Normally, to indicate EOF to a program attached to standard input on a Linux terminal, I need to press Ctrl+D once if I just pressed Enter, or twice otherwise. I noticed that th
I've managed to confirm that this is due to an unambiguous bug in glibc versions prior to 2.28 (commit 2cc7bad). Relevant quotes from the C standard:
The byte input/output functions — those functions described in this subclause that perform input/output: [...],
fread
The byte input functions read characters from the stream as if by successive calls to the
fgetc
function.If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file indicator for the stream is set and the
fgetc
function returnsEOF
. Otherwise, thefgetc
function returns the next character from the input stream pointed to bystream
.
(emphasis on "or" mine)
The following program demonstrates the bug with fgetc
:
#include
int main(void) {
while(fgetc(stdin) != EOF) {
puts("Read and discarded a character from stdin");
}
puts("fgetc(stdin) returned EOF");
if(!feof(stdin)) {
/* Included only for completeness. Doesn't occur in my testing. */
puts("Standard violation! After fgetc returned EOF, the end-of-file indicator wasn't set");
return 1;
}
if(fgetc(stdin) != EOF) {
/* This happens with glibc in my testing. */
puts("Standard violation! When fgetc was called with the end-of-file indicator set, it didn't return EOF");
return 1;
}
/* This happens with musl in my testing. */
puts("No standard violation detected");
return 0;
}
To demonstrate the bug:
The exact bug is that if the end-of-file stream indicator is set, but the stream is not at end-of-file, glibc's fgetc will return the next character from the stream, rather than EOF as the standard requires.
Since fread
is defined in terms of fgetc
, this is the cause of what I originally saw. It's previously been reported as glibc bug #1190 and has been fixed since commit 2cc7bad in February 2018, which landed in glibc 2.28 in August 2018.