Compilation error of Variable Length Array with GCC

Question

How to make a compilation successful for a program with a variable length array?(currently, Showing error : Variable sized array). I am using gcc in linux. How to make compiler compatible to c99 standard ? PLease help me in this. THanks in advance.

With a new enough version of GCC it will automatically use a later version of the C standard. Which version are you using? What are you trying to do? Please [edit] your question to include a [mcve] of your code, together with a full and complete copy-paste (as text) of your errors (with comments added in the code where you get the errors). — Some programmer dude, Jul 06 '20 at 04:44
Also please take some time to read [the help pages](http://stackoverflow.com/help), take the SO [tour], read [ask], as well as [this question checklist](https://codeblog.jonskeet.uk/2012/11/24/stack-overflow-question-checklist/). — Some programmer dude, Jul 06 '20 at 04:45
it would have been faster to read the diagnostic message that tells you what to do, than write this question ... — M.M, Jul 06 '20 at 04:45
@M.M Usually a VLA warning is just to hint that one uses VLAs in their code. It does not tell you what to do to satisfy the compiler. If you don't know what a VLA is nor what its advantages or disadvantages are you got a problem. Asking for that is absolutely appropriate. — RobertS supports Monica Cellio, Jul 06 '20 at 07:57

Rohan Bari · Answer 1 · 2020-07-09T14:56:57.967

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How to make compiler compatible to c99 standard?

By default, the compiler defaults to the most compatible version of C version is installed. Do define the compilation version explicitly, compile the program with the following command-line:

$ gcc -std=c99 -o my_program my_program.c

By defining the -std=c99, the compiler will be using C99 standard.

Edit: If you're still getting the warning and not the error, then you need to provide your code to know what exactly is wrong.

edited Jul 09 '20 at 14:56

answered Jul 06 '20 at 05:15

Rohan Bari

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Yes , I tried it . Still no changes in the result. Thank You!!! – Dhana Prakaash Jul 06 '20 at 06:48
@DhanaPrakaash Then you need to show your code, we don't exactly know what your code does and the `-std=c99` flag must work correctly. Also, edit the question and add all the information after typing `gcc -v` command in your terminal. – Rohan Bari Jul 06 '20 at 06:50
@RohanBari Even with `-std=c99` the *warning* won't go away. The compiler is just hinting. – RobertS supports Monica Cellio Jul 06 '20 at 07:19
@RobertSsupportsMonicaCellio the VLAs are supported in C99 standard, ain't it? – Rohan Bari Jul 06 '20 at 07:25
@RohanBari https://stackoverflow.com/questions/61825661/why-does-clang-complain-about-using-variable-length-arrays-with-std-c99-flag – RobertS supports Monica Cellio Jul 06 '20 at 07:26
@RobertSsupportsMonicaCellio the question is tagged with `gcc` I guess. It's not clang. – Rohan Bari Jul 06 '20 at 07:26
@RohanBari It covers in the answers that any compiler can inform you about what it likes to do. I asked for clang, but the answers can be understood for any compiler for the *most* part. – RobertS supports Monica Cellio Jul 06 '20 at 07:29
@RohanBari It also explains why at OP's execution the presumed *error* does not go away -> "*Still no changes in the result*". I'm still a kind of confused about the "error" OP seems to get. The use of VLAs shouldn't cause any *error*. – RobertS supports Monica Cellio Jul 06 '20 at 07:40
@RobertSsupportsMonicaCellio Yes, that's obviously not an error. I simply answered - *how to make compiler compatible to c99 standard* and that's the possible method to remove the VLA warning. However, I already asked for some code from the OP in order to understand what's actually going on. – Rohan Bari Jul 06 '20 at 07:42
@RohanBari I got nothing against your answer. It could have been highly possible that the use of `-std=c99` had removed the warning. I just try to find out what is going on here. Yes, the missing code is unfortunate and disadvantageous. – RobertS supports Monica Cellio Jul 06 '20 at 07:52
@RohanBari Also your answer answers perfectly: "*How to make compiler compatible to c99 standard?*" - Maybe you should emphasize this and separate with that both questions. – RobertS supports Monica Cellio Jul 06 '20 at 08:02

RobertS supports Monica Cellio · Answer 2 · 2020-07-06T08:04:19.003

"How to make a compilation successful for a program with a variable length array? (currently, Showing error : Variable sized array)."

Usually it isn't an error to compile a code with a VLA unless you compile with -Werror flag.

The diagnostic you get is high-probably "only" a warning that you use a VLA inside of it, which is risky.

Thus, the compiler informs you about that.

So, you indeed can compile a program with VLAs without any error.

If you got errors they must belong to anything else. We can't find those out since you showed no specific code.

Take a look at this question of mine, not so long ago (even if it is for Clang, it covers the same topic as the answers suggest that a compiler is free to complain about whatever it likes):

Why does clang complain about using variable-length arrays with '-std=c99' flag?

All useful information you can find there.

VLAs are not portable. Try to use alternatives, for example dynamically allocated arrays by using malloc().

"How to make compiler compatible to (the) C99 standard?"

As Rohan in his answer already said, you can use the -std-c99 flag at the invocation of gcc for that. But it probably won't solve your problem to do so.

Compilation error of Variable Length Array with GCC

2 Answers2