Dynamic String Input - using scanf("%as")

Question

I am trying to read input using scanf and storing into char * dynamically as specified by GCC manual, But it is giving a compile time error.

  char *string;
  if (scanf ("%as",&string) != 1){
    //some code
  }
  else{
   printf("%s\n", *string);
   free(string);
   //some code
  }

Nietzche-jou · Accepted Answer · 2010-02-24 22:36:28Z

10

The a modifier to scanf won't work if you are compiling with the -std=c99 flag; make sure you aren't using that.

If you have at least version 2.7 of glibc, you can and should use the m modifier in place of a.

Also, it is your responsibility to free the buffer.

edited Feb 24, 2010 at 22:36

answered Feb 24, 2010 at 22:28

Nietzche-jou

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4 Comments

N 1.1 Over a year ago

compiling with '-ansi' or '--std=c98' works with scanf("%as").

Michael Burr Over a year ago

Some background on the m flag, since the GCC/glibc docs don't mention it: gcc.gnu.org/ml/gcc-patches/2007-09/msg01342.html

user339222 Over a year ago

To use the a modifier, pass -D_GNU_SOURCE to gcc, but of course better use m.

Peter Cordes Over a year ago

Confusingly, with _GNU_SOURCE defined (in the .c before any #includes), gcc 4.9.2 with -std=gnu99 warns that %a wants a float*, but you're passing it a char **. But the behaviour matches %m[. It took me a while to remember that %a was a dynamic allocation conversion, because the man page doesn't mention the old GNU meaning anywhere near the other modifiers. Even the new POSIX.1-2008 %ms / %m[ only gets mentioned in a dense paragraph in with the field width stuff. Really easy to miss. :/

Tronic · Accepted Answer · 2010-02-24 22:03:07Z

1

Do you have GNU extensions enabled? Standard C doesn't have a modifier at all.

answered Feb 24, 2010 at 22:03

Tronic

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Comments

N 1.1 · Accepted Answer · 2010-03-02 10:58:09Z

1

'Dynamic String Input' with scanf("%as") will work if the -ansi or -std=c89 flag is enabled.
Compile using gcc -ansi

Or else you can use scanf("%ms")

edited Mar 2, 2010 at 10:58

answered Feb 24, 2010 at 22:26

N 1.1

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Comments

score 0 · Accepted Answer · 2010-02-24 22:10:18Z

0

I've had limited experience with GCC, but I've never seen a %a modifier for scanf. Have you tried replacing the %a with %s in the third line you provided?

edited Feb 24, 2010 at 22:10

answered Feb 24, 2010 at 22:02

user65628

4 Comments

N 1.1 Over a year ago

Please refer to the link provided. FYI %c stores only 1 char. I am trying to dynamically allocate memory for storing a complete string of 0-9a-zA-z characters.

user65628 Over a year ago

I know what %c does - I just missed that bit. What happens when you use %s instead of %a?

N 1.1 Over a year ago

%s will work if you already have allocated memory. whereas %as (with a flag) allocates required memory itself to *variable which can later be freed()

user65628 Over a year ago

@nvl - That I did not know. Thanks for the info :)

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