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SetSharedVariable
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SetSharedVariable

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SetSharedVariable[s1,s2,…]

declares the symbols si as shared variables whose values are synchronized among all parallel kernels.

Details

  • The unique value of a shared variable is maintained by the master kernel and every access on a parallel subkernel is synchronized through the master kernel.
  • Shared variables without a value evaluate to Null.
  • SetSharedVariable has attribute HoldAll.

Examples

open allclose all

Basic Examples  (1)Summary of the most common use cases

Make xs be a (global) shared variable:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-n57t1

Each subkernel increments the (global) shared variable value:

In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-d9uenc
Out[2]=2
In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-bdvuu
Out[3]=3

Without sharing, each subkernel has its own (local) copy of the variable:

In[4]:=4
✖
https://wolfram.com/xid/0bh0oifvcgpggm-o9v42l
In[5]:=5
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https://wolfram.com/xid/0bh0oifvcgpggm-df9qc
Out[5]=5
In[6]:=6
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https://wolfram.com/xid/0bh0oifvcgpggm-r1say
Out[6]=6

Scope  (8)Survey of the scope of standard use cases

Assignment  (3)

Share a single value:

In[8]:=8
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https://wolfram.com/xid/0bh0oifvcgpggm-japih9
In[9]:=9
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https://wolfram.com/xid/0bh0oifvcgpggm-evd4bb
Out[9]=9

Assign a new value to a shared variable:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-dgjsfc
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-k20jmh
Out[2]=2

Only the last assignment performed remains:

In[3]:=3
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https://wolfram.com/xid/0bh0oifvcgpggm-cn0nbs
Out[3]=3

Append to a shared list:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-pvu52o
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-esmj3q
In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-r9li36
Out[3]=3

Parts of a shared variable  (3)

Access a single element of a shared list efficiently:

In[4]:=4
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https://wolfram.com/xid/0bh0oifvcgpggm-i89vqf
In[5]:=5
✖
https://wolfram.com/xid/0bh0oifvcgpggm-jpocrz
Out[5]=5

Change a single element of a shared variable:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-5ibgne
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-ricidu
In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-7xc9qs
Out[3]=3

Modify a part of a shared variable:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-sm8p7v
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-e24wqh
Out[2]=2

Associations  (2)

Assign to a specific key:

In[26]:=26
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https://wolfram.com/xid/0bh0oifvcgpggm-m8bxpu
In[34]:=34
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https://wolfram.com/xid/0bh0oifvcgpggm-49kcfw
Out[35]=35

If the key is a string, Key can be omitted:

In[36]:=36
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https://wolfram.com/xid/0bh0oifvcgpggm-eqq9bd
Out[37]=37

Modify a value in an association:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-p0s7kp
In[2]:=2
✖
https://wolfram.com/xid/0bh0oifvcgpggm-wcdsj5
Out[2]=2

Generalizations & Extensions  (1)Generalized and extended use cases

Delayed values defined on the master kernel are evaluated on the master kernel:

In[1]:=1
✖
https://wolfram.com/xid/0bh0oifvcgpggm-nbcrg
In[2]:=2
✖
https://wolfram.com/xid/0bh0oifvcgpggm-i2ikwi
In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-jsi3o5
Out[3]=3
In[4]:=4
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https://wolfram.com/xid/0bh0oifvcgpggm-bn4oww
Out[4]=4

Delayed values defined on a subkernel are evaluated on the kernel asking for the value:

In[5]:=5
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https://wolfram.com/xid/0bh0oifvcgpggm-e88i3m
In[6]:=6
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https://wolfram.com/xid/0bh0oifvcgpggm-kmadv
In[7]:=7
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https://wolfram.com/xid/0bh0oifvcgpggm-kxfl1n
Out[7]=7
In[8]:=8
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https://wolfram.com/xid/0bh0oifvcgpggm-ceo4n3
Out[8]=8

Applications  (5)Sample problems that can be solved with this function

Use shared variables to synchronize input and output in an infinite search:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-gje3ls

Find such that is prime, until the computation is manually aborted:

In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-ddeli7
Out[2]=2

Use a shared variable to signal all evaluations to stop as soon as a prime has been found:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-w537e
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-2s6d0
Out[2]=2

Use a shared variable to hand out tasks to do:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-fdk4rt
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-f10gsn
Out[2]=2
In[3]:=3
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https://wolfram.com/xid/0bh0oifvcgpggm-3c84
Out[3]=3

Monitor the progress of a calculation, counting major calculation steps:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-yj41g
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-bkx6ba
Out[2]=2

Return results as they are found:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-chy7ji
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-crvvfb
Out[2]=2

Watch the results appear in real time:

In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-chzxg2
Out[3]=3

Properties & Relations  (3)Properties of the function, and connections to other functions

$KernelCount is effectively a shared variable:

