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It might be a bit late, but I recently encountered the same issue and did some research. Based on your URL reference to fcntl's definition, I noticed it's actually implemented as:

extern int __REDIRECT (fcntl, (int __fd, int __cmd, ...), fcntl64);

Digging deeper, the __REDIRECT macro uses __asm__ ("xyz"), a technique that modifies symbol names at the assembly level.

Why not leverage this same approach to bypass glibc's behavior? Here's a code snippet I used:

#if defined (__GLIBC__) && defined (__GNUC__) && defined (__USE_FILE_OFFSET64)
extern int fcntl_legacy(int __fd, int __cmd, ...) __asm__("fcntl");
#else
#define fcntl_legacy fcntl
#endif

inline int set_nonblocking(int fd)
{
    int flags = fcntl_legacy(fd, F_GETFL, 0);
    if (flags == -1) {
        return -1;
    }

    return fcntl_legacy(fd, F_SETFL, flags | O_NONBLOCK);
}

Tested with -D_FILE_OFFSET_BITS=64 + GCC 14.2.1 + glibc 2.40 on amd64, this successfully redirects the dependency from fcntl64 to the legacy fcntl.

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