How to initialize a char array without the null terminator?

Question

The char array is a part of network message, which has well defined length, so the null terminator is not needed.

struct Cmd {
    char cmd[4];
    int arg;
}

struct Cmd cmd { "ABCD" , 0 }; // this would be buffer overflow

How can I initialize this cmd member char array? without using functions like strncpy?

You need to initialize the structure variable like this:- struct Cmd cmd{ {'A','B','C','D'}, 0}; . If you give "ABCD", it is considered as string literal and will be stored in const char pointer and string literal ends with '\0' . — sonulohani
– sonulohani, Commented May 13, 2019 at 4:31
@sonulohani no, this is not "needed". Besides the missing = the given initialization is fine. — Jens Gustedt
– Jens Gustedt, Commented May 13, 2019 at 5:55
@JensGustedt The trick with C silently skipping the null terminator is dangerous - a language flaw in my opinion. Providing a {'A','B','C','D'} initializer is therefore good practice: it is self-documenting code showing that the programmer is aware of the lack of null termination, but it also allows compilers/static analysers to give a warning elsewhere in the code, when a string literal initializer results in a string that is not null terminated. — Lundin
– Lundin, Commented May 13, 2019 at 7:03
@JensGustedt You wrote that the initialization is fine. But it is not, since it contains a potential subtle bug. — Lundin
– Lundin, Commented May 14, 2019 at 6:28

P.W · Accepted Answer · 2019-05-13 04:52:57Z

15

Terminating null character is ignored if the size of the char array is the same as the number of characters in the initializer. So cmd will not have the null terminator.

The relevant section in the C11 standard (n1570) is 6.7.9/14:

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.

And the statement:

struct Cmd cmd { "ABCD" , 0 };

should be:

struct Cmd cmd  = { "ABCD" , 0 };

edited May 13, 2019 at 4:52

answered May 13, 2019 at 4:41

P.W

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3 Comments

Jonathan Leffler Over a year ago

Note that this is yet another area where C and C++ have different views on how things should work. C++11 (ISO/IEC 14882:2012) §8.5 Initializers, §8.5.2 Character arrays, ¶2: 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’. —end example ]

fluter Over a year ago

does that means C++ is different than C on this issue?

P.W Over a year ago

@fluter:Yes. In C++, it is ill-formed. Meaning, it is not a valid program.(program constructed according to the syntax rules, diagnosable semantic rules, and the One Definition Rule)

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