SEMOP(3P)

💖 SEMOP(3P) 💖

okay bestie, gather round, because this is the POSIX Programmer's Manual page for semop, and it's lowkey a whole journey. ✨

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🌸 PROLOG 🌸

so like, real talk first: this manual page is part of the POSIX Programmer's Manual. the Linux implementation of this interface may differ — so if you're on Linux, go consult the corresponding Linux manual page for the actual tea, okay? and also... the interface may not be implemented on Linux at all. 😭 not me finding that out the hard way. anyway.

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✨ NAME ✨

semop — XSI semaphore operations 🎀

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🫶 SYNOPSIS 🫶

#include <sys/sem.h>

int semop(int semid, struct sembuf *sops, size_t nsops);

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💅 DESCRIPTION 💅

so the semop() function operates on XSI semaphores (see the Base Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore). and here's a lil caveat bestie: it is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime. so like, don't count on it, iykyk.

the semop() function shall perform atomically a user-defined array of semaphore operations in array order on the set of semaphores associated with the semaphore identifier specified by the argument semid. atomic, periodt. no half-doing it. 💅

the argument sops is a pointer to a user-defined array of semaphore operation structures. and the implementation shall not modify elements of this array unless the application uses implementation-defined extensions. so hands off, basically. 🙈

the argument nsops is the number of such structures in the array. cute.

each structure, sembuf, includes the following members:

• unsigned short sem_num — Semaphore number.
• short sem_op — Semaphore operation.
• short sem_flg — Operation flags.

each semaphore operation specified by sem_op is performed on the corresponding semaphore specified by semid and sem_num. 🌈

the variable sem_op specifies one of three semaphore operations, and this is the part where it's giving main character energy:

1. if sem_op is a negative integer and the calling process has alter permission, one of the following shall occur:
   • if semval (see <sys/sem.h>) is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from semval. also, if (sem_flg & SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the semadj value of the calling process for the specified semaphore. 💖
   • if semval is less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT) is non-zero, semop() shall return immediately. bye bestie. 👋
   • if semval is less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT) is 0, semop() shall increment the semncnt associated with the specified semaphore and suspend execution of the calling thread until one of the following conditions occurs:
     • the value of semval becomes greater than or equal to the absolute value of sem_op. when this occurs, the value of semncnt associated with the specified semaphore shall be decremented, the absolute value of sem_op shall be subtracted from semval and, if (sem_flg & SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the semadj value of the calling process for the specified semaphore. ✨
     • the semid for which the calling thread is awaiting action is removed from the system. when this occurs, errno shall be set to [EIDRM] and -1 shall be returned. 💀
     • the calling thread receives a signal that is to be caught. when this occurs, the value of semncnt associated with the specified semaphore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction(). 🦋
2. if sem_op is a positive integer and the calling process has alter permission, the value of sem_op shall be added to semval and, if (sem_flg & SEM_UNDO) is non-zero, the value of sem_op shall be subtracted from the semadj value of the calling process for the specified semaphore. 🎀
3. if sem_op is 0 and the calling process has read permission, one of the following shall occur:
   • if semval is 0, semop() shall return immediately. slay.
   • if semval is non-zero and (sem_flg & IPC_NOWAIT) is non-zero, semop() shall return immediately.
   • if semval is non-zero and (sem_flg & IPC_NOWAIT) is 0, semop() shall increment the semzcnt associated with the specified semaphore and suspend execution of the calling thread until one of the following occurs:
     • the value of semval becomes 0, at which time the value of semzcnt associated with the specified semaphore shall be decremented. 🌸
     • the semid for which the calling thread is awaiting action is removed from the system. when this occurs, errno shall be set to [EIDRM] and -1 shall be returned. 😭
     • the calling thread receives a signal that is to be caught. when this occurs, the value of semzcnt associated with the specified semaphore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction().

upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops shall be set to the process ID of the calling process. also, the sem_otime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description. ⭐

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👑 RETURN VALUE 👑

upon successful completion, semop() shall return 0; otherwise, it shall return -1 and set errno to indicate the error. that's the whole vibe check. 💜

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🔥 ERRORS 🔥

the semop() function shall fail if:

• E2BIG — The value of nsops is greater than the system-imposed maximum.
• EACCES — Operation permission is denied to the calling process; see Section 2.7, XSI Interprocess Communication.
• EAGAIN — The operation would result in suspension of the calling process but (sem_flg & IPC_NOWAIT) is non-zero.
• EFBIG — The value of sem_num is greater than or equal to the number of semaphores in the set associated with semid.
• EIDRM — The semaphore identifier semid is removed from the system.
• EINTR — The semop() function was interrupted by a signal.
• EINVAL — The value of semid is not a valid semaphore identifier, or the number of individual semaphores for which the calling process requests a SEM_UNDO would exceed the system-imposed limit.
• ENOSPC — The limit on the number of individual processes requesting a SEM_UNDO would be exceeded.
• ERANGE — An operation would cause a semval to overflow the system-imposed limit, or an operation would cause a semadj value to overflow the system-imposed limit.

ngl bestie, ERANGE and EINVAL are the ones that sneak up on you. read the limits. 👀

the following sections are informative.

