Showing posts with label programming. Show all posts
Showing posts with label programming. Show all posts

Sunday, December 2, 2012

Thread safe programming







There are several ways in which a function can be thread safe.
It can be reentrant. This means that a function has no state, and does not touch any global or static variables, so it can be called from multiple threads simultaneously. The term comes from allowing one thread to enter the function while another thread is already inside it.
It can have a critical section. This term gets thrown around a lot, but frankly I prefer critical data. A critical section occurs any time your code touches data that is shared across multiple threads. So I prefer to put the focus on that critical data.
If you use a mutex properly, you can synchronize access to the critical data, properly protecting from thread unsafe modifications. Mutexes and Locks are very useful, but with great power comes great responsibility. You must not lock the same mutex twice within the same thread (that is a self-deadlock). You must be careful if you acquire more than one mutex, as it increases your risk for deadlock. You must consistently protect your data with mutexes.
If all of your functions are thread safe, and all of your shared data properly protected, your application should be thread safe.
As Crazy Eddie said, this is a huge subject. I recommend reading up on boost threads, and using them accordingly.
low-level caveat: compilers can reorder statements, which can break thread safety. With multiple cores, each core has its own cache, and you need to properly sync the caches to have thread safety. Also, even if the compiler doesn't reorder statements, the hardware might. So, full, guaranteed thread safety isn't actually possible today. You can get 99.99% of the way there though, and work is being done with compiler vendors and cpu makers to fix this lingering caveat.
Anyway, if you're looking for a checklist to make a class thread-safe:
  • Identify any data that is shared across threads (if you miss it, you can't protect it)
  • create a member boost::mutex m_mutex and use it whenever you try to access that shared member data (ideally the shared data is private to the class, so you can be more certain that you're protecting it properly).
  • clean up globals. Globals are bad anyways, and good luck trying to do anything thread-safe with globals.
  • Beware the static keyword. It's actually not thread safe. So if you're trying to do a singleton, it won't work right.
  • Beware the Double-Checked Lock Paradigm. Most people who use it get it wrong in some subtle ways, and it's prone to breakage by the low-level caveat.
That's an incomplete checklist. I'll add more if I think of it, but hopefully it's enough to get you started.

source: http://stackoverflow.com/questions/5125241/how-to-make-an-application-thread-safe

http://en.wikipedia.org/wiki/Thread_safety
Thread safety is a computer programming concept applicable in the context of multi-threaded programs. A piece of code is thread-safe if it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time. There are various strategies for making thread-safe data structures.[1][2]
A key challenge in multi-threaded programming, thread safety was not a concern for most application developers until the 1990s when operating systems began to expose multiple threads for code execution. Today, a program may execute code on several threads simultaneously in a sharedaddress space where each of those threads has access to virtually all of the memory of every other thread. Thread safety is a property that allows code to run in multi-threaded environments by re-establishing some of the correspondences between the actual flow of control and the text of the program, by means of synchronization.

Levels of thread safety

Software libraries can provide certain thread-safety guarantees. For example, concurrent reads might be guaranteed to be thread-safe, but concurrent writes might not be. Whether or not a program using such a library is thread-safe depends on whether it uses the library in a manner consistent with those guarantees.
Different vendors use slightly different terminology for thread-safety:[3][4][5][6]
  • Thread safe: Implementation is guaranteed to be free of race conditions when accessed by multiple threads simultaneously.
  • Conditionally safe: Different threads can access different objects simultaneously, and access to shared data is protected from race conditions.
  • Not thread safe: Code should not be accessed simultaneously by different threads.
Thread safety guarantees usually also include design steps to prevent or limit the risk of different forms of deadlocks, as well as optimizations to maximize concurrent performance. However, deadlock-free guarantees can not always be given, since deadlocks can be caused by callbacks and violation of architectural layering independent of the library itself.

[edit]Implementation approaches

There are a several approaches for avoiding race conditions to achieve thread safety. The first class of approaches focuses on avoiding shared state, and includes:
Re-entrancy 
Writing code in such a way that it can be partially executed by a thread, reexecuted by the same thread or simultaneously executed by another thread and still correctly complete the original execution. This requires the saving of state information in variables local to each execution, usually on a stack, instead of in static or global variables or other non-local state. All non-local state must be accessed through atomic operations and the data-structures must also be reentrant.
Thread-local storage 
Variables are localized so that each thread has its own private copy. These variables retain their values across subroutine and other code boundaries, and are thread-safe since they are local to each thread, even though the code which accesses them might be executed simultaneously by another thread.
The second class of approaches are synchronization-related, and are used in situations where shared state cannot be avoided:
Mutual exclusion
Access to shared data is serialized using mechanisms that ensure only one thread reads or writes to the shared data at any time. Incorporation of mutal exclusion needs to be well thought out, since improper usage can lead to side-effects like deadlockslivelocks and resource starvation.
Atomic operations 
Shared data are accessed by using atomic operations which cannot be interrupted by other threads. This usually requires using special machine language instructions, which might be available in a runtime library. Since the operations are atomic, the shared data are always kept in a valid state, no matter how other threads access it. Atomic operations form the basis of many thread locking mechanisms, and are used to implement mutual exclusion primitives.
Immutable objects 
The state of an object cannot be changed after construction. This implies that only read-only data is shared and inherent thread safety. Mutable (non-const) operations can then be implemented in such a way that they create new objects instead of modifying existing ones. This approach is used by the string implementations in Java, C# and python.[7]

