Pass by value and pass by reference

With the exception of arrays and functions (see below), C always passes arguments `by value': a copy of the value of each argument is passed to the function; the function cannot modify the actual argument passed to it:

void foo(int j) { j = 0; /* modifies the copy of the argument received by the function */ } int main(void) { int k=10; foo(k); /* k still equals 10 */ }

If you do want a function to modify its argument you can obtain the desired effect using pointer arguments instead:

void foo(int *j) { *j = 0; } int main(void) { int k=10; foo(&k); /* k now equals 0 */ }

This is sometimes known as `pass by reference' in other languages.