Typically it's the other way around, but we can't have shared library named libc.so.6 in target/release directory.
cargo includes 'target/release' in LD_LIBRARY_PATH for build script, so even if clean build runs fine,
every subsquent run will make build script link with relibc.
It seams that stdout of ld.so is not that much of an issue but actually
it unfortunately is. The major problem here is that sometimes programs
generate header files in stdout (./getmy_custom_headers > header.h) and
we need to keep that cleen. and this is very very popular in gcc.
This patch avoids collecting symbols, resolving relocs if they are
already done (usually for example libc.so during a dlopen for another
libfoo.so). This patch is purely for performance boost.
It is usually not optimal to load a library twice and for specifics,
it is **terrible** idea to load libc twice it was enough trouble
dealing with libc statically linked into ld.so. So What this patch does
it check for soname and if a library is already loaded it won't get
loaded again. Why soname ? because unfortunately some bins gets linked
againt libc.so while of their dependencies gets linked against
libc.so.6 while one is usually symbolic link for the other.
Usually it is possible to refer to library either by the file name or by
elf "soname" soname is very similar for specifying something like
(LIB/API version) combination so if for example you have ./prog that
loads libx.so which is version 5.1.1 and there is ./plugin.so that ./prog
would load that requires libx.so version 5.1.2 both libx.so should have
the same soname to hint that they offer the exact same functionality.
And this patch specifies the soname for relibc libc.so.
struct flock is posix defined locking mechanism on *nix platform
Example usage (copied from https://gavv.github.io/articles/file-locks/) :
#include <fcntl.h>
struct flock fl;
memset(&fl, 0, sizeof(fl));
// lock in shared mode
fl.l_type = F_RDLCK;
// lock entire file
fl.l_whence = SEEK_SET; // offset base is start of the file
fl.l_start = 0; // starting offset is zero
fl.l_len = 0; // len is zero, which is a special value representing end
// of file (no matter how large the file grows in future)
fl.l_pid = 0; // F_SETLK(W) ignores it; F_OFD_SETLK(W) requires it to be zero
// F_SETLKW specifies blocking mode
if (fcntl(fd, F_SETLKW, &fl) == -1) {
exit(1);
}
// atomically upgrade shared lock to exclusive lock, but only
// for bytes in range [10; 15)
//
// after this call, the process will hold three lock regions:
// [0; 10) - shared lock
// [10; 15) - exclusive lock
// [15; SEEK_END) - shared lock
fl.l_type = F_WRLCK;
fl.l_start = 10;
fl.l_len = 5;
// F_SETLKW specifies non-blocking mode
if (fcntl(fd, F_SETLK, &fl) == -1) {
exit(1);
}
// release lock for bytes in range [10; 15)
fl.l_type = F_UNLCK;
if (fcntl(fd, F_SETLK, &fl) == -1) {
exit(1);
}
// close file and release locks for all regions
// remember that locks are released when process calls close()
// on any descriptor for a lock file
close(fd);
Mateusz Tabaka