用于生成ftok（）键的公式是什么？

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;
}

Ie 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.