Hello,
I’ve got a question about shared memory under Linux. When I allocate e.g. 20 Bytes of shm with shmget()&shmat(), I’m able to write up to ~5200 Bytes of data until it crashes. (with symbol lookup error?!)
When I write ~5900 Byte, I get a segmentation fault.
Under Solaris 10 I’m able to write up to PAGE_SIZE until I get a segmentation fault. (and no symbol lookup errors)
getconf PAGE_SIZE results in 4k on my Linux machine.
Well, my question is: How many Bytes are allocated at once of the OS? 4k or 8k or something like that would IMHO make sense for me, but these ~5500 Bytes are quiet strange. :
And why are there symbol lookup errors? (and no, this isn’t my homework and yeah I’m aware that this is a silly question 
)
I tested this with the following program:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
#include <sys/shm.h>
int main(int argc, char* argv])
{
int shmid;
char* message;
long limit = 0;
long real = 0;
/* Parameter handling */
if (argc == 3) {
real = atol(argv[1]);
limit = atol(argv[2]);
} else {
printf("Usage: %s <size of shared memory> <nr of bytes to write>
", argv[0]);
exit(1);
}
if ((limit <= 0) || (real <= 0)) {
printf("Please specify a number above 0
");
exit(1);
}
/* Allocate and attach "real"-Bytes of shared memory */
shmid = shmget(42, (size_t) real, IPC_CREAT | 0640);
if (shmid == -1) {
perror("Couldn't create shared memory");
exit(errno);
}
message = (char*) shmat(shmid, NULL, 0);
if (message == NULL) {
shmctl(shmid, IPC_RMID, 0);
perror("Couldn't attach shared memory");
exit(errno);
}
/* Write "limit"-Bytes to shared memory */
memset(message, '\0', (size_t) limit);
printf("%ld Bytes successfully written to shared memory
", limit);
/* Detach and free shared memory */
if (shmdt((void*) message) == -1) {
perror("Couldn't detach shared memory");
exit(errno);
}
if (shmctl(shmid, IPC_RMID, 0) == -1) {
perror("Couldn't remove shared memory");
exit(errno);
}
return EXIT_SUCCESS;
}
With the following output:
> ./producer 20 4096
4096 Bytes successfully written to shared memory
> ./producer 20 5100
5100 Bytes successfully written to shared memory
> ./producer 20 5300
./producer: symbol lookup error: ./producer: undefined symbol: printf, version GLIBC_2.0
> ./producer 20 5900
Speicherzugriffsfehler
(segmentation fault)