Why does gcc allow char array initialization with string literal larger than array?

Question

int main()
{
    char a[7] = "Network";
    return 0;
}

A string literal in C is terminated internally with a nul character. So, the above code should give a compilation error since the actual length of the string literal Network is 8 and it cannot fit in a char[7] array.

However, gcc (even with -Wall) on Ubuntu compiles this code without any error or warning. Why does gcc allow this and not flag it as compilation error?

gcc only gives a warning (still no error!) when the char array size is smaller than the string literal. For example, it warns on:

char a[6] = "Network";

[Related] Visual C++ 2012 gives a compilation error for char a[7]:

1>d:\main.cpp(3): error C2117: 'a' : array bounds overflow
1> d:\main.cpp(3) : see declaration of 'a'

Answer 1

Initializing a char array with a string literal that is larger than it is fine in C, but wrong in C++. That explains the difference in behavior between gcc and VC++.

You would get no error if you compiled the same as a C file with VC++. And you would get an error if you compiled it as a C++ file with g++.

The C standard says:

An array of character type may be initialized by a character string literal or UTF−8 string literal, optionally enclosed in braces. Successive bytes of the string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

[...]

EXAMPLE 8

The declaration

char s[] = "abc", t[3] = "abc";

defines ‘‘plain’’ char array objects s and t whose elements are initialized with character string literals. This declaration is identical to

char s[] = { 'a', 'b', 'c', '\0' },
     t[] = { 'a', 'b', 'c' };

(Section 6.7.9 of the C11 draft standard, actual wording in final standard might be different.)

This means that it's perfectly correct to drop the termination character if the array doesn't have room for it. It's maybe unexpected, but it's exactly how the language is supposed to work, and a (at least to me) well-known feature.

On the contrary, the C++ standard says:

There shall not be more initializers than there are array elements.

Example:

 char cv[4] = "asdf"; // error

is ill-formed since there is no space for the implied trailing '\0'.

(8.5.2 of the C++ 2011 draft n3242.)

Answer 2

In the early days of C and Unix, memory and disk were small, so not storing the NUL byte at the end of the string was actually a technique that was used. If the string variable is seven characters long, you could store a seven-character string in it, and since seven was the max length, you knew the string ended there, even without the terminator character. This is why strncpy works the way it does.

Answer 3

While unwind's answer explains why gcc doesn't warn about this, it doesn't say what you can do about it.

gcc's -Wc++-compat warning option will detect this particular issue with the message:

foo.c: In function ‘main’:
foo.c:3:17: warning: initializer-string for array chars is too long for C++ [-Wc++-compat]

That's the only option that will cause gcc to warn about this problem. You can write a short script to quickly grep the warning options out of gcc's man page, try compiling with each, and see if it complains.

$ time for F in $(man gcc | grep -o -- '-W[^= ]*')
    do if gcc -c "${F}" foo.c |& grep :3 >& /dev/null; then
         echo "${F}"; gcc -c "${F}" foo.c
    fi
  done
man gcc | grep -o -- '-W[^= ]*')
man gcc | grep -o -- '-W[^= ]*'
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wc++-compat
foo.c: In function ‘main’:
foo.c:3:17: warning: initializer-string for array chars is too long for C++ [-Wc++-compat]
-Wunused
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused-variable
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wtraditional
foo.c: In function ‘main’:
foo.c:3:5: warning: traditional C rejects automatic aggregate initialization [-Wtraditional]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused-variable
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused-variable
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wunused
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wtraditional
foo.c: In function ‘main’:
foo.c:3:5: warning: traditional C rejects automatic aggregate initialization [-Wtraditional]
-Wtraditional
foo.c: In function ‘main’:
foo.c:3:5: warning: traditional C rejects automatic aggregate initialization [-Wtraditional]
-Wc++-compat
foo.c: In function ‘main’:
foo.c:3:17: warning: initializer-string for array chars is too long for C++ [-Wc++-compat]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wtraditional
foo.c: In function ‘main’:
foo.c:3:5: warning: traditional C rejects automatic aggregate initialization [-Wtraditional]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wall
foo.c: In function ‘main’:
foo.c:3:10: warning: unused variable ‘a’ [-Wunused-variable]
-Wtraditional
foo.c: In function ‘main’:
foo.c:3:5: warning: traditional C rejects automatic aggregate initialization [-Wtraditional]

real    0m26.399s
user    0m5.128s
sys 0m15.329s

---

In general, a lint-like tool such as splint will warn you about all sorts of potential issues. In this case, it will say:

foo.c:3:17: String literal with 8 characters is assigned to char [7] (no room
               for null terminator): "Network"
  A string literal is assigned to a char array that is not big enough to hold
  the null terminator. (Use -stringliteralnoroom to inhibit warning)
foo.c:3:10: Variable a declared but not used

Answer 4

The preffered way of declaring a string literal is usually:

   char a[] = "Network";
   printf("size of a: %d\n", sizeof a); // The compiler 'knows' the size of a.
   // this prints '8'

Let the compiler figure it out. It's cumbersome to manually specify the array size and keep it in sync with the string literal's actual length...

So I guess GCC doesn't really bother with anything more than a warning.