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Imagine a constructor like this:
int object_new(void **out) {
void *my_object;
int r;
...
r = ioctl(...);
if (r < 0)
return -errno;
...
*out = my_object;
return 0;
}
We have a lot of those in systemd. If you now call those, gcc might inline
the call and optimize it. However, gcc cannot know that "errno" is
negative if "r" is. Therefore, a caller like this will produce warnings:
r = object_new(&obj);
if (r < 0)
return r;
obj->xyz = "foobar";
In case the ioctl in the constructor fails, gcc might assume "errno" is 0
and thus the error-handling is not triggered. Therefore, "obj" is
uninitialized, but accessed. Gcc will warn about that.
The new negative_errno() helper can be used to mitigate those warnings.
The helper is guaranteed to return a negative integer. Furthermore, it
spills out runtime warnings if "errno" is non-negative.
Instead of returning "-errno", you can use:
return negative_errno();
gcc will no longer assume that this can return >=0, thus, it will not warn
about it.
Use this new helper in libsystemd-terminal to fix some grdev-drm warnings.