On 04/02/2018 06:25 PM, Stephen Hemminger wrote:
On Sat, 31 Mar 2018 14:48:55 -0400
Neil Horman <[email protected]> wrote:
On Sat, Mar 31, 2018 at 06:21:41PM +0200, Gaëtan Rivet wrote:
On Sat, Mar 31, 2018 at 11:27:55AM -0400, Neil Horman wrote:
On Sat, Mar 31, 2018 at 05:09:47PM +0200, Gaëtan Rivet wrote:
On Sat, Mar 31, 2018 at 09:33:43AM -0400, Neil Horman wrote:
On Fri, Mar 30, 2018 at 10:47:09PM +0800, Tonghao Zhang wrote:
I rebuild it on ubuntu 17.10 and cash it. I use the 'RTE_SET_USED' to fix it.
diff --git a/lib/librte_vhost/fd_man.c b/lib/librte_vhost/fd_man.c
index 771675718..f11803191 100644
--- a/lib/librte_vhost/fd_man.c
+++ b/lib/librte_vhost/fd_man.c
@@ -279,7 +279,8 @@ fdset_pipe_read_cb(int readfd, void *dat __rte_unused,
int *remove __rte_unused)
{
char charbuf[16];
- read(readfd, charbuf, sizeof(charbuf));
+ int r = read(readfd, charbuf, sizeof(charbuf));
+ RTE_SET_USED(r);
}
void
@@ -319,5 +320,6 @@ fdset_pipe_init(struct fdset *fdset)
void
fdset_pipe_notify(struct fdset *fdset)
{
- write(fdset->u.writefd, "1", 1);
+ int r = write(fdset->u.writefd, "1", 1);
+ RTE_SET_USED(r);
}
A better option might be to use _Pragma
Something like this perhaps
#define ALLOW_UNUSED(x) \
_Pragma(push) \
_Pragma(diagnostic ignored "-Wunused-result") \
#x;\
_Pragma(pop)
This is of course untested, so it probably needs some tweaking, but this method
avoids the need to declare an additional stack variable, which i don't think can
be eliminated due to the cast. I believe that this method should also work
accross compilers (the gcc and clang compilers support this, and i think the
intel compiler should as well)
Neil
It would be nice to avoid the definition of a useless variable.
An alternative could be
if (read() < 0) {
/* Failure here is acceptable for such and such reason. */
}
to ensure all-around compatibility, and the definition or another macro.
Just a suggestion.
That would be a good alternative, but I think its effectiveness is dependent on
when the compiler does with the return value check. Without any code inside the
conditional, the compiler may optimize the check out, meaning the warning will
still be asserted. If it doesn't optimize the check out, then you have a
useless compare and jump instruction left in the code path.
Best
Neil
I tested quickly, I see no difference with the three methods:
gcc seems to be sufficiently smart to optimize out the conditional, clang not so
much:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
__attribute__((warn_unused_result))
int wur(void)
{
printf("CALLING WUR!\n");
return read(0, NULL, 0);
}
#define UNUSED_RESULT(x) if (x) {}
int main(void)
{
UNUSED_RESULT(wur());
return 0;
}
[nhorman@neilslaptop ~]$ gcc -g -Wunused-result -Werror ./test.c
[nhorman@neilslaptop ~]$ objdump -d -S a.out > ./results
[nhorman@neilslaptop ~]$ cat results
...
000000000040054b <main>:
#define UNUSED_RESULT(x) if (x) {}
int main(void)
{
40054b: 55 push %rbp
40054c: 48 89 e5 mov %rsp,%rbp
UNUSED_RESULT(wur());
40054f: e8 d3 ff ff ff callq 400527 <wur>
return 0;
400554: b8 00 00 00 00 mov $0x0,%eax
}
400559: 5d pop %rbp
40055a: c3 retq
40055b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
[nhorman@neilslaptop ~]$ clang -g -Wunused-result -Werror ./test.c
[nhorman@neilslaptop ~]$ objdump -d -S a.out > ./results
[nhorman@neilslaptop ~]$ cat results
...
0000000000400570 <main>:
}
#define UNUSED_RESULT(x) if (x) {}
int main(void)
{
400570: 55 push %rbp
400571: 48 89 e5 mov %rsp,%rbp
400574: 48 83 ec 10 sub $0x10,%rsp
400578: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%rbp)
UNUSED_RESULT(wur());
40057f: e8 ac ff ff ff callq 400530 <wur>
400584: 83 f8 00 cmp $0x0,%eax
400587: 0f 84 05 00 00 00 je 400592 <main+0x22>
40058d: e9 00 00 00 00 jmpq 400592 <main+0x22>
400592: 31 c0 xor %eax,%eax
return 0;
400594: 48 83 c4 10 add $0x10,%rsp
400598: 5d pop %rbp
400599: c3 retq
40059a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
There is an additional compare and two jump statements there. I'm sure
eventually most compilers will figure out how to eliminate this, and it might
even do so now with the right optimization flags, but I think its best to just
organize the source such that no conditional branching is implied. Assuming the
intel compiler supports it (which I think it should, can someone with access to
it confirm), the _Pragma utility is probably the most clear way to do that.
Regards
Neil
Rather than wallpapering over the unused result, why not do real error checking?
If the program was run in a non-Linux environment (such as WSL etc), maybe an
error
could occur. Best to return an error; or at least call rte_exit().
Do we really want to call rte_exit() in a library?
