reset the file location using lseek() system call

Question

I tried to use the system call lseek() to get back the beginning of a file or reach the end of the file.

The exact code I used is:

int location = lseek(fd, 0, SEEK_SET) //get back to the beginning
int location = lseek(fd, 0, SEEK_END) //reach to the end

However, after the file location has been reset, whenever I tried to use read(), the return value of read() is always set to -1, which means something was wrong. Furthermore, the errno message I got was Bad file descriptor. Does anyone know what should I do?

PS: I tried to close and reopen the file to help me get back to the beginning of the file and it worked. But I have no ideas on how should I get to the end of the file and read the entire file in the reverse order without using lseek().

Plus: a reproducible example would be:

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

int main(void)
{
    int fd;
    char buffer[1000];

    fd = creat("newFile", 0777);

    memset(buffer, 'a', 500);
    write(fd, buffer, 500); // fill up

    int location = lseek(fd, 0, SEEK_SET); //get back to the beginning

    int read_bytes = read(fd, buffer, 500);
    // this should return the bytes it reads but it actually returns -1
    printf("%d\n", read_bytes);
    return 0;
}

Please show a full compilable example, the problem cannot be explained from those two lines of code — Ctx, Sep 02 '19 at 22:37
@Fred The reproducible example *must have a `main` function and no syntax errors*. That means a *complete program*. — S.S. Anne, Sep 02 '19 at 22:50
the return value of lseek() seems good. It's the new file location as it should be. — Fred, Sep 02 '19 at 22:51
@Fred OK, here's a minimal reproducible example for you. Next time, do it yourself. — S.S. Anne, Sep 02 '19 at 23:17

S.S. Anne · Accepted Answer · 2019-09-02T23:09:19.730

5

The creat function does not allow you to read from the file. It only allows you to write to it.

From creat(2):

creat()
A call to creat() is equivalent to calling open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.

The important part here is O_WRONLY. That means "write only".

If you want to open the file (and create it) for reading and writing, then you can use open like so:

int fd = open("newFile", O_CREAT|O_RDWR|O_TRUNC, 0777);

The important part here is O_RDWR. That means "read and write".
If you want to have open give an error if the file already exists, add the O_EXCL flag; this causes -1 to be returned and errno to be set to EEXIST if the file already exists when you try to create it.

edited Sep 02 '19 at 23:09

answered Sep 02 '19 at 22:47

S.S. Anne

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So after I created a file and wrote something into it, I should use open() before I can use read()? But I thought the second parameter, which I specified to be 0777, already gives me the right to read, write and execute. – Fred Sep 02 '19 at 22:55
1

@Fred You can open it for reading and writing in the first place by using `open()` and the correct flags – Ctx Sep 02 '19 at 22:55
1

@Fred Those are just the permissions on the file. You still have to `open` the file with the adequate permissions. – S.S. Anne Sep 02 '19 at 22:59

score 0 · Answer 2 · edited Sep 08 '19 at 13:58

0

The following proposed code:

cleanly compiles
properly declares the variable types
properly creates the file
properly includes the needed header files
properly checks for error indications from C library functions (and properly handles any error)

And now, the proposed code:

#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <string.h>
#include <unistd.h>

int main(void)
{
    int fd;
    char buffer[1000];

    // fd = creat("newFile", 0777);
    fd = open("newFile", O_CREAT|O_RDWR|O_TRUNC, 0777);
    if( fd < 0 )
    {
        perror( "open failed" );
        exit( EXIT_FAILURE );
    }

    memset(buffer, 'a', 500);
    write(fd, buffer, 500); // fill up

    off_t location = lseek(fd, 0, SEEK_SET); //get back to the beginning

    printf( "%ld\n", location );

    ssize_t read_bytes = read(fd, buffer, 500);

    if( read_bytes < 0 )
    {
        perror( "read failed" );
        exit( EXIT_FAILURE );
    }


    // this should return the bytes it reads but it actually returns -1
    printf("%ld\n", read_bytes);

    return 0;
}

A run of the program results in:

0
500

Suggest reading/understanding the MAN pages for any C library functions your code uses

edited Sep 08 '19 at 13:58

S.S. Anne

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answered Sep 04 '19 at 02:51

user3629249

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*BZZT* wrong: `__off_t location`. Just define `_POSIX_C_SOURCE` to an appropriate value and use `off_t`. – S.S. Anne Sep 04 '19 at 10:56
@JL2210, Per the MAN page, the returned type from `lseek()` is `__off_t` And the posted code runs with no warnings, errors – user3629249 Sep 04 '19 at 23:39
Then there's something wrong with that man page. `__off_t` is glibc-specific. – S.S. Anne Sep 04 '19 at 23:39
Here's the definition from the Linux man-page: `off_t lseek(int fd, off_t offset, int whence);` