Passing by reference in C

Question

If C does not support passing a variable by reference  why does this work    include  lt stdio h gt   void f int  j        j        int main       int i   20    int  p    amp i    f p     printf  i    d n   i      return 0      Output     gcc -std c99 test c   a exe i   21

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You re not passing an int by reference  you re passing a pointer-to-an-int by value  Different syntax  same meaning

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What you are doing is pass by value not pass by reference  Because you are sending the value of a variable  p  to the function  f   in main as f p     The same program in C with pass by reference will look like     this program gives 2 errors as pass by reference is not supported in C    include  lt stdio h gt   void f int  amp j         j is reference variable to i same as int  amp j   i   j       int main       int i   20    f i     printf  i    d n   i      return 0      Output -   3 12  error  expected          or     before     token              void f int  j                             9 3   warning  implicit declaration of function  f                 f a

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You re passing a pointer address location  by value   It s like saying  here s the place with the data I want you to update

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pointers and references are two different thigngs   A couple of things I have not seen mentioned   A pointer is the address of something  A pointer can be stored and copied like any other variable  It thus have a size   A reference should be seen as an ALIAS of something  It does not have a size and cannot be stored  It MUST reference something  ie  it cannot be null or changed  Well  sometimes the compiler needs to store the reference as a pointer  but that is an implementation detail   With references you don t have the issues with pointers  like ownership handling  null checking  de-referencing on use

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Pass by reference   by using pointers  has been in C from the beginning  Why do you think it s not

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Your example works because you are passing the address of your variable to a function that manipulates its value with the dereference operator    While C does not support reference data types  you can still simulate passing-by-reference by explicitly passing pointer values  as in your example   The C   reference data type is less powerful but considered safer than the pointer type inherited from C  This would be your example  adapted to use C   references   void f int  amp j      j       int main       int i   20    f i     printf  i    d n   i      return 0

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p is a pointer variable  Its value is the address of i  When you call f  you pass the value of p  which is the address of i

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In C  to pass by reference you use the address-of operator  amp  which should be used against a variable  but in your case  since you have used the pointer variable p  you do not need to prefix it with the address-of operator  It would have been true if you used  amp i as the parameter  f  amp i    You can also add this  to dereference p and see how that value matches i   printf  p  d  n   p

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No pass-by-reference in C  but p  refers  to i  and you pass p by value

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Because you re passing the value of the pointer to the method and then dereferencing it to get the integer that is pointed to

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Because you re passing a pointer memory address  to the variable p into the function f  In other words you are passing a pointer not a reference

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In C everything is pass-by-value  The use of pointers gives us the illusion that we are passing by reference because the value of the variable changes  However  if you were to print out the address of the pointer variable  you will see that it doesn t get affected  A copy of the value of the address is passed-in to the function  Below is a snippet illustrating that    void add number int  a         a    a   2     int main int argc  char  argv         int a   2      printf  before pass by reference  a     i n   a      add number  amp a      printf  after  pass by reference  a     i n   a       printf  before pass by reference  a     p n    amp a      add number  amp a      printf  after  pass by reference  a     p n    amp a       before pass by reference  a    2 after  pass by reference  a    4 before pass by reference  a    0x7fff5cf417ec after  pass by reference  a    0x7fff5cf417ec

