How to get flags of opened fd in C?

I want to get flags of fd was opened before in C.

But I use fcntl(fd,F_GETFD,0) reference by fcntl man page, it always return 1 to me.

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

#define XSZ(x) (int)(sizeof(x)*2)

int main()
{
    int ret;
    static const char str [] = "hello c program!\n";
    int fd = open("test.txt", O_RDWR | O_APPEND | O_CREAT, 0777);
    if(fd < 0)
    {
        perror("open");
        return -1;
    }
    printf("fd = %d\n", fd);

    ret = fcntl(fd, F_GETFD, 0);
    printf("flag:%d\n",ret);
    write(fd, str, strlen(str)-1);

    return 0;
}

It always print:

fd = 3
flag:1

What I thought the ret is the sum of O_RDWR | O_APPEND | O_CREAT

Solution

You should use F_GETFL instead of F_GETFD. Also remember to print in octal to compare.

One important thing is not all flags are returned by fcntl. Only the access mode flags are remembered, like O_RDWR. However O_CREATE/O_TRUNC etc. are operating mode or open-time flags which are not remembered by the system and hence not returned.

Here is your modified code

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

#define XSZ(x) (int)(sizeof(x)*2)

int main()
{
    int ret;
    int fd = open("test.txt", O_RDWR | O_APPEND | O_CREAT, 0777);
    if(fd < 0)
    {
        perror("open");
        return -1;
    }
    printf("fd = %d\n", fd);
    ret = fcntl(fd, F_GETFL);
    perror("fcntl");
    printf("flag:%o\n",ret);
    printf("flag:%o\n",O_RDWR|O_APPEND);
    write(fd, "hello c program\n", strlen("hello c program!\n"));

    return 0;
}

here is the output of the above code

fd = 3
fcntl: Success
flag:102002
flag:2002