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@(#)fread.3 8.2 (Berkeley) 3/8/94
$FreeBSD: stable/11/lib/libc/stdio/fread.3 357852 2020-02-13 03:13:29Z kevans $

.Dd January 23, 2020 .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 n 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 .

p 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 .

p 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).

p 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 .