The issue is discussed in the blog post Solaris: Workaround to stdio's 255 open file descriptors limitation.

If the system is running any existing major customer releases of Solaris 10 i.e., Solaris 10 3/05 through Solaris 10 11/06, extended FILE facility can be installed on these systems by applying the kernel patch, 125100-04 or later & libc patch, 120473-05 or later.

Systems running Solaris Express (SX) or any OpenSolaris distribution after build 39 does not need the patches mentioned above to get the extended FILE solution for stdio's 256 open files limitation.

Do not forget to read the man pages of extendedFILE(5), enable_extended_FILE_stdio(3C), fopen(3C), fdopen(3C) and popen(3C) to enable the extended FILE solution by pre-loading /usr/lib/extendedFILE.so.1 (run-time solution) or by using the enhanced stdio's interfaces, fopen(), fdopen(), popen() or with the new interface enable_extended_FILE_stdio().

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Acknowledgments:
Peter Shoults, Sun Microsystems
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