🎀 EXAMPLES 🎀

Okay bestie, this is the part where it all clicks. Two whole examples, real code, we love to see it. 💖

💅 Setting Values in Semaphores

So the goal here: set the values of the two semaphores that belong to the semid identifier to whatever's chilling in the sb array. Slay.

#include <sys/sem.h>
...
int semid;
struct sembuf sb[2];
int nsops = 2;
int result;

/* Code to initialize semid. */
...

/* Adjust value of semaphore in the semaphore array semid. */
sb[0].sem_num = 0;
sb[0].sem_op = -1;
sb[0].sem_flg = SEM_UNDO | IPC_NOWAIT;
sb[1].sem_num = 1;
sb[1].sem_op = 1;
sb[1].sem_flg = 0;

result = semop(semid, sb, nsops);

🦋 Creating a Semaphore Identifier

This one's the main character fr. ✨ Here's the plot: the program grabs a unique semaphore key with the ftok() function, then gets a semaphore ID tied to that key using the semget() function. And bestie — that first call also doubles as a vibe check to make sure the semaphore actually exists. If it doesn't exist? The program just makes it, which is what the second call to semget() is doing. When it's creating the semaphore for the queuing process, the program tries to create one semaphore with read/write permission for all. It also uses the IPC_EXCL flag, which forces semget() to fail if the semaphore is already there (no duplicates, periodt).

Once the semaphore exists, the program calls semctl() and semop() to initialize it to the values sitting in the sbuf array. The number of processes that can run concurrently without queuing is initially set to 2. And that final call to semget() creates a semaphore identifier you can use later in the program. Iconic.

Processes that get semid without creating it check that sem_otime is non-zero — that's how they make sure the creating process has finished the semop() initialization. Trust the process, but verify. 😌

The final call to semop() acquires the semaphore and waits until it's free. And the SEM_UNDO option releases the semaphore when the process exits, waiting until there are less than two processes running concurrently. Gorgeous.

#include <stdio.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdlib.h>
...
key_t semkey;
int semid;
struct sembuf sbuf;
union semun {
    int val;
    struct semid_ds *buf;
    unsigned short *array;
} arg;
struct semid_ds ds;
...
/* Get unique key for semaphore. */
if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
    perror("IPC error: ftok"); exit(1);
}

/* Get semaphore ID associated with this key. */
if ((semid = semget(semkey, 0, 0)) == -1) {

    /* Semaphore does not exist - Create. */
    if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
        S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
    {
        /* Initialize the semaphore. */
        arg.val = 0;
        sbuf.sem_num = 0;
        sbuf.sem_op = 2;  /* This is the number of runs without queuing. */
        sbuf.sem_flg = 0;
        if (semctl(semid, 0, SETVAL, arg) == -1
            || semop(semid, &sbuf, 1) == -1) {
            perror("IPC error: semop"); exit(1);
        }
    }
    else if (errno == EEXIST) {
        if ((semid = semget(semkey, 0, 0)) == -1) {
            perror("IPC error 1: semget"); exit(1);
        }
        goto check_init;
    }
    else {
        perror("IPC error 2: semget"); exit(1);
    }
}
else
{
    /* Check that semid has completed initialization. */
    /* An application can use a retry loop at this point rather than
       exiting. */
    check_init:
    arg.buf = &ds;
    if (semctl(semid, 0, IPC_STAT, arg) < 0) {
        perror("IPC error 3: semctl"); exit(1);
    }
    if (ds.sem_otime == 0) {
        perror("IPC error 4: semctl"); exit(1);
    }
}
...
sbuf.sem_num = 0;
sbuf.sem_op = -1;
sbuf.sem_flg = SEM_UNDO;
if (semop(semid, &sbuf, 1) == -1) {
    perror("IPC Error: semop"); exit(1);
}

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🌸 APPLICATION USAGE 🌸

Okay so real talk: the POSIX Realtime Extension defines alternative interfaces for interprocess communication. So if you're an app dev who needs to use IPC, do yourself a favor and design your applications so that any modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use those alternative interfaces later. Future-proofing, we love her. 💜

💀 RATIONALE 💀

None.

(Yeah bestie, there's literally nothing here. Moving on. 🙈)

⭐ FUTURE DIRECTIONS ⭐

None.

(Also nothing. The void. We respect it. ✨)

👑 SEE ALSO 👑

• Section 2.7, XSI Interprocess Communication
• Section 2.8, Realtime
• exec
• exit()
• fork()
• semctl()
• semget()
• sem_close()
• sem_destroy()
• sem_getvalue()
• sem_init()
• sem_open()
• sem_post()
• sem_trywait()
• sem_unlink()

And from the Base Definitions volume of POSIX.1‐2017:

• Section 4.17, Semaphore
• <sys_ipc.h>
• <sys_sem.h>
• <sys_types.h>

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💖 COPYRIGHT 💖

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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And that's a wrap, bestie!! 🫶 This whole thing came to you from IEEE/The Open Group, dated 2017, and the page itself is SEMOP(3P). Go forth and semaphore responsibly. No cap. 💅🌈