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.\"     @(#)fread.3	8.2 (Berkeley) 3/8/94
.\"
.Dd February 1, 2020
.Dt FREAD 3
.Os
.Sh NAME
.Nm fread ,
.Nm fwrite
.Nd binary stream input/output
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft size_t
.Fn fread "void * restrict ptr" "size_t size" "size_t nmemb" "FILE * restrict stream"
.Ft size_t
.Fn fwrite "const void * restrict ptr" "size_t size" "size_t nmemb" "FILE * restrict stream"
.Sh DESCRIPTION
The function
.Fn fread
reads
.Fa nmemb
objects, each
.Fa size
bytes long, from the stream pointed to by
.Fa stream ,
storing them at the location given by
.Fa ptr .
.Pp
The function
.Fn fwrite
writes
.Fa nmemb
objects, each
.Fa size
bytes long, to the stream pointed to by
.Fa stream ,
obtaining them from the location given by
.Fa ptr .
.Pp
Mixing
.Fn fread
and
.Fn fwrite
calls without setting the file position explicitly using
.Xr fsetpos 3
between read and write or write and read operations will lead to unexpected
results because of buffering the file pointer not being set to the
expected position after each operation completes.
This behavior is allowed by ANSI C for efficiency and it will not be changed.
.Sh RETURN VALUES
The functions
.Fn fread
and
.Fn fwrite
advance the file position indicator for the stream
by the number of bytes read or written.
They return the number of objects read or written.
If
.Fa size
or
.Fa nmemb
is 0, the functions return 0 and the state of
.Fa stream
remains unchanged.
.Pp
If the product of
.Fa size
and
.Fa nmemb
results in size_t overflow, 0 is returned and errno
is set to
.Er EOVERFLOW .

If an error occurs, or the end-of-file is reached,
the return value is a short object count (or zero).
.Pp
The function
.Fn fread
does not distinguish between end-of-file and error, and callers
must use
.Xr feof 3
and
.Xr ferror 3
to determine which occurred.
The function
.Fn fwrite
returns a value less than
.Fa nmemb
only if a write error has occurred.
.Sh SEE ALSO
.Xr read 2 ,
.Xr write 2
.Sh STANDARDS
The functions
.Fn fread
and
.Fn fwrite
conform to
.St -ansiC .