Best way to copy data of one structure to the other

[aboosoyeed]

Assuming i have a pointer to a structure

StructA a* which contains value. Now in between computation i want to keep a backup of the structure by storing the a in b which is also an object of StructA i,e StructA b*.
What is the best way to copy the structure .
i cant to b=a coz that wud point reference b to a and not a backup.
I tried memcopy(b,a,sizeof(a)) but i realised sizeof(a) gives the size of the structure and not the contents

[cas]

*b = *a;

[Salem]

Just do
structA b = *a;

Assuming your struct does NOT contain any pointers.

[aboosoyeed]

Just do
structA b = *a;

Assuming your struct does NOT contain any pointers.

Unfortunately it does .. i am using libpq module .

using
structA b = *a; (in my case PGresult op=*res
results in
pgdb.c:76:5: error: variable ‘op’ has initializer but incomplete type
pgdb.c:76:18: error: dereferencing pointer to incomplete type
pgdb.c:76:15: error: storage size of ‘op’ isn’t known
which is understandable

[Salem]

Then you're stuck.

If the type is incomplete, there is no way you can make "a backup copy", since you can't find out what size it is, or what it contains.

[iMalc]

Those errors mean that you are missing actual definition of the struct at that point, which likely means you are missing some #include for it.
If the struct itself contains pointer then you're best to write a "copy" method for the struct and use that.
Internally within the copy method I would start with a *this = *that; (i.e. one line complete copy) and then just correct the members that are pointers.

[QuantumPete]

Unfortunately it does .. i am using libpq module .

Have you checked whether this module has an pre-built copy function for this struct?