$ cat double-close.c

#include <fcntl.h>
#include <unistd.h>
void test()
{
  int fd = open("test.txt", O_RDONLY);
  close(fd);
  close(fd);
}

int main() {}

$ gcc-11.2.0 double-close.c -fanalyzer

double-close.c: In function ‘test’:
double-close.c:7:3: warning: double close on fd [-Wanalyzer-double-fclose]
    7 |   close(fd);
      |   ^~~~~~~~~
  ‘test’: events 1-3
    |
    |    5 |   int fd = open("test.txt", O_RDONLY);
    |      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |            |
    |      |            (1) opened here
    |    6 |   close(fd);
    |      |   ~~~~~~~~~ 
    |      |   |
    |      |   (2) first ‘close’ here
    |    7 |   close(fd);
    |      |   ~~~~~~~~~ 
    |      |   |
    |      |   (3) second ‘close’ was here; first ‘close’ was at (2)
    |

$ cat dup.c

#include <fcntl.h>
#include <unistd.h>

void test() {
	int fd = open("test.txt", O_RDONLY);
	close(fd);
	int fd2 = dup(fd);
}
int main(){}

$ gcc-11.2.0 dup.c -fanalyzer

dup.c: In function ‘test’:
dup.c:7:19: warning: dup on closed fd [-Wanalyzer-double-fclose]
    7 |         int fd2 = dup(fd);
      |                   ^~~~~~~
  ‘test’: events 1-3
    |
    |    5 |         int fd = open("test.txt", O_RDONLY);
    |      |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                  |
    |      |                  (1) opened here
    |    6 |         close(fd);
    |      |         ~~~~~~~~~ 
    |      |         |
    |      |         (2) first ‘close’ here
    |    7 |         int fd2 = dup(fd);
    |      |                   ~~~~~~~
    |      |                   |
    |      |                   (3) dup on closed file-descriptor ‘fd’ was called here which was closed at (2)
    |

$ cat read-write.c

#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>

void test ()
{
	int fd = open ("text", O_RDONLY);
	close(fd);
	write(fd, "hello", 5); //write in closed fd
	char * s = (char *) malloc(2);
	read(fd, s, 2); //read on closed fd
	free(s);
}

void main(){}

$ gcc-11.2.0 read-write.c -fanalyzer

read-write.c: In function ‘test’:
read-write.c:9:9: warning: write on closed fd [-Wanalyzer-double-fclose]
    9 |         write(fd, "hello", 5); //write in closed fd
      |         ^~~~~~~~~~~~~~~~~~~~~
  ‘test’: events 1-3
    |
    |    7 |         int fd = open ("text", O_RDONLY);
    |      |                  ^~~~~~~~~~~~~~~~~~~~~~~
    |      |                  |
    |      |                  (1) opened here
    |    8 |         close(fd);
    |      |         ~~~~~~~~~ 
    |      |         |
    |      |         (2) first ‘close’ here
    |    9 |         write(fd, "hello", 5); //write in closed fd
    |      |         ~~~~~~~~~~~~~~~~~~~~~
    |      |         |
    |      |         (3) write on closed file-descriptor ‘fd’ was called here which was closed at (2)
    |
read-write.c:11:9: warning: read on closed fd [-Wanalyzer-double-fclose]
   11 |         read(fd, s, 2); //read on closed fd
      |         ^~~~~~~~~~~~~~
  ‘test’: events 1-3
    |
    |    7 |         int fd = open ("text", O_RDONLY);
    |      |                  ^~~~~~~~~~~~~~~~~~~~~~~
    |      |                  |
    |      |                  (1) opened here
    |    8 |         close(fd);
    |      |         ~~~~~~~~~ 
    |      |         |
    |      |         (2) first ‘close’ here
    |......
    |   11 |         read(fd, s, 2); //read on closed fd
    |      |         ~~~~~~~~~~~~~~
    |      |         |
    |      |         (3) read on closed file-descriptor ‘fd’ was called here which was closed at (2)
    |