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``shmop system call example'' is a menu-driven program. It allows all possible combinations of using the shmat and shmdt system calls to be exercised.
From studying this program, you can observe the method of passing arguments and receiving return values. The user-written program requirements are pointed out.
This program begins (lines 5-9) by including the required header files as specified by the shmop(2) manual page. Note that in this program errno is declared as an external variable; therefore, the sys/errno.h header file does not have to be included.
Variable and structure names have been chosen to be as close
as possible to
those in the synopsis.
Their declarations are self explanatory.
These names
make the program more readable and are perfectly
valid since they are local to the program.
The variables declared for this program and what they are used for are as follows:
This example program combines both the shmat and shmdt system calls. The program prompts for the number of attachments and enters a loop until they are done for the specified shared memory identifiers. Then, the program prompts for the number of detachments to be performed and enters a loop until they are done for the specified shared memory segment addresses.
The program prompts for the number of attachments to be performed, and the value is stored at the address of the attach variable (lines 19-23).
A loop is entered using the attach variable and the i counter (lines 23-72) to perform the specified number of attachments.
In this loop, the program prompts for a shared memory segment identifier (lines 26-29); it is stored in the shmid variable (line 30). Next, the program prompts for the address where the segment is to be attached (lines 32-36); it is stored in the laddr variable (line 37) and converted to a pointer (line 39). Then, the program prompts for the desired flags to be used for the attachment (lines 40-47), and the code representing the flags is stored in the flags variable (line 48). The flags variable is tested to determine the code to be stored for the shmflg variable used to pass them to the shmat system call (lines 49-60). The system call is executed (line 63). If successful, a message stating so is displayed along with the attach address (lines 68-70). If unsuccessful, a message stating so is displayed and the error code is displayed (line 65). The loop then continues until it finishes.
After the attach loop completes, the program prompts for the number of detach operations to be performed (lines 73-77) and the value is stored in the detach variable (line 76).
A loop is entered using the detach variable and the i counter (lines 80-98) to perform the specified number of detachments.
In this loop, the program prompts for the address of the shared memory segment to be detached (lines 81-85); it is stored in the laddr variable (line 86) and converted to a pointer (line 88). Then, the shmdt system call is performed (line 89). If successful, a message stating so is displayed along with the address that the segment was detached from (lines 95, 96). If unsuccessful, the error number is displayed (line 92). The loop continues until it finishes.
The example program for the shmop system calls follows. We suggest that you name the source program file shmop.c and the executable file shmop.
1 /*This is a program to illustrate 2 *the shared memory operations, shmop(), 3 *system call capabilities. 4 */5 /*Include necessary header files.*/ 6 #include <stdio.h> 7 #include <sys/types.h> 8 #include <sys/ipc.h> 9 #include <sys/shm.h> 10 /*Start of main C language program*/ 11 main() 12 { 13 extern int errno; 14 void *addr; 15 long laddr; 16 int flags, i, attach; 17 int shmid, shmflg, retrn, detach;
18 /*Loop for attachments by this process.*/ 19 printf("Enter the number of\n"); 20 printf("attachments for this\n"); 21 printf("process (1-4).\n"); 22 printf(" Attachments = ");
23 scanf("%d", &attach); 24 printf("Number of attaches = %d\n", attach);
25 for(i = 1; i <= attach; i++) { 26 /*Enter the shared memory ID.*/ 27 printf("\nEnter the shmid of\n"); 28 printf("the shared memory segment to\n"); 29 printf("be operated on = "); 30 scanf("%d", &shmid); 31 printf("\nshmid = %d\n", shmid);
32 /*Enter the value for shmaddr.*/ 33 printf("\nEnter the value for\n"); 34 printf("the shared memory address\n"); 35 printf("in hexadecimal:\n"); 36 printf(" Shmaddr = "); 37 scanf("%lx", &laddr); 38 addr = (void*) laddr; 39 printf("The desired address = 0x%lx\n", (long)addr);
40 /*Specify the desired flags.*/ 41 printf("\nEnter the corresponding\n"); 42 printf("number for the desired\n"); 43 printf("flags:\n"); 44 printf("SHM_RND = 1\n"); 45 printf("SHM_RDONLY = 2\n"); 46 printf("SHM_RND and SHM_RDONLY = 3\n"); 47 printf(" Flags = "); 48 scanf("%d", &flags);
49 switch(flags) 50 { 51 case 1: 52 shmflg = SHM_RND; 53 break; 54 case 2: 55 shmflg = SHM_RDONLY; 56 break; 57 case 3: 58 shmflg = SHM_RND | SHM_RDONLY; 59 break; 60 } 61 printf("\nFlags = 0%o\n", shmflg);
62 /*Do the shmat system call.*/ 63 addr = shmat(shmid, addr, shmflg); 64 if(addr == (char*) -1) { 65 printf("\nShmat failed, error = %d\n", errno); 66 } 67 else { 68 printf ("\nShmat was successful\n"); 69 printf("for shmid = %d\n", shmid); 70 printf("The address = 0x%lx\n", (long)addr); 71 } 72 }
73 /*Loop for detachments by this process.*/ 74 printf("Enter the number of\n"); 75 printf("detachments for this\n"); 76 printf("process (1-4).\n"); 77 printf(" Detachments = ");
78 scanf("%d", &detach); 79 printf("Number of attaches = %d\n", detach); 80 for(i = 1; i <= detach; i++) {
81 /*Enter the value for shmaddr.*/ 82 printf("\nEnter the value for\n"); 83 printf("the shared memory address\n"); 84 printf("in hexadecimal:\n"); 85 printf(" Shmaddr = "); 86 scanf("%lx", &laddr); 87 addr = (void*) laddr; 88 printf("The desired address = 0x%lx\n", (long)addr);
89 /*Do the shmdt system call.*/ 90 retrn = shmdt(addr); 91 if(retrn == -1) { 92 printf("Error = %d\n", errno); 93 } 94 else { 95 printf ("\nShmdt was successful\n"); 96 printf("for address = 0x%lx\n", (long)addr);
97 } 98 } 99 }
shmop system call example