FTOK(3) - Linux man page online | Library functions

Convert a pathname and a project identifier to a System V IPC key.

FTOK(3) Linux Programmer's Manual FTOK(3)


ftok - convert a pathname and a project identifier to a System V IPC key


#include <sys/types.h> #include <sys/ipc.h> key_t ftok(const char *pathname, int proj_id);


The ftok() function uses the identity of the file named by the given pathname (which must refer to an existing, accessible file) and the least significant 8 bits of proj_id (which must be nonzero) to generate a key_t type System V IPC key, suitable for use with msgget(2), semget(2), or shmget(2). The resulting value is the same for all pathnames that name the same file, when the same value of proj_id is used. The value returned should be different when the (simultaneously existing) files or the project IDs differ.


On success, the generated key_t value is returned. On failure -1 is returned, with errno indicating the error as for the stat(2) system call.


For an explanation of the terms used in this section, see attributes(7). ┌──────────┬───────────────┬─────────┐ │Interface │ Attribute │ Value │ ├──────────┼───────────────┼─────────┤ │ftok() │ Thread safety │ MT-Safe │ └──────────┴───────────────┴─────────┘


POSIX.1-2001, POSIX.1-2008.


On some ancient systems, the prototype was: key_t ftok(char *pathname, char proj_id); Today, proj_id is an int, but still only 8 bits are used. Typical usage has an ASCII character proj_id, that is why the behavior is said to be undefined when proj_id is zero. Of course, no guarantee can be given that the resulting key_t is unique. Typically, a best-effort attempt combines the given proj_id byte, the lower 16 bits of the inode num‐ ber, and the lower 8 bits of the device number into a 32-bit result. Collisions may eas‐ ily happen, for example between files on /dev/hda1 and files on /dev/sda1.


msgget(2), semget(2), shmget(2), stat(2), svipc(7)


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GNU 2015-08-08 FTOK(3)
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ftok(3) referred by IPC::SysV(3perl) | ipcrm(1) | migratepages(8) | msgget(2) | numactl(8) | semget(2) | semget(2freebsd) | shmctl(2freebsd) | shmget(2) | shmget(2freebsd) | svipc(7)
refer to attributes(7) | msgget(2) | semget(2) | shmget(2) | stat(2) | svipc(7)
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