[edit]Examples

In the following piece of C code, the function is thread-safe, but not reentrant:
#include 
 
int increment_counter ()
{
        static int counter = 0;
        static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
 
        pthread_mutex_lock(&mutex);
 
        // only allow one thread to increment at a time
        ++counter;
        // store value before any other threads increment it further
        int result = counter;   
 
        pthread_mutex_unlock(&mutex);
 
        return result;
}
In the above, increment_counter can be called by different threads without any problem since a mutex is used to synchronize all access to the shared counter variable. But if the function is used in a reentrant interrupt handler and a second interrupt arises inside the function, the second routine will hang forever. As interrupt servicing can disable other interrupts, the whole system could suffer.
The same function can be implemented to be both thread-safe and reentrant using the lock-free atomics in C++11:
#include 
 
int increment_counter ()
{
        static std::atomic<int> counter(0);
 
        // increment is guaranteed to be done atomically
        int result = ++counter;
 
        return result;
}

Wednesday, June 6, 2012

errno, perror, strerr




Diagrams
http://bramp.github.io/js-sequence-diagrams/


IDE one:
http://ideone.com
Ideone is an online compiler and debugging tool which allows you to compile source code and execute it online in more than 60 programming languages.



errno.h - C Error Codes in Linux


cat /usr/include/asm-generic/errno-base.h
#ifndef _ASM_GENERIC_ERRNO_BASE_H
#define _ASM_GENERIC_ERRNO_BASE_H

#define EPERM            1      /* Operation not permitted */
#define ENOENT           2      /* No such file or directory */
#define ESRCH            3      /* No such process */
#define EINTR            4      /* Interrupted system call */
#define EIO              5      /* I/O error */
#define ENXIO            6      /* No such device or address */
#define E2BIG            7      /* Argument list too long */
#define ENOEXEC          8      /* Exec format error */
#define EBADF            9      /* Bad file number */
#define ECHILD          10      /* No child processes */
#define EAGAIN          11      /* Try again */
#define ENOMEM          12      /* Out of memory */
#define EACCES          13      /* Permission denied */
#define EFAULT          14      /* Bad address */
#define ENOTBLK         15      /* Block device required */
#define EBUSY           16      /* Device or resource busy */
#define EEXIST          17      /* File exists */
#define EXDEV           18      /* Cross-device link */
#define ENODEV          19      /* No such device */
#define ENOTDIR         20      /* Not a directory */
#define EISDIR          21      /* Is a directory */
#define EINVAL          22      /* Invalid argument */
#define ENFILE          23      /* File table overflow */
#define EMFILE          24      /* Too many open files */
#define ENOTTY          25      /* Not a typewriter */
#define ETXTBSY         26      /* Text file busy */
#define EFBIG           27      /* File too large */
#define ENOSPC          28      /* No space left on device */
#define ESPIPE          29      /* Illegal seek */
#define EROFS           30      /* Read-only file system */
#define EMLINK          31      /* Too many links */
#define EPIPE           32      /* Broken pipe */
#define EDOM            33      /* Math argument out of domain of func */
#define ERANGE          34      /* Math result not representable */

#endif


cat /usr/include/asm-generic/errno.h
#ifndef _ASM_GENERIC_ERRNO_H
#define _ASM_GENERIC_ERRNO_H

#include 

#define EDEADLK         35      /* Resource deadlock would occur */
#define ENAMETOOLONG    36      /* File name too long */
#define ENOLCK          37      /* No record locks available */
#define ENOSYS          38      /* Function not implemented */
#define ENOTEMPTY       39      /* Directory not empty */
#define ELOOP           40      /* Too many symbolic links encountered */
#define EWOULDBLOCK     EAGAIN  /* Operation would block */
#define ENOMSG          42      /* No message of desired type */
#define EIDRM           43      /* Identifier removed */
#define ECHRNG          44      /* Channel number out of range */
#define EL2NSYNC        45      /* Level 2 not synchronized */
#define EL3HLT          46      /* Level 3 halted */
#define EL3RST          47      /* Level 3 reset */
#define ELNRNG          48      /* Link number out of range */
#define EUNATCH         49      /* Protocol driver not attached */
#define ENOCSI          50      /* No CSI structure available */
#define EL2HLT          51      /* Level 2 halted */
#define EBADE           52      /* Invalid exchange */
#define EBADR           53      /* Invalid request descriptor */
#define EXFULL          54      /* Exchange full */
#define ENOANO          55      /* No anode */
#define EBADRQC         56      /* Invalid request code */
#define EBADSLT         57      /* Invalid slot */

