| .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) |
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| .\" Modified 2001-11-28, by Michael Kerrisk, <mtk.manpages@gmail.com> |
| .\" Changed data type of proj_id; minor fixes |
| .\" aeb: further fixes; added notes. |
| .\" |
| .TH FTOK 3 2015-08-08 "GNU" "Linux Programmer's Manual" |
| .SH NAME |
| ftok \- convert a pathname and a project identifier to a System V IPC key |
| .SH SYNOPSIS |
| .nf |
| .B #include <sys/types.h> |
| .B #include <sys/ipc.h> |
| .fi |
| .sp |
| .BI "key_t ftok(const char *" pathname ", int " proj_id ); |
| .SH DESCRIPTION |
| The |
| .BR ftok () |
| function uses the identity of the file named by the given |
| .I pathname |
| (which must refer to an existing, accessible file) |
| and the least significant 8 bits of |
| .I proj_id |
| (which must be nonzero) to generate a |
| .I key_t |
| type System V IPC key, suitable for use with |
| .BR msgget (2), |
| .BR semget (2), |
| or |
| .BR shmget (2). |
| .LP |
| The resulting value is the same for all pathnames that |
| name the same file, when the same value of |
| .I proj_id |
| is used. |
| The value returned should be different when the |
| (simultaneously existing) files or the project IDs differ. |
| .SH RETURN VALUE |
| On success, the generated |
| .I key_t |
| value is returned. |
| On failure \-1 is returned, with |
| .I errno |
| indicating the error as for the |
| .BR stat (2) |
| system call. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lb lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR ftok () |
| T} Thread safety MT-Safe |
| .TE |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| On some ancient systems, the prototype was: |
| .sp |
| .RS |
| .BI "key_t ftok(char *" pathname ", char " proj_id ); |
| .RE |
| .PP |
| Today, |
| .I proj_id |
| is an |
| .IR int , |
| but still only 8 bits are used. |
| Typical usage has an ASCII character |
| .IR proj_id , |
| that is why the behavior is said to be undefined when |
| .I proj_id |
| is zero. |
| .LP |
| Of course, no guarantee can be given that the resulting |
| .I key_t |
| is unique. |
| Typically, a best-effort attempt combines the given |
| .I proj_id |
| byte, the lower 16 bits of the inode number, and the |
| lower 8 bits of the device number into a 32-bit result. |
| Collisions may easily happen, for example between files on |
| .I /dev/hda1 |
| and files on |
| .IR /dev/sda1 . |
| .SH SEE ALSO |
| .BR msgget (2), |
| .BR semget (2), |
| .BR shmget (2), |
| .BR stat (2), |
| .BR svipc (7) |