error: ‘for‘ loop initial declarations are only allowed in C99 mode

The phenomenon

   In use today  gcc  Compile the code and report ：

error: 'for' loop initial declarations are only allowed in C99 mode

note: use option -std=c99 or -std=gnu99 to compile your code

reason

   Compilation error , This is because in the gcc Directly in for Increment... Is initialized in the loop ：

for(int i=0; i<len; i++) 
{
}

The first method

This grammar is in gcc Is wrong , You must first define i Variable ：

int i;
for(i=0;i<len;i++)
{ 
}

This is because gcc be based on c89 standard , If you change into c99 The standard can be in for Define within the loop i The variable .

The second solution

It's in writable makefile When compiling instructions , take  gcc for.c -o for.out  Change it to  gcc -std=c99 for.c -o for.out