Question

The following code is expected to write “some text” to demo.txt, but it doesn’t work:

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

int main(int argc, char *argv[])
{
    FILE *fp;
    int fd;

    if ((fd = open("demo.txt", O_RDWR)) == -1) {
        perror("open");
        exit(1);
    }
    fp = fdopen(fd, "w");
    fprintf(fp, "some text\n");
    close(fd);
    return 0;
}

Answer 1

You should use fflush(fp) to clear the buffer before closing the file.

When you write to the file descriptor fp, the data is buffered.
But you are closing the file with close(fd) before the buffered data can be written written to the file demo.txt. Hence, buffered data is lost.
If you do fflush(fp), it will ensure the buffered data is written to demo.txt immediately.

You should not call close() before doing fclose() for all the opened files.

Proper way is to do fclose(fp) first and then do close(fd).