pread() return 0 if read beyond end of file in linux, but not zero in cygwin.
I have a small code to show the problem:
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
int main()
{
const char* file = "/home/xliu/work/exome/a.bam";
struct stat st;
stat(file, &st);
char buf[65536];
int fd = open(file, O_RDONLY);
int ret = pread(fd, buf, sizeof buf, st.st_size);
fprintf(stderr, "filesize %ld, after eof pread() return = %d, errno = %d\n",
st.st_size, ret, errno);
lseek(fd, st.st_size, SEEK_SET);
ret = read(fd, buf, sizeof buf);
fprintf(stderr, "filesize %ld, after eof read() return = %d, errno = %d\n",
st.st_size, ret, errno);
}
$ ./a.exe
filesize 6126093048, after eof pread() return = 3, errno = 0
filesize 6126093048, after eof read() return = 0, errno = 0
The issue is that NtReadFile() return EOF status, but doesn't set
io.information to 0. As a result, the current pread() implementation could
return an arbitrary number in the stack. The fix is a one line fix: reset io
status block.
diff --git a/winsup/cygwin/fhandler_disk_file.cc
b/winsup/cygwin/fhandler_disk_file.ccindex bc8fead..525cb32 100644---
a/winsup/cygwin/fhandler_disk_file.cc+++
b/winsup/cygwin/fhandler_disk_file.cc@@ -1525,6 +1525,7 @@
fhandler_disk_file::pread (void *buf, size_t count, off_t offset)
IO_STATUS_BLOCK io; LARGE_INTEGER off = { QuadPart:offset };
+ memset(&io, 0, sizeof(io)); if (!prw_handle && prw_open (false))
goto non_atomic; status = NtReadFile (prw_handle, NULL, NULL, NULL,
&io, buf, count,
