Tutorial :sizeof abuse : get the size of a const table



Question:

When declaring a const table, it is possible to get the size of the table using sizeof. However, once you stop using the symbol name, it does not work anymore. is there a way to have the following program output the correct size for table A, instead of 0 ?

#include <stdio.h>    struct mystruct {      int a;      short b;  };    const struct mystruct tableA[] ={      {           .a = 1,          .b = 2,      },      {           .a = 2,          .b = 2,      },      {           .a = 3,          .b = 2,      },  };    const struct mystruct tableB[] ={      {           .a = 1,          .b = 2,      },      {           .a = 2,          .b = 2,      },  };      int main(int argc, char * argv[]) {      int tbl_sz;      const struct mystruct * table;        table = tableA;      tbl_sz = sizeof(table)/sizeof(struct mystruct);      printf("size of table A : %d\n", tbl_sz);        table = tableB;      tbl_sz = sizeof(tableB)/sizeof(struct mystruct);      printf("size of table B : %d\n", tbl_sz);        return 0;  }  

Output is :

size of table A : 0  size of table B : 2  

This is the intended behavior of sizeof. But is there a way for a compiler to know the size of a const table, given a pointer to the table instead of the symbol name ?


Solution:1

Is there a way for a compiler to know the size of a const table, given a pointer to the table instead of the symbol name?

No, because sizeof() is evaluated at compile-time (unless it is a VLA, but a VLA is not a constant table), and the compiler cannot, in general, tell which table the pointer is pointing to. Granted, in the scenario shown, it might be possible in some hypothetical variation of the C language, but that would mean varying definitions of what sizeof() returns, which would be a bigger problem than not getting the answer you might like but do not get.

So, as everyone else ably pointed out, when you take the size of a pointer, you get the size of the pointer. Assuming a standard 32-bit machine since the results are consistent with that assumption, your structure is 8 bytes and your pointers are 4 bytes, so the result of the division is zero, as expected.


Solution:2

you are asking for the sizeof a pointer. That is always the pointer size (ie usually 4 bytes on a 32-bit machine and 8 bytes on a 64-bit machine). In the 2nd attempt you are asking for the sizeof the array and hence you get the result you'd expect.


Solution:3

No - you're asking for the sizeof() a pointer. But since what you're really trying to get is the number of elements in an array, you can use a macro that will return that value but will generally give you an error if you pass a pointer instead of an array:

#define COUNT_OF(x) ((sizeof(x)/sizeof(0[x])) / ((size_t)(!(sizeof(x) % sizeof(0[x])))))  

See this SO answer for more details: Is there a standard function in C that would return the length of an array?

For an even safer solution when using C++ instead of C, see this SO answer that uses templates to ensure that trying to get an array count on a pointer will always generate an error: Compile time sizeof_array without using a macro


Solution:4

Short answer is no; if all you have is a pointer, then there's no (standard) way to get the size of the thing being pointed to through that pointer.


Solution:5

Although syntactically correct, your sample is more conventionally written as:

const struct mystruct tableA[] = {      {1, 2},      {2, 2},      {3, 3},   };  

Which is less verbose and therefore more readable.


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