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HP-UX 11i Version 3: February 2007
fread(), fwrite() — buffered binary input/output to a stream file
size_t fread(void *__restrict ptr, size_t size, size_t nitems, FILE *__restrict stream);
size_t fwrite(const void *__restrict ptr, size_t size, size_t nitems, FILE *__restrict stream);
fread() copies, into an array pointed to by ptr, up to nitems items of data from the named input stream, where an item of data is a sequence of bytes (not necessarily terminated by a null byte) of length size. fread() stops appending bytes if an end-of-file or error condition is encountered while reading stream, or if nitems items have been read. fread() leaves the file pointer in stream, if defined, pointing to the byte following the last byte read if there is one. fread() does not change the contents of stream.
fwrite() appends at most nitems items of data from the array pointed to by ptr to the named output stream. fwrite() stops appending when it has appended nitems items of data or if an error condition is encountered on stream. fwrite() does not change the contents of the array pointed to by ptr.
The argument size is typically sizeof(*ptr) where the pseudo-function sizeof specifies the length of an item pointed to by ptr.
After fread() or fwrite() is applied to a stream, the stream becomes byte-oriented (see orientation(5)).
fread(), fwrite(), fread_unlocked(), and fwrite_unlocked() return the number of items read or written. If size or nitems is 0, no characters are read or written and 0 is returned.
The value returned will be less than nitems only if a read error or end-of-file is encountered.
When the file corresponding to an open stream gets extended after the end-of-file is reached, any subsequent calls to fread() will succeed and the end-of-file indicator will remain set. However, in the UNIX2003 standards environment (see standards(5)), this function will return zero and the end-of-file indicator will remain set.
The ferror() or feof() functions must be used to distinguish between an error condition and an end-of-file condition.
Refer to getc(3S) for a description of errors returned by fread().
Refer to putc(3S) for a description of errors returned by fwrite().
fread_unlocked() and fwrite_unlocked() are obsolescent interfaces supported only for compatibility with existing DCE applications. New multithreaded applications should use fread() and fwrite().