#define EDEADLOCK       EDEADLK

#define EBFONT          59      /* Bad font file format */
#define ENOSTR          60      /* Device not a stream */
#define ENODATA         61      /* No data available */
#define ETIME           62      /* Timer expired */
#define ENOSR           63      /* Out of streams resources */
#define ENONET          64      /* Machine is not on the network */
#define ENOPKG          65      /* Package not installed */
#define EREMOTE         66      /* Object is remote */
#define ENOLINK         67      /* Link has been severed */
#define EADV            68      /* Advertise error */
#define ESRMNT          69      /* Srmount error */
#define ECOMM           70      /* Communication error on send */
#define EPROTO          71      /* Protocol error */
#define EMULTIHOP       72      /* Multihop attempted */
#define EDOTDOT         73      /* RFS specific error */
#define EBADMSG         74      /* Not a data message */
#define EOVERFLOW       75      /* Value too large for defined data type */
#define ENOTUNIQ        76      /* Name not unique on network */
#define EBADFD          77      /* File descriptor in bad state */
#define EREMCHG         78      /* Remote address changed */
#define ELIBACC         79      /* Can not access a needed shared library */
#define ELIBBAD         80      /* Accessing a corrupted shared library */
#define ELIBSCN         81      /* .lib section in a.out corrupted */
#define ELIBMAX         82      /* Attempting to link in too many shared libraries */
#define ELIBEXEC        83      /* Cannot exec a shared library directly */
#define EILSEQ          84      /* Illegal byte sequence */
#define ERESTART        85      /* Interrupted system call should be restarted */
#define ESTRPIPE        86      /* Streams pipe error */
#define EUSERS          87      /* Too many users */
#define ENOTSOCK        88      /* Socket operation on non-socket */
#define EDESTADDRREQ    89      /* Destination address required */
#define EMSGSIZE        90      /* Message too long */
#define EPROTOTYPE      91      /* Protocol wrong type for socket */
#define ENOPROTOOPT     92      /* Protocol not available */
#define EPROTONOSUPPORT 93      /* Protocol not supported */
#define ESOCKTNOSUPPORT 94      /* Socket type not supported */
#define EOPNOTSUPP      95      /* Operation not supported on transport endpoint */
#define EPFNOSUPPORT    96      /* Protocol family not supported */
#define EAFNOSUPPORT    97      /* Address family not supported by protocol */
#define EADDRINUSE      98      /* Address already in use */
#define EADDRNOTAVAIL   99      /* Cannot assign requested address */
#define ENETDOWN        100     /* Network is down */
#define ENETUNREACH     101     /* Network is unreachable */
#define ENETRESET       102     /* Network dropped connection because of reset */
#define ECONNABORTED    103     /* Software caused connection abort */
#define ECONNRESET      104     /* Connection reset by peer */
#define ENOBUFS         105     /* No buffer space available */
#define EISCONN         106     /* Transport endpoint is already connected */
#define ENOTCONN        107     /* Transport endpoint is not connected */
#define ESHUTDOWN       108     /* Cannot send after transport endpoint shutdown */
#define ETOOMANYREFS    109     /* Too many references: cannot splice */
#define ETIMEDOUT       110     /* Connection timed out */
#define ECONNREFUSED    111     /* Connection refused */
#define EHOSTDOWN       112     /* Host is down */
#define EHOSTUNREACH    113     /* No route to host */
#define EALREADY        114     /* Operation already in progress */
#define EINPROGRESS     115     /* Operation now in progress */
#define ESTALE          116     /* Stale NFS file handle */
#define EUCLEAN         117     /* Structure needs cleaning */
#define ENOTNAM         118     /* Not a XENIX named type file */
#define ENAVAIL         119     /* No XENIX semaphores available */
#define EISNAM          120     /* Is a named type file */
#define EREMOTEIO       121     /* Remote I/O error */
#define EDQUOT          122     /* Quota exceeded */

#define ENOMEDIUM       123     /* No medium found */
#define EMEDIUMTYPE     124     /* Wrong medium type */
#define ECANCELED       125     /* Operation Canceled */
#define ENOKEY          126     /* Required key not available */
#define EKEYEXPIRED     127     /* Key has expired */
#define EKEYREVOKED     128     /* Key has been revoked */
#define EKEYREJECTED    129     /* Key was rejected by service */

/* for robust mutexes */
#define EOWNERDEAD      130     /* Owner died */
#define ENOTRECOVERABLE 131     /* State not recoverable */

#endif

Tuesday, May 11, 2010

Programming Tools Guide




Programming Tools Guide




Wednesday, March 24, 2010

Modular programming in C




What is Modular programming ?

- A programming technique to break down program functions into separate modules/parts/layers.
- Module, have to accomplishes one function by containing the source codes and input/output variables needed to accomplish that function.

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