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Short answer  Yes  C does implement parameter passing by reference using pointers   While implementing parameter passing  designers of programming languages use three different strategies  or semantic models   transfer data to the subprogram  receive data from the subprogram  or do both  These models are commonly known as in mode  out mode  and inout mode  correspondingly   Several models have been devised by language designers to implement these three elementary parameter passing strategies    Pass-by-Value  in mode semantics  Pass-by-Result  out mode semantics  Pass-by-Value-Result  inout mode semantics  Pass-by-Reference  inout mode semantics  Pass-by-Name  inout mode semantics   Pass-by-reference is the second technique for inout-mode parameter passing   Instead of copying data back and forth between the main routine and the subprogram  the runtime system sends a direct access path to the data for the subprogram  In this strategy the subprogram has direct access to the data effectively sharing the data with the main routine  The main advantage with this technique is that its absolutely efficient in time and space because there is no need to duplicate space and there is no data copying operations   Parameter passing implementation in C  C implements pass-by-value and also pass-by-reference  inout mode  semantics using pointers as parameters  The pointer is send to the subprogram and no actual data is copied at all  However  because a pointer is an access path to the data of the main routine  the subprogram may change the data in the main routine  C adopted this method from ALGOL68   Parameter passing implementation in C    C   also implements pass-by-reference  inout mode  semantics using pointers and also using a special kind of pointer  called reference type  Reference type pointers are implicitly dereferenced inside the subprogram but their semantics are also pass-by-reference   So the key concept here is that pass-by-reference implements an access path to the data instead of copying the data into the subprogram  Data access paths can be explicitly dereferenced pointers or auto dereferenced pointers  reference type    For more info please refer to the book Concepts of Programming Languages by Robert Sebesta  10th Ed   Chapter 9

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I think C in fact supports pass by reference   Most languages require syntactic sugar to pass by reference instead of value   C   for example requires  amp  in the parameter declaration     C also requires syntactic sugar for this  It s   in the parameter type declaration and  amp  on the argument  So   and  amp  is the C syntax for pass by reference    One could now argue that real pass by reference should only require syntax on the parameter declaration  not on the argument side    But now comes C  which does support by reference passing and requires syntactic sugar on both parameter and argument sides     The argument that C has no by-ref passing cause the the syntactic elements to express it exhibit the underlying technical implementation is not an argument at all  as this applies more or less to all implementations   The only remaining argument is that passing by ref in C is not a monolithic feature but combines two existing features   Take ref of argument by  amp   expect ref to type by     C  for example does require two syntactic elements  but they can t be used without each other    This is obviously a dangerous argument  cause lots of other features in languages are composed of other features   like string support in C

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Because there is no pass-by-reference in the above code  Using pointers  such as void func int  p   is pass-by-address  This is pass-by-reference in C    won t work in C    void func int amp  ref   ref   4        int a  func a      a is 4 now

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That is not pass-by-reference  that is pass-by-value as others stated      The C language is pass-by-value without exception  Passing a pointer   as a parameter does not mean pass-by-reference    The rule is the following      A function is not able to change the actual parameters value      Let s try to see the differences between scalar and pointer parameters of a function   Scalar variables  This short program shows pass-by-value using a scalar variable  param is called the formal parameter and variable at function invocation is called actual parameter  Note incrementing param in the function does not change variable    include  lt stdio h gt   void function int param        printf  I ve received value  d n   param       param       int main void        int variable   111       function variable       printf  variable  d m   variable       return 0      The result is  I ve received value 111 variable 111   Illusion of pass-by-reference  We change the piece of code slightly  param is a pointer now    include  lt stdio h gt   void function2 int  param        printf  I ve received value  d n    param         param        int main void        int variable   111       function2  amp variable       printf  variable  d n   variable       return 0      The result is  I ve received value 111 variable 112   That makes you believe that the parameter was passed by reference  It was not  It was passed by value  the param value being an address  The int type value was incremented  and that is the side effect that make us think that it was a pass-by-reference function call   Pointers - passed-by-value  How can we show prove that fact  Well  maybe we can try the first example of Scalar variables  but instead of scalar we use addresses  pointers   Let s see if that can help    include  lt stdio h gt   void function2 int  param        printf  param s address  d n   param       param   NULL     int main void        int variable   111      int  ptr    amp variable       function2 ptr       printf  ptr s address  d n   ptr       return 0      The result will be that the two addresses are equal  don t worry about the exact value    Example result   param s address -1846583468 ptr s address -1846583468   In my opinion this proves clearly that pointers are passed-by-value  Otherwise ptr would be NULL after function invocation

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In C  Pass-by-reference is simulated   by passing the address of a variable    a pointer  and dereferencing that   address within the function to read or   write the actual variable   This will   be referred to as  C style   pass-by-reference     Source  www-cs-students stanford edu

[c] Passing by reference in C

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