[LIP] a strange problem.

kindly go through the following snippet:

 ...
 /* opens a file, initialises to 0 and maps into shared-memory */
 /*if((fd=open(*(argstr+1),O_RDWR|O_CREAT,FILE_MODE))<0)*/
 if((fd=open(*(argstr+1),O_RDONLY,FILE_MODE))<0)
 {
  fprintf(stderr,"file creation error for %s.\n%u: %s\n",*(argstr+1),\
          (unsigned)errno,strerror(errno));
  return 1;
 }
 /*if((write(fd,&shared,sizeof(struct shared)))<0)
 {
  fprintf(stderr,"file write error for %s.\n%u: %s\n",*(argstr+1),\
          (unsigned)errno,strerror(errno));
  return 1;
 }*/
 if((ptr=(struct shared *)mmap((void *)NULL,sizeof(struct shared),
          PROT_READ,MAP_SHARED,fd,0))==(struct shared *)MAP_FAILED)
 ...

i'm coming across a strange problem.
when a file is opened in  O_RDONLY and a call to mmap() with PROT
paramter value 
as PROT_READ|PROT_WRITE is made, it's giving an error: permission
denied, which 
is very appropriate.

but when i open a file in O_RDWR|O_CREAT and make a call to mmap() with
PROT 
paramter value as PROT_READ, it's letting me pass through the mmap() 
call. i'm expecting the same permission denied at this juncture.

can anyone please let me what's happenin' here?
.


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