CODEWITHSUNDEEP
c++gccgcc-warning
Peter VARGA
Peter VARGA

Reputation: 5186

gcc 4.9.2 bug in -Werror=sizeof-pointer-memaccess?

#include <string.h>

void test(char charArray [100])
{
    strncpy(charArray, "some text", sizeof(charArray));
}

int main()
{
    char charArray [100];
    test(charArray);
    // EDIT: According to comment from P0W I added this line - where is the difference?
    strncpy(charArray, "some text", sizeof(charArray)); // compiles OK
}

Compiled with gcc 4.9.2 on SLES 11 SP2 with this command line g++ gcc-warning-bug-2.cpp -Wall -Wextra -c -Werror I get this warning. Due the -Werror flag I cannot compile the project:

gcc-warning-bug-2.cpp: In function ‘void test(char*)’:
gcc-warning-bug-2.cpp:5:40: error: argument to ‘sizeof’ in ‘char* strncpy(char*, const char*, size_t)’ call is the same expression as the destination; did you mean to provide an explicit length? [-Werror=sizeof-pointer-memaccess]
  strncpy(charArray, "some text", sizeof(charArray));
                                        ^
cc1plus: all warnings being treated as errors

According to the actual gcc 4.9.2 documentation https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses sizeof.
This warning warns e.g. about memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and suggests a possible fix, or about memcpy (&foo, ptr, sizeof (&foo));. This warning is enabled by -Wall.

this should be compiled fine because charArray is an array!

Bug? Should I report it to GNU gcc developer team?

Upvotes: 3

Views: 5871

Answers (1)

gnasher729
gnasher729

Reputation: 52538

You fell straight into the trap.

In C, C++, Objective-C, Objective-C++, a parameter with a declaration that looks like "array of T" actually has type T*.

Your parameter charArray has a declaration that looks like "array of 100 chars", but the declaration is in fact "pointer to char".

Therefore, your third parameter to strncpy has a value of (most likely) 4 or 8, and not the 100 that you seem to expecct.

BTW. strncpy is highly dangerous the way you use it.

Upvotes: 11

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