+.PP
+In order to support the same number of open files as does the system,
+.I fopen
+must allocate additional memory for data structures using
+.I calloc
+after 20 files have been opened.
+This confuses some programs which use their own memory allocators.
+An undocumented routine,
+.IR f_prealloc ,
+may be called to force immediate allocation of all internal memory
+except for buffers.