Using Open, Write and Close System Calls in C

Question

I'm having problems with very basic use of Open, Write and Close system calls in C, the following code is compiling but showing 0 bytes written to the file and no content being output into the file. What am I doing wrong?!

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

int main(void)
{
    int fd;
    ssize_t numWritten;

    if(fd = open("test.txt", O_RDWR | O_CREAT | O_APPEND) == - 1)
        printf("There has been an error opening the file\n");
    else
    {
        if(numWritten = write(fd, "Test Text\n", 10) == -1)
            printf("There has been an error writing to the file!\n");
        else
            printf("You have written %ld bytes\n", (long)numWritten);
    }

    close(fd);

    return 0;
}

Also if `open` failed the file descriptor is invalid, so you shouldn't try to `close` it (move `close` into the `else` block). — Antti Haapala -- Слава Україні, Nov 15 '17 at 22:14
What is the boolean result of 'open("test.txt", O_RDWR | O_CREAT | O_APPEND) == - 1' likely to be, and why are you assigning it to an fd? — Martin James, Nov 15 '17 at 22:17
"If the file does not exist, create it. If the O_CREAT option is used, then you must include the third parameter." See http://codewiki.wikidot.com/c:system-calls:open — Richard Chambers, Nov 15 '17 at 22:18
Ah and naturally you have the parentheses missing in num_written too — Antti Haapala -- Слава Україні, Nov 15 '17 at 22:19
@RichardChambers yes it is. If there are *more* duplicates, point them out and I will add them too — Antti Haapala -- Слава Україні, Nov 15 '17 at 22:20
@MartinJames `open("test.txt", O_RDWR | O_CREAT | O_APPEND) == - 1` will evaluate to `0`. So the string is written to standard *input* which is opened to terminal. Therefore the string is displayed there, because even `stdin` is opened in read mode. — Antti Haapala -- Слава Україні, Nov 15 '17 at 22:27
Owait - the file opens OK, the compare is false and so fd gets loaded with 0, so that the write is to stdin.. :( — Martin James, Nov 15 '17 at 22:28
yes. OP just misses two sets of parentheses and the mode argument, for example `0644`. — Antti Haapala -- Слава Україні, Nov 15 '17 at 22:28
To answer the "What am I doing wrong?!" part, Add a parenthesis pair in your if-write line and everything works fine. from: if(numWritten = write(fd, "Test Text\n", 10) == -1) to: if( (numWritten = write(fd, "Test Text\n", 10)) == -1) — Neilvert Noval, Nov 15 '17 at 22:33

Using Open, Write and Close System Calls in C

0 Answers0