On 11/27/20 9:30 AM, Michael Kerrisk (man-pages) wrote: > Hi Alex, > > On 11/26/20 1:28 PM, Alejandro Colomar (mailing lists; readonly) wrote: >> >> >> On 11/26/20 10:31 AM, Michael Kerrisk (man-pages) wrote: >>> Hello Davide, >>> >>> On Thu, 26 Nov 2020 at 01:01, Davide Giorgio <davide@xxxxxxxxxxxxxxxx> wrote: >>>> >>>> Good morning, >>>> >>>> reading the pivot_root man page >>>> (https://man7.org/linux/man-pages/man2/pivot_root.2.html) >>>> there seems to be an error in the example source program >>>> "pivot_root_demo.c". >>>> In particular, there is a wrong check on the return value of mount(2). >>>> https://man7.org/linux/man-pages/man2/mount.2.html#RETURN_VALUE >>>> >>>> The error is in this line >>>> if (mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) == 1) >>>> >>>> that should be >>>> if (mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) == -1) >>>> >>>> >>>> Thank you for your work, kind regards >>> >>> Thanks! Fixed! >> >> Hi Michael, >> >> What about fixing this from a different approach: >> >> instead of comparing against -1 >> for functions that either return either 0 or -1, >> we can include those functions in the greater family of >> functions that return either 0 or non-zero (error code). >> I propose comparing against 0: >> >> - if (mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) == 1) >> + if (mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) != 0) >> >> I consider this to be safer, simpler, >> and although negligible, also faster. >> >> What are your thoughts? > > History and the standards say -1. (Okay, mount(2) is not in > POSIX, but the statement is true for syscalls generally.) So, I > prefer to use -1 (and always do so in my own code.) > > The check "ret != 0" does not work for system calls that > return a nonnegative value on success (e.g., open()). > > The check "ret < 0" does not work for system calls that > can legitimately return a value less than zero on success. > (getpriority() is the most notable example, but there > are one or two other cases also.) > > The check "ret == -1" is clear, and--in a standards > sense--precise. Though one must still be careful, since, > for example, getpriority() can return -1 on success. > (See the manual page for info on how to fdeal with this.) Hi Michael, Hmmm, I understand. Thanks, Alex > > Thanks, > > Michael >
