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some more C-lib specific advice

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Huzaifa Sidhpurwala 2021-09-18 11:00:46 +05:30
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modules/ROOT/pages/programming-languages/C-Libc.adoc
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@ -288,7 +288,8 @@ them to find out what the length actually means.
=== Using tricky syscalls or library functions === Using tricky syscalls or library functions
==== `readlink` ==== `readlink`
This is the hardest system call to use correctly because of everything you have to do This is the hardest system call to use correctly because of everything you have to do
* The buf should be of PATH_MAX length, that includes space for the terminating NUL character. * The buf should be of PATH_MAX length, that includes space for the terminating NUL character.
* The bufsize should be `sizeof(buf) - 1` * The bufsize should be `sizeof(buf) - 1`
* `readlink` return value should be caught as a signed integer (ideally type `ssize_t`). * `readlink` return value should be caught as a signed integer (ideally type `ssize_t`).
@ -304,4 +305,7 @@ This is the hardest system call to use correctly because of everything you have
* If `S_ISLNK` macro is used, the stat buffer MUST come from lstat or from fstatat with `AT_SYMLINK_NOFOLLOW` * If `S_ISLNK` macro is used, the stat buffer MUST come from lstat or from fstatat with `AT_SYMLINK_NOFOLLOW`
* If you are doing something really important, call fstat after opening and compare the before and after stat buffers before trusting them. * If you are doing something really important, call fstat after opening and compare the before and after stat buffers before trusting them.
=== `setgid`, `setuid`:
* Call these in the right order: groups and then uid.
* Always check the return code.
* If `setgid` & `setuid` are used, supplemental groups are not reset. This must be done with setgroups or initgroups before the uid change.