SizeofPtr -- Use of sizeof() on a Pointer Type

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SizeofPtr warns about unintended use of sizeof() on pointer expressions.

The code calls sizeof() on a malloced pointer type, which always returns wordsize/8. This can produce an unexpected result if the programmer intended to determine how much memory has been allocated.

Example 1

Care should be taken to ensure sizeof returns the size of the data structure itself, and not the size of the pointer to the data structure.

In this example, sizeof(foo) returns the size of the pointer.

 double *foo;
 foo = (double *)malloc(sizeof(foo));

In this example, sizeof(*foo) returns the size of the data structure and not the size of the pointer.

 double *foo;
 foo = (double *)malloc(sizeof(*foo));

Example 2

This example defines a fixed username and password. The AuthenticateUser() function is intended to accept a username and a password from an untrusted user, and check to ensure that it matches the username and password. If the username and password match, AuthenticateUser() is intended to indicate that authentication succeeded.

 char *username = "admin";
 char *pass = "password";
 int AuthenticateUser(char *inUser, char *inPass) {
   printf("Sizeof username = %d\n", sizeof(username));
   printf("Sizeof pass = %d\n", sizeof(pass));
   if (strncmp(username, inUser, sizeof(username))) {
     printf("Auth failure of username using sizeof\n");
   /* sizeof returns 4 on many platforms and architectures. */
   if (! strncmp(pass, inPass, sizeof(pass))) {
     printf("Auth success of password using sizeof\n");
   else {
     printf("Auth fail of password using sizeof\n");
 int main (int argc, char **argv)
   int authResult;
   if (argc < 3) {
     ExitError("Usage: Provide a username and password");
   authResult = AuthenticateUser(argv[1], argv[2]);
   if (authResult != AUTH_SUCCESS) {
     ExitError("Authentication failed");
   else {

In AuthenticateUser(), because sizeof() is applied to a parameter with an array type, the sizeof() call might return 4 on many modern architectures. As a result, the strncmp() call only checks the first four characters of the input password, resulting in a partial comparison, leading to improper authentication.

Because of the partial comparison, any of these passwords would still cause authentication to succeed for the "admin" user:


Because only 4 characters are checked, this significantly reduces the search space for an attacker, making brute force attacks more feasible.

The same problem also applies to the username, so values such as adminXYZ and administrator will succeed for the username.

See Also