What is wrong with the function shmdt()? - c

Question

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#define BUFSZ 2048
int main()
{
int shmid,i,fd,nwrite,nread;
char *shmadd;
char buf[5];
buf[5] = '\0';
if((shmid=shmget(IPC_PRIVATE,BUFSZ,0x666))<0)
{
perror("shmget");
exit(1);
}
else
printf("created shared-memory: %d\n",shmid);
if((shmadd=shmat(shmid,0,0))<(char *)0)
{
perror("shmat");
exit(1);
}
else
printf("attached shared-memory\n");
shmadd="Hello";
if((fd = open("share",O_CREAT | O_RDWR,0666))<0)
{
perror("open");
exit(1);
}
else
printf("open success!\n");
if((nwrite=write(fd,shmadd,5))<0)
{
perror("write");
exit(1);
}
else
printf("write success!\n");
lseek( fd, 0, SEEK_SET );
if((nread=read(fd,buf,5))<0)
{
perror("read");
exit(1);
}
else
printf("read %d form file:%s\n",nread,buf);
if(close(fd) == -1)
printf("close fd fails!\n");
else
printf("close fd succeeds!\n");
if((shmdt(shmadd))<0)
{
perror("shmdt");
exit(1);
}
else
printf("deleted shared-memory\n");
exit(0);
}
Above is the code which is to demo the shared memory in Linux. And the running result is below:
$ ./ex2
created shared-memory: 1572887
attached shared-memory
open success!
write success!
read 5 form file:Hello
close fd succeeds!
shmdt: Invalid argument
As you can see, everything goes fine except shmdt(). Why it fails?
Further：
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
#define BUFSZ 2048
int main()
{
int shmid,i,fd,nwrite,nread;
char *shmadd;
char buf[5];
buf[5] = '\0';
if((shmid=shmget(IPC_PRIVATE,BUFSZ,0x666)) < 0)
{
perror("shmget");
exit(1);
}
else
printf("created shared-memory: %d\n",shmid);
if((shmadd=shmat(shmid,0,0)) < (char *)0)
{
perror("shmat");
exit(1);
}
else
printf("attached shared-memory\n");
strcpy(shmadd, "Hello");
if((fd = open("share",O_CREAT | O_RDWR,0666)) < 0)
{
perror("open");
exit(1);
}
else
printf("open success!\n");
if((nwrite=write(fd,shmadd,5)) < 0)
{
perror("write");
exit(1);
}
else
printf("write success!\n");
lseek( fd, 0, SEEK_SET );
if((nread=read(fd,buf,5)) < 0)
{
perror("read");
exit(1);
}
else
printf("read %d form file:%s\n",nread,buf);
if(close(fd) == -1)
printf("close fd fails!\n");
else
printf("close fd succeeds!\n");
if((shmdt(shmadd))<0)
{
perror("shmdt");
exit(1);
}
else
printf("deleted shared-memory\n");
exit(0);
}
Following your answers, I changed the code as above. But now I got new error!
$ ./ex2
created shared-memory: 2129948
attached shared-memory
Segmentation fault (core dumped)
I seems that strcpy causes the error. But why?

Answer 1

This is the problem:
shmadd="Hello";
This changes the shmadd pointer to point to a string in memory. I think you intended to copy the string into the shared memory. To do that, you would do:
strcpy(shmadd,"Hello");
Also note that your check for errors is wrong, it should be:
if((shmadd=shmat(shmid,0,0)) == (void *)-1) { ... error ... }
And your permissions should be octal, not hex:
if((shmid=shmget(IPC_PRIVATE,BUFSZ,0666)) < 0)

Answer 2

Your check for an error with shmadd in this line is incorrect:
if((shmadd=shmat(shmid,0,0)) < (char *)0)
You need to do an explicit comparison to -1 as per the man page, as a -1 cast to a pointer becomes an unsigned value (0xFFFFFFFF on 32-bit systems). Replace that line with this one:
if((shmadd=shmat(shmid,0,0)) == (char *)-1)
and you'll get a permission denied error because you are not root.