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.\"     @(#)fread.3	8.2 (Berkeley) 3/8/94
.\"
.Dd April 2, 2022
.Dt FREAD 3
.Os
.Sh NAME
.Nm fread ,
.Nm fread_unlocked ,
.Nm fwrite ,
.Nm fwrite_unlocked
.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 fread_unlocked "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"
.Ft size_t
.Fn fwrite_unlocked "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
The
.Fn fread_unlocked
and
.Fn fwrite_unlocked
functions are equivalent to
.Fn fread
and
.Fn fwrite
respectively, except that the caller is responsible for locking the stream
with
.Xr flockfile 3
before calling them.
These functions may be used to avoid the overhead of locking the stream
and to prevent races when multiple threads are operating on the same stream.
.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 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 -isoC .
.Sh HISTORY
The functions
.Fn fread
and
.Fn fwrite
first appeared in
.At v7 .