What is the formula used to produce a ftok() key?

Question

What is the formula used to produce a key that ftok() produces? ftok is a Linux function for creating keys for SYSTEM V IPC.

Answer 1

In ftok from glibc 2.29:

key_t
ftok (const char *pathname, int proj_id)
{
  struct stat64 st;
  key_t key;

  if (__xstat64 (_STAT_VER, pathname, &st) < 0)
    return (key_t) -1;

  key = ((st.st_ino & 0xffff) | ((st.st_dev & 0xff) << 16)
     | ((proj_id & 0xff) << 24));

  return key;
}

I.e. it's creating a 32-bit key_t by taking the upper 8 bits from the lower 8 bits of proj_id, the second upper 8 bits from the lower 8 bits of the device number of the provided pathname, and the lower 16 bits from the lower 16 bits of the inode number of the provided pathname.

musl libc uses the same algorithm:

key_t ftok(const char *path, int id)
{
    struct stat st;
    if (stat(path, &st) < 0) return -1;

    return ((st.st_ino & 0xffff) | ((st.st_dev & 0xff) << 16) | ((id & 0xffu) << 24));
}

Answer 2

The ftok() source code in glibc library is :

#include <sys/ipc.h>
#include <sys/stat.h>
key_t
ftok (const char *pathname, int proj_id)
{
  struct stat64 st;
  key_t key;
  if (__xstat64 (_STAT_VER, pathname, &st) < 0)
    return (key_t) -1;
  key = ((st.st_ino & 0xffff) | ((st.st_dev & 0xff) << 16)
         | ((proj_id & 0xff) << 24));
  return key;
}

Other functions are available here too.