In[1]:=1
✖
https://wolfram.com/xid/0bh0oifvcgpggm-dpb4sr
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-eh7zw8
Out[2]=2

Shared variables are listed in $SharedVariables:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-g28wjc
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-gz26v3
Out[2]=2

To share definitions of functions, use SetSharedFunction:

In[1]:=1
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https://wolfram.com/xid/0bh0oifvcgpggm-pwjmsa
In[2]:=2
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https://wolfram.com/xid/0bh0oifvcgpggm-wumyn9

Shared functions are effectively evaluated on the master kernel:

In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-kk9pqp
Out[3]=3

Possible Issues  (3)Common pitfalls and unexpected behavior

The single-bracket notation for association access is not supported by shared variables:

In[60]:=60
✖
https://wolfram.com/xid/0bh0oifvcgpggm-bqmq1n
In[61]:=61
✖
https://wolfram.com/xid/0bh0oifvcgpggm-0ym672
Out[61]=61

Use the Part notation instead:

In[63]:=63
✖
https://wolfram.com/xid/0bh0oifvcgpggm-x6eywn
Out[64]=64

Separate read and write operations are not thread-safe:

In[1]:=1
✖
https://wolfram.com/xid/0bh0oifvcgpggm-kxo973
In[2]:=2
✖
https://wolfram.com/xid/0bh0oifvcgpggm-fybf7m
Out[2]=2
In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-n1d1rr
Out[3]=3

Use atomic updates for synchronization:

In[4]:=4
✖
https://wolfram.com/xid/0bh0oifvcgpggm-fguo0j
In[5]:=5
✖
https://wolfram.com/xid/0bh0oifvcgpggm-dt6vib
Out[5]=5
In[6]:=6
✖
https://wolfram.com/xid/0bh0oifvcgpggm-cgpwbj
Out[6]=6

Alternatively, use CriticalSection to make a whole code section atomic:

In[7]:=7
✖
https://wolfram.com/xid/0bh0oifvcgpggm-dyz7v5
In[8]:=8
✖
https://wolfram.com/xid/0bh0oifvcgpggm-hdb28c
Out[8]=8
In[9]:=9
✖
https://wolfram.com/xid/0bh0oifvcgpggm-b7qr5n
Out[9]=9

Create a shared variable whose value is a large matrix:

In[1]:=1
✖
https://wolfram.com/xid/0bh0oifvcgpggm-lbcte3

Accessing its elements causes the whole matrix to be transferred to the subkernels repeatedly:

In[2]:=2
✖
https://wolfram.com/xid/0bh0oifvcgpggm-g60jkc
Out[2]=2

Use Unevaluated to allow the special code for parts of shared variables to see the variable:

In[3]:=3
✖
https://wolfram.com/xid/0bh0oifvcgpggm-cawvvr
Out[3]=3
Wolfram Research (2008), SetSharedVariable, Wolfram Language function, https://reference.wolfram.com/language/ref/SetSharedVariable.html.
✖
Wolfram Research (2008), SetSharedVariable, Wolfram Language function, https://reference.wolfram.com/language/ref/SetSharedVariable.html.

Text

Wolfram Research (2008), SetSharedVariable, Wolfram Language function, https://reference.wolfram.com/language/ref/SetSharedVariable.html.

✖
Wolfram Research (2008), SetSharedVariable, Wolfram Language function, https://reference.wolfram.com/language/ref/SetSharedVariable.html.

CMS

Wolfram Language. 2008. "SetSharedVariable." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/SetSharedVariable.html.

✖
Wolfram Language. 2008. "SetSharedVariable." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/SetSharedVariable.html.

APA

Wolfram Language. (2008). SetSharedVariable. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/SetSharedVariable.html

✖
Wolfram Language. (2008). SetSharedVariable. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/SetSharedVariable.html

BibTeX

@misc{reference.wolfram_2025_setsharedvariable, author="Wolfram Research", title="{SetSharedVariable}", year="2008", howpublished="\url{https://reference.wolfram.com/language/ref/SetSharedVariable.html}", note=[Accessed: 30-April-2025 ]}

✖
@misc{reference.wolfram_2025_setsharedvariable, author="Wolfram Research", title="{SetSharedVariable}", year="2008", howpublished="\url{https://reference.wolfram.com/language/ref/SetSharedVariable.html}", note=[Accessed: 30-April-2025 ]}

BibLaTeX

@online{reference.wolfram_2025_setsharedvariable, organization={Wolfram Research}, title={SetSharedVariable}, year={2008}, url={https://reference.wolfram.com/language/ref/SetSharedVariable.html}, note=[Accessed: 30-April-2025 ]}

✖
@online{reference.wolfram_2025_setsharedvariable, organization={Wolfram Research}, title={SetSharedVariable}, year={2008}, url={https://reference.wolfram.com/language/ref/SetSharedVariable.html}, note=[Accessed: 30-April-2025 ]}