C Language and Libraries Help (clang.hlp) (Table of Contents; Topic list)
_filelength
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─────Run-Time Library───────────────────────────────────────────────────────
 
     The _filelength function returns the length, in bytes, of the
     target file associated with <handle>.
 
     Return Value
 
     The return value is described above. A return value of -1L
     indicates an error, and an invalid handle sets errno to EBADF.
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