is not. Finding the
correct function prototypes is only the first of many challenges.
It’s a big package as well, so the first time you compile, it will
take some time (glibc 2.2.2 has 8,552 files and 1,775,440 lines of
code, including comments).

“Better than rebuilding glibc is selectively overriding a
function. Many of the modern Unixes support the concept of
preloading user defined libraries. These libraries can be either
complete replacements (that is, a private version of glibc) or
subsets — even a single function. You can use a private version of
glibc by setting the LD_LIBRARY_PATH to include your private
version of the library first. You can use a subset of library
routines that you write by using the LD_PRELOAD environmental
value. Both LD_LIBRARY_PATH and LD_PRELOAD are controlled by the
dynamic ELF linker/loader. It uses a first match to satisfy any
symbol name. By preloading your version of a library or function
you short circuit the normal path, allowing you to override
it…”

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