How do I fix "for loop initial declaration used outside C99 mode" GCC error?

Question

I'm trying to solve the 3n+1 problem and I have a for loop that looks like this:

for(int i = low; i <= high; ++i)
        {
                res = runalg(i);
                if (res > highestres)
                {
                        highestres = res;
                }

        }

Unfortunately I'm getting this error when I try to compile with GCC:

3np1.c:15: error: 'for' loop initial declaration used outside C99 mode

I don't know what C99 mode is. Any ideas?

dhilt · Accepted Answer · 2017-11-08 07:20:39Z

161

I'd try to declare i outside of the loop!

Good luck on solving 3n+1 :-)

Here's an example:

#include <stdio.h>

int main() {

   int i;

   /* for loop execution */
   for (i = 10; i < 20; i++) {
       printf("i: %d\n", i);
   }   

   return 0;
}

2 Comments

Erik W Over a year ago

then why don't you use while loop instead?

user25 Over a year ago

@ErikW your question is very bad

Ivaylo Strandjev · Accepted Answer · 2013-11-15 09:39:45Z

125

There is a compiler switch which enables C99 mode, which amongst other things allows declaration of a variable inside the for loop. To turn it on use the compiler switch -std=c99

Or as @OysterD says, declare the variable outside the loop.

edited Nov 15, 2013 at 9:39

Ivaylo Strandjev

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answered Aug 24, 2008 at 6:46

JamesSugrue

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2 Comments

Spudd86 Over a year ago

actually -std=gnu99 is probably more desirable since that way you still get gcc extensions (gcc defaults to -std=gnu89, however this will be changing to gnu99 at some point in the next few versions)

Keith Thompson Over a year ago

@Spudd86: Whether you want to enable gcc extensions depends on what you're doing. Disabling them is helpful if your goal is to write code that's portable to compilers other than gcc.

akelec · Accepted Answer · 2017-04-02 12:17:30Z

23

To switch to C99 mode in CodeBlocks, follow the next steps:

Click Project/Build options, then in tab Compiler Settings choose subtab Other options, and place -std=c99 in the text area, and click Ok.

This will turn C99 mode on for your Compiler.

I hope this will help someone!

edited Apr 2, 2017 at 12:17

answered Dec 1, 2014 at 9:57

akelec

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3 Comments

Ankur Shah Over a year ago

This helped in codeblocks 16.1 under the Other Compiler options! Though I used -std=gnu99 as depicted in some answers above!

Ankur Shah Over a year ago

But this helped temporarily! I have to do this every time I compile code!...:(

akelec Over a year ago

No, you have not! This must be done for each new project only once.

Blorgbeard · Accepted Answer · 2013-09-13 01:25:37Z

14

I've gotten this error too.

for (int i=0;i<10;i++) { ..

is not valid in the C89/C90 standard. As OysterD says, you need to do:

int i;
for (i=0;i<10;i++) { ..

Your original code is allowed in C99 and later standards of the C language.

edited Sep 13, 2013 at 1:25

answered Aug 24, 2008 at 6:50

Blorgbeard

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6 Comments

Matt J Over a year ago

For gcc, throw it a "-std=c99". For additional features, see Imran's answer.

Keith Thompson Over a year ago

Yes, it is valid C; it's just not valid C89/C90.

Blorgbeard Over a year ago

@KeithThompson: clarified.

Keith Thompson Over a year ago

You're still saying it's not valid C, only with a qualification. It is perfectly valid C; the 1999 standard superseded and replaced the 1990 standard, and the 2011 standard superseded and replaced the 1999 standard (though the latter didn't exist when you originally posted this answer).

Blorgbeard Over a year ago

@KeithThompson Well, that's not what I thought I was saying. Would you consider "is not valid in the C89/C90 standard" correct?

|

Community · Accepted Answer · 2017-05-23 12:34:18Z

13

@Blorgbeard:

New Features in C99

inline functions
variable declaration no longer restricted to file scope or the start of a compound statement
several new data types, including long long int, optional extended integer types, an explicit boolean data type, and a complex type to represent complex numbers
variable-length arrays
support for one-line comments beginning with //, as in BCPL or C++
new library functions, such as snprintf
new header files, such as stdbool.h and inttypes.h
type-generic math functions (tgmath.h)
improved support for IEEE floating point
designated initializers
compound literals
support for variadic macros (macros of variable arity)
restrict qualification to allow more aggressive code optimization

http://en.wikipedia.org/wiki/C99

A Tour of C99

edited May 23, 2017 at 12:34

CommunityBot

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answered Aug 25, 2008 at 11:41

Imran

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Comments

Roberto Cuadros · Accepted Answer · 2013-04-08 05:25:12Z

6

if you compile in C change

for (int i=0;i<10;i++) { ..

to

int i;
for (i=0;i<10;i++) { ..

You can also compile with the C99 switch set. Put -std=c99 in the compilation line:

gcc -std=c99 foo.c -o foo

REF: http://cplusplus.syntaxerrors.info/index.php?title='for'_loop_initial_declaration_used_outside_C99_mode

answered Apr 8, 2013 at 5:25

Roberto Cuadros

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Comments

Mink · Accepted Answer · 2018-06-27 15:37:50Z

6

For anyone attempting to compile code from an external source that uses an automated build utility such as Make, to avoid having to track down the explicit gcc compilation calls you can set an environment variable. Enter on command prompt or put in .bashrc (or .bash_profile on Mac):

export CFLAGS="-std=c99"

Note that a similar solution applies if you run into a similar scenario with C++ compilation that requires C++ 11, you can use:

export CXXFLAGS="-std=c++11"

answered Jun 27, 2018 at 15:37

Mink

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Comments

Dark Burrow · Accepted Answer · 2018-10-07 06:51:25Z

2

Jihene Stambouli answered OP question most directly... Question was; why does

for(int i = low; i <= high; ++i)
{
    res = runalg(i);
    if (res > highestres)
    {
        highestres = res;
    }
}

produce the error;

3np1.c:15: error: 'for' loop initial declaration used outside C99 mode

for which the answer is

for(int i = low...

should be

int i;
for (i=low...

answered Oct 7, 2018 at 6:51

Dark Burrow

415 bronze badges

Comments

Jobayer Ahmmed · Accepted Answer · 2019-07-29 10:57:10Z

2

Enable C99 mode in Code::Blocks 16.01

Go to Settings-> Compiler...
In Compiler Flags section of Compiler settings tab, select checkbox 'Have gcc follow the 1999 ISO C language standard [-std=c99]'

answered Jul 29, 2019 at 10:57

Jobayer Ahmmed

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Comments

James · Accepted Answer · 2013-09-12 23:32:33Z

1

I had the same problem and it works you just have to declare the i outside of the loop:

int i;

for(i = low; i <= high; ++i)

{
        res = runalg(i);
        if (res > highestres)
        {
                highestres = res;
        }

}

edited Sep 12, 2013 at 23:32

James

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answered Sep 12, 2013 at 22:57

Jihene Stambouli

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Comments

demiurg_spb · Accepted Answer · 2013-05-29 14:27:19Z

0

For Qt-creator: just add next lines to *.pro file...

QMAKE_CFLAGS_DEBUG = \
    -std=gnu99

QMAKE_CFLAGS_RELEASE = \
    -std=gnu99

answered May 29, 2013 at 14:27

demiurg_spb

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