Thread-safety refers to whether a block of code can be executed simultaneously by multiple threads without errors. While functions written by a programmer can be identified as thread-safe or not, system library functions may not be thread-safe. The strtok() string function from the C standard library, for example, is not thread-safe because it caches input in a static variable that is shared across threads, leading to unpredictable results when called simultaneously from multiple threads. Making functions re-entrant by avoiding shared static variables can make them thread-safe.

1. CH 5.10
THREAD-SAFETY
BY :MOHAMED RAMADAN

2. Thread-Safety
• A block of code is thread-safe if it can be simultaneously executed by
multiple threads without causing problems.
• When you program your own functions, you know if they are safe to
be called by multiple threads or not.
• You may forget to check if system library functions used are thread-
safe.
Unsafe function: strtok()from C string.h library
Other example.
–The random number generator random in stdlib.h.
–The time conversion function localtime in time.h.

3. Example of using strtok()
•“Tokenize” a English text file
# Tokens are contiguous sequences of characters separated by a
white-space, a tab, or a newline.
#Example: “Welcome back”
Two tokens: “Welcome”, “back”
• Divide the input file into lines of text and assign the lines to the threads
in a round-robin fashion.
#Each thread tokenizes a line using strtok()
#Line 1 > thread 0, Line 2 > thread 1, . . . , the t th goes to thread t,
the t +1st goes to thread 0, etc.

4. The strtok function
• The first time it’s called,
# the string argument is the text to be tokenized (Our line of input)
# strtok caches a static pointer to string
• For subsequent calls, it returns successive tokens taken from the
cached copy#
# the first argument should be NULL.

6. Run the program with a single thread
• When we run the program with a single thread, it correctly tokenizes
the input stream.
Pease porridge hot.
Pease porridge cold.
Pease porridge in the pot.
Nine days old.
• It correctly tokenizes the input stream with 1 thread
Pease
porridge
hot
…

7. Run the program with a two threads
The second time we run it with this input, we get the following output:
Thread 0 > line 0 = Pease porridge hot.
Thread 1 > line 1 = Pease porridge cold.
Thread 0 > token 0 = Pease
Thread 1 > token 0 = Pease
Thread 0 > token 1 = porridge
Thread 1 > token 1 = cold.
Thread 0 > line 2 = Pease porridge in the pot
Thread 1 > line 3 = Nine days old.
Thread 0 > token 0 = Pease here
Thread 1 > token 0 = Nine
Thread 0 > token 1 = days
Thread 1 > token 1 = old.

8. What happened?
• strtok caches the input line by declaring a variable to have static
(persistent) storage class.
# Unfortunately this cached string is shared, not private.
So the strtok function is not thread-safe. If multiple threads call it
simultaneously, the output may not be correct.

9. “re-entrant” (thread safe) functions
• In some cases, the C standard specifies an alternate, thread-safe,
version of a function.

10. Thanks