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int (*log_fn)(const char *, ...) = printf;
/* ... */
log_fn("foo");

If a vulnerability exists in this program that allows an attacker to overwrite the log_fn function pointer, such as a buffer overflow or arbitrary memory write, the attacker may be able to overwrite the value of printf with the location of an arbitrary function.

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#include <Windows.h>
 
void *log_fn = EncodePointer(printf);
/* ... */
int (*fn)(const char *, ...) = (int (*)(const char *, ...))DecodePointer(log_fn);

fn("foo");

Note that DecodePointer() does not return success or failure.  If an attacker has overwritten the pointer contained in log_fn, the pointer returned will be invalid and cause your application to crash.  However, this is preferable to giving an attacker the ability to execute arbitrary code. 

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Bibliography

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