Hi folks,
I'm starting to learn a little C, and I figured I'd learn from the
masters ;-) I needed to read in some data from the network, and I
figured the safe_* calls would be a good example of how to do it correctly.
So, I took a look, and found:
static void safe_read(int fd, void *buffer, unsigned size)
{
int n = 0;
while (n < size) {
int ret = xread(fd, (char *) buffer + n, size - n);
if (ret < 0)
die("read error (%s)", strerror(errno));
if (!ret)
die("The remote end hung up unexpectedly");
n += ret;
}
}
Surely size and 'n' should have the same signed-ness?
And, in fact, shouldn't they actually be size_t, rather than 'int',
since xread is defined as:
static inline ssize_t xread(int fd, void *buf, size_t len)
{
ssize_t nr;
while (1) {
nr = read(fd, buf, len);
if ((nr < 0) && (errno == EAGAIN || errno == EINTR))
continue;
return nr;
}
}
And finally, 'ret' in safe_read should be a 'ssize_t', not an int, right?
Or is it just a case that we don't really care, since we control the
ranges of the values, and the underlying types are int anyway? Patches
to follow if I get an indication that anyone cares, otherwise I'd be
posting my question to a C newbies group. ;-)
Rogan
Rogan Dawes [off-list ref] wrote:
I'm starting to learn a little C, and I figured I'd learn from the
masters ;-) I needed to read in some data from the network, and I
figured the safe_* calls would be a good example of how to do it correctly.
...
static void safe_read(int fd, void *buffer, unsigned size)
{
int n = 0;
while (n < size) {
int ret = xread(fd, (char *) buffer + n, size - n);
...
Surely size and 'n' should have the same signed-ness?
Gah. Yes. And ret should be ssize_t.
And, in fact, shouldn't they actually be size_t, rather than 'int',
since xread is defined as:
Yes.
static inline ssize_t xread(int fd, void *buf, size_t len)
...
And finally, 'ret' in safe_read should be a 'ssize_t', not an int, right?
Oh, I see you noticed that too. ;-)
Or is it just a case that we don't really care, since we control the
ranges of the values, and the underlying types are int anyway? Patches
to follow if I get an indication that anyone cares, otherwise I'd be
posting my question to a C newbies group. ;-)
It is sort of a case we don't care. These probably should be fixed.
A patch would be nice. You want to learn C... ;-)
We currently assume that sizeof(unsigned) == sizeof(int) == 4,
and that nobody is crazy enough to call this functions with values
over ~2,000,000,000 so we don't practically have signed/unsigned
issues here. Right now anyway. But it shouldn't be like this.
So size_t/ssize_t are the right types.
The one that cracks me up is what moron declared read(2) to take
size_t as the input argument and ssize_t as the return value.
So I can pass in a value that if successfully read by the kernel
will actually be < 0 upon return, making my code think the read
call failed - but it didn't. Riiiiiiiiight.
xread was just following that standard, broken model.
--
Shawn.
Shawn O. Pearce wrote:
quoted
Or is it just a case that we don't really care, since we control the
ranges of the values, and the underlying types are int anyway? Patches
to follow if I get an indication that anyone cares, otherwise I'd be
posting my question to a C newbies group. ;-)
It is sort of a case we don't care. These probably should be fixed.
A patch would be nice. You want to learn C... ;-)
Ok. So I did a little patch, but I'm not sure whether I solved anything.
Now we have an implicit cast from size_t to ssize_t in packet_read_line.
I guess this echoes your comment about requesting a large size_t, and
getting a ssize_t result back. I suppose in theory we should be refusing
to handle a length greater than that which would fit into a ssize_t? Or
simply making sure to always return data smaller than max(ssize_t)?
Patch to follow.
Rogan