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Old 09-15-2008, 03:18 PM   #1
LinuxInfo
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I need to know the reason why 'State' has been declared as 'Long' in Linux PCB.


Dear Sir,

Please refer to the following Linux PCB code and let me know why State has been declared Long.

/* memory management info */
struct mm_struct *mm;
/* open file information */
struct files_struct *files;
/* tss for this task */
struct thread_struct tss;
int pid;
volatile long state; /* -1
unrunnable, 0 runnable, >0 stopped
*/
long priority;
unsigned short uid,euid,suid,fsuid;
#ifdef __SMP__
int processor;
#endif
struct task_struct *p_opptr, *p_pptr,
*p_cptr, *p_ysptr, *p_osptr;
/* limits */
struct rlimit rlim[RLIM_NLIMITS];
long utime, stime, cutime, cstime,
start_time;
 
Old 09-15-2008, 05:01 PM   #2
pinniped
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You'll have to look at how 'state' is used to find out. 'long' is typically used for a variable which may be 32-bit on a 32-bit CPU and 64-bit on a 64-bit CPU (at least GCC allows this behavior).
 
Old 09-15-2008, 09:30 PM   #3
LinuxInfo
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Quote:
Originally Posted by pinniped View Post
You'll have to look at how 'state' is used to find out. 'long' is typically used for a variable which may be 32-bit on a 32-bit CPU and 64-bit on a 64-bit CPU (at least GCC allows this behavior).
Thanks for the reply.

But please see the below code:

struct task_struct{
long state; /*0 for dispatchable*/ pid_t pid;
long priority;
unsigned long signal;
unsigned long blocked;
unsigned long flags;

Here we could also have taken int as we might be using 0 value or we could also have taken Double but it seems like there is a reason we used Long everywhere. Please if anyone could answer me this, it will be a great help to me.

Thanks.

Last edited by LinuxInfo; 09-15-2008 at 09:37 PM.
 
Old 09-15-2008, 11:14 PM   #4
graemef
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Well since it is used as a flag then an integer type (rather than a float) makes sense. Since the comment indicates >0 maybe the use of long is to avoid an overflow problem. To confirm this you will need to look into the code.
 
Old 09-16-2008, 12:02 AM   #5
Mr. C.
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long is used to maintain architecture alignment; as mentioned, this will be 32-bit on 32-bit platforms, and 64-bit on 64-bit platforms. If int were used, 64-bit platforms would be accessing unaligned data (relative to register size).
 
  


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