Understanding a simple semaphore program

[pythoNewbie]

Dear experts,

I am trying to understand a simple semaphore program written in C, and trying to insert some printf statements in the code , but there is no output at all.


#include<stdio.h>
#include<phtread.h>
#include<sys/ipc.h>
#include<sys/sem.h>

int i, nb_place;
int semid;
struct sembuf operation;
 
void reservation()
{
/*prinff*/
/*operation p */
   operation.sem_num = 0;
operation.sem_op = -1;
operation.sem_flg = 0;
semop(semid, &operation, 1);
nb_place = nb_place -1;
 
/*operation v */
operation.sem_num = 0;
operation.sem_op = 1;
operation.sem_flg = 0;
semop (semid, &operation, 1);
}
 
main()
{
/*prinf*/
  pthread_t num_thread[3];
/*creation of a semaphore initialised to value 1 */
 
semid = semget(12,1,IPC_CREAT|IPC_EXCL|0600);
semctl(semid,0,SETVAL,1);
 
for(i=0; i<3; i++)
{
  /*printf*/
pthread_create(&num_thread[i], NULL, (void *(*)())reservation, NULL);
pthread_join(num_thread, NULL);
 
semctl(semid,0,IPC_RMID,0);
}
}
 

In the previous when I locate a printf statement in the /*printf*/ nothing is displayed, why is this? and i would be grateful if you give a small explanation of what is supposed to be happening in this program.

[weaknessforcats]

I don't see any printf statements in your code.

/*printf*/ is a comment.

[pythoNewbie]

sorry for not being obvious, the printf statements are flags but they are not displayed when i execute the program. the following code is more obvious

int i, nb_place;
int semid;
struct sembuf operation;
void reservation()
{
    printf("FLAG1");
    /*operation p */
    operation.sem_num = 0;
    operation.sem_op = -1;
    operation.sem_flg = 0;
    semop(semid, &operation, 1);
    nb_place = nb_place -1;
    /*operation v */
    operation.sem_num = 0;
    operation.sem_op = 1;
    operation.sem_flg = 0;
    semop (semid, &operation, 1);
}
 
main()
{
    printf("FLAG 2");
    pthread_t num_thread[3];
    /*creation of a semaphore initialised to value 1 */
    semid = semget(12,1,IPC_CREAT|IPC_EXCL|0600);
    semctl(semid,0,SETVAL,1);
    for(i=0; i<3; i++)
    {
        printf("FLAG 3");
        pthread_create(&num_thread[i], NULL, (void *(*)())reservation, NULL);
        pthread_join(num_thread, NULL);
        semctl(semid,0,IPC_RMID,0);
    }
} 
 

[JosAH]

Just to be sure print a newline character at the end of your output strings; stdout may be line buffered so nothing will show up for a while. Do this instead: printf("FLAG 1\n");

kind regards,

Jos

[pythoNewbie]

Thank you JosAH very much
> so nothing will show up for a while
well, actually nothing is showed up always. the newline character works fine.

I have a question about the semget function, I want to understand it better, isn't there any SIMPLE tutorials for that?

semid = semget(12,1,IPC_CREAT|IPC_EXCL|0600);

[JosAH]

@pythoNewbie
Unix programmers don't need tutorials; they read the manual pages.

kind regards,

Jos ;-)

[pythoNewbie]

Aha

I will try this one, thank you very much