Question

I have written wrappers for both open() and open64(). Now I run vi by preloading my wrapper library using LD_PRELOAD environment variable and I see that the open64() wrapper is used instead of the open(). But when I strace vi I see that the system calls made is to open() (of course including the other system calls). What is the issue here?

Answer 1

The strace utility traces system calls (syscall) and open happens to be both a syscall and a library function. Both the open() and open64() library functions use the open syscall internally to request services from the kernel. It is my understanding that using open() with the O_LARGEFILE flag is equivalent to using open64() to support large files in 32-bit applications.

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If the call to open64() were to call the open() function internally, your open() wrapper would not be called because you cannot interpose internal library function calls. They are resolved before runtime.