lkml.org 
[lkml]   [2014]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patches in this message
/
Date
From
Subject[RFC] Easier printing of unknown-size quantities

We have a number of types whose sizes are architecture- or
config-dependent such as pgoff_t or sector_t. The recommendation for
printing them is to cast them to (unsigned long long) and print them with
%Lu/%llx/... That's entirely reasonable except that it's so verbose.
I was wishing that C had a more succinct way to express that, when it
struct me that we can fix this without changing the compiler or language
spec, viz:

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index ecb8754..011b55e 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -28,7 +28,7 @@
#define LLONG_MIN (-LLONG_MAX - 1)
#define ULLONG_MAX (~0ULL)
#define SIZE_MAX (~(size_t)0)
+#define ULL(x) ((unsigned long long)(x))

#define STACK_MAGIC 0xdeadbeef


For demonstration purposes only (because I have another patch series
that would conflict with this):
diff --git a/fs/buffer.c b/fs/buffer.c
index 6024877..50e2596 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -148,7 +148,7 @@ static void buffer_io_error(struct buffer_head *bh)
{
char b[BDEVNAME_SIZE];
printk(KERN_ERR "Buffer I/O error on device %s, logical block %Lu\n",
- bdevname(bh->b_bdev, b),
- (unsigned long long)bh->b_blocknr);
+ bdevname(bh->b_bdev, b), ULL(bh->b_blocknr));
}

/*
@@ -257,8 +256,7 @@ __find_get_block_slow(struct block_device *bdev, sector_t block)

printk("__find_get_block_slow() failed. "
"block=%llu, b_blocknr=%llu\n",
- (unsigned long long)block,
- (unsigned long long)bh->b_blocknr);
+ ULL(block), ULL(bh->b_blocknr));
printk("b_state=0x%08lx, b_size=%zu\n",
bh->b_state, bh->b_size);
printk("device %s blocksize: %d\n", bdevname(bdev, b),
@@ -1083,8 +1081,7 @@ grow_buffers(struct block_device *bdev, sector_t block, int size)
char b[BDEVNAME_SIZE];

printk(KERN_ERR "%s: requested out-of-range block %llu for "
- "device %s\n",
- __func__, (unsigned long long)block,
+ "device %s\n", __func__, ULL(block),
bdevname(bdev, b));
return -EIO;
}
Seems like a clear win to me. What do others think? Might create a bit
of churn, but I find the "after" version easier to read than the former.

This is an RFC, not a PATCH, so I have deliberately mangled the patches to
not apply. I'm happy to supply working patches after we've bikeshedded
this one around a bit. I'm also happy to introduce it, but forbid any
mass conversion patches for a year until code has had the chance to
start using it.

In fact, let me help start the bikeshedding. I was trying to decide
what I wanted this to print:

unsigned int x, y;
x = y = 0x80000000;
printf("%llx\n", ULL(x+y));
I decided that I didn't want to magically fix the overflow, but rather
expose it. After all, if the user wants the promotion, they can always
write ULL(x)+y. That's why there are extra parens in the definition
of ULL.

I also considered naming the macro Lu(x) so you could use it like this:

printf("%Lu\n", Lu(x));

which is kind of cute, but it seemed clearer to me to name it the same
as the integer suffixes.

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."


\
 
 \ /
  Last update: 2014-01-09 15:21    [W:1.634 / U:0.108 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site