Can someone elaborate on the following gcc error?
$ gcc -o Ctutorial/temptable.out temptable.c
temptable.c: In function ‘main’:
temptable.c:5: error: ‘for’ loop initial declaration used outside C99 mode
temptable.c:
...
/* print Fahrenheit-Celsius Table */
main()
{
for(int i = 0; i <= 300; i += 20)
{
printf("F=%d C=%d\n",i, (i-32) / 9);
}
}
P.S: I vaguely recall that int i
should be declared before a for
loop. I should state that I am looking for an answer that gives a historical context of C standard.
for (int i = 0; ...)
is a syntax that was introduced in C99. In order to use it you must enable C99 mode by passing -std=c99
(or some later standard) to GCC. The C89 version is:
int i;
for (i = 0; ...)
EDIT
Historically, the C language always forced programmers to declare all the variables at the begin of a block. So something like:
{
printf("%d", 42);
int c = 43; /* <--- compile time error */
must be rewritten as:
{
int c = 43;
printf("%d", 42);
a block is defined as:
block := '{' declarations statements '}'
C99, C++, C#, and Java allow declaration of variables anywhere in a block.
The real reason (guessing) is about allocating internal structures (like calculating stack size) ASAP while parsing the C source, without go for another compiler pass.