Let us consider the following code:
#include <stdio.h>
int main(){
int size,i;
scanf("%d",&size);
int x[size];
for(i=0;i<size;i++){
x[i] = i;
}
for(i=0;i<size;i++){
printf("%d\n", x[i]);
}
return 0;
}
I think that there are versions in that we cannot declare an array using a variable for defining its size, like in:
int x[size];
But I'm not sure about which version is. This is allowed in the so called ANSI C?
In C89/C90, you can't interleave declarations and statements, such as putting the int x[size]; declaration (definition) after the scanf() — even if you change the dimension to a compile time constant.
In C89/C90, you cannot use a variable length array — so the int x[size]; definition is not legal because size is not a compile time constant.
C99 compilers are required to support both declarations at (almost) arbitrary points in a function (they still can't be preceded by a label) and variable length array (VLA) definitions.
C11 compilers are required to support variable definitions (almost) anywhere in a function. C11 compilers may optionally support VLAs (§6.10.8.3 Conditional feature macros and §6.7.6.2 Array declarators), and should define __STDC_NO_VLA__ if they do not support them.
C18 is equivalent to C11 in all aspects of this discussion.
Just for the record:
Of course, the only unusual thing about X3.159-1989 was that it was published by ANSI before there was an ISO standard. However, ANSI has adopted each subsequent ISO standard too, as have other national standards bodies such as BSI (British Standards Institute) and DIN (Deutsches Institut für Normung or German Institute for Standardization).
There was also an Amendment 1 (to ISO 9899:1990) finalized in 1994 and published in 1995. That added headers and introduced digraphs and made sundry other changes. It is almost never considered separately, especially not now, 25 years later.
Note that GCC treats the -ansi option as equivalent to -std=c90, which can lead to confusion over terminology. ANSI originally published 'the ANSI C standard' a year or so before ISO did, but the intent was always to have a common standard, and ANSI endorsed the ISO 9899:1990 standard when it was published.
ANSI-C (C89) does not allow initialize an array using a variable.
For example:
int x = 5;
int ia[x];
This above example is illegal. ANSI-C restricts the array intialization size to be constant. However you can use macros to define the length of an array.
Example:
#define MAX_ARRAY_SIZE 5
int ia[MAX_ARRAY_SIZE];
This kind of array, called variable length array was added in C99. Check out this nice article about modern C programming:
