How do you compare structs for equality in C

Question

How do you compare two instances of structs for equality in standard C

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Greg is correct that one must write explicit comparison functions in the general case   It is possible to use memcmp if    the structs contain no floating-point fields that are possibly NaN  the structs contain no padding  use -Wpadded with clang to check this  OR the structs are explicitly initialized with memset at initialization  there are no member types  such as Windows BOOL  that have distinct but equivalent values    Unless you are programming for embedded systems  or writing a library that might be used on them   I would not worry about some of the corner cases in the C standard   The near vs  far pointer distinction does not exist on any 32- or 64- bit device   No non-embedded system that I know of has multiple NULL pointers   Another option is to auto-generate the equality functions   If you lay your struct definitions out in a simple way  it is possible to use simple text processing to handle simple struct definitions   You can use libclang for the general case  ndash  since it uses the same frontend as Clang  it handles all corner cases correctly  barring bugs    I have not seen such a code generation library   However  it appears relatively simple   However  it is also the case that such generated equality functions would often do the wrong thing at application level   For example  should two UNICODE STRING structs in Windows be compared shallowly or deeply

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You may be tempted to use memcmp  amp a   amp b  sizeof struct foo    but it may not work in all situations  The compiler may add alignment buffer space to a structure  and the values found at memory locations lying in the buffer space are not guaranteed to be any particular value   But  if you use calloc or memset the full size of the structures before using them  you can do a shallow comparison with memcmp  if your structure contains pointers  it will match only if the address the pointers are pointing to are the same

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If you do it a lot I would suggest writing a function that compares the two structures   That way  if you ever change the structure you only need to change the compare in one place     As for how to do it     You need to compare every element individually

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memcmp does not compare structure  memcmp compares the binary  and there is always garbage in the struct  therefore it always comes out False in comparison   Compare element by element its safe and doesn t fail

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It depends on whether the question you are asking is    Are these two structs the same object  Do they have the same value    To find out if they are the same object  compare pointers to the two structs for equality  If you want to find out in general if they have the same value you have to do a deep comparison  This involves comparing all the members  If the members are pointers to other structs you need to recurse into those structs too   In the special case where the structs do not contain pointers you can do a memcmp to perform a bitwise comparison of the data contained in each without having to know what the data means   Make sure you know what  equals  means for each member - it is obvious for ints but more subtle when it comes to floating-point values or user-defined types

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if the 2 structures variable are initialied with calloc or they are set with 0 by memset so you can compare your 2 structures with memcmp and there is no worry about structure garbage and this will allow you to earn time

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If the structs only contain primitives or if you are interested in strict equality then you can do something like this    int my struct cmp const struct my struct   lhs  const struct my struct   rhs        return memcmp lhs  rsh  sizeof struct my struct        However  if your structs contain pointers to other structs or unions then you will need to write a function that compares the primitives properly and make comparison calls against the other structures as appropriate   Be aware  however  that you should have used memset  amp a  sizeof struct my struct   1  to zero out the memory range of the structures as part of your ADT initialization

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C provides no language facilities to do this - you have to do it yourself and compare each structure member by member

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You can t use memcmp to compare structs for equality due to potential random padding characters between field in structs        bad   memcmp  amp struct1   amp struct2  sizeof struct1      The above would fail for a struct like this   typedef struct Foo     char a       padding      double d       padding      char e       padding      int f    Foo     You have to use member-wise comparison to be safe

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This compliant example uses the  pragma pack compiler extension from Microsoft Visual Studio to ensure the structure members are packed as tightly as possible    include  lt string h gt    pragma pack push  1  struct s     char c    int i    char buffer 13       pragma pack pop   void compare const struct s  left  const struct s  right       if  0    memcmp left  right  sizeof struct s

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Note you can use memcmp   on non static stuctures without worrying about padding  as long as you don t initialise all members  at once   This is defined by C90   http   www pixelbeat org programming gcc auto init html

[c] How do you compare structs for equality in C?

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