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 charac

Answer 1

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