错误：'for' 循环声明仅在 c99 模式下

Question

I am trying to compile this C code in my linux system (I am new to all of this) and I keep getting this error:

ForkCall.c: In function ‘main’:
  ForkCall.c:70:1: error: ‘for’ loop initial declarations are only allowed in C99 mode
    for (int i=1; i<=5; i++)
    ^
  ForkCall.c:70:1: note: use option -std=c99 or -std=gnu99 to compile your code

I tried declaring int i before the for loop, but got this error:

ForkCall.c: In function ‘main’:
  ForkCall.c:69:1: error: a label can only be part of a statement and a declaration is 
  not a statement
   int i;

   ^

This is the code:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main()
{
    pid_t pid, pid2;
    int n;

    printf("Select a number [1-5]: ") ;
    scanf("%d", &n);
    printf("\n\n");

    switch(n)
    {
    case 1:
        fork();
        printf("This is process %d\n", getpid());
        break;

    case 2:
        fork(); fork();
        printf("This is process %d\n", getpid());
        break;

    case 3:
        fork(); fork(); fork();
        printf("This is process %d\n", getpid());
        break;

    case 4:
        if((pid=fork()) && (pid2 = fork())) {fork();}
        if((pid=fork()) && (pid2 = fork())) {fork();}
        if ((pid=fork()) && (pid2 = fork())) {fork();}
        printf("This is process %d\n", getpid());
        break;

    case 5:
        for (int i=1; i<=5; i++)
        {
            fork();
        }
        printf("This is process %d\n", getpid());
        break;

    default:
        printf("Number not in range [1-5] !\n");

    }
}
``

Answer 1

Either set the option of the compiler that allows to compile code as a C99 code (or C11, or C18). Or rewrite this part of the code:

    case 5:
        for (int i=1; i<=5; i++)
        {
        fork();
        }
        printf("This is process %d\n", getpid());
        break;

the following way

    case 5:
    {
        int i = 1;
        for ( ; i<=5; i++)
        {
            fork();
        }
        printf("This is process %d\n", getpid());
        break;
    }

That is, enclose the compound statement in this code snippet in braces. Then the label will not precede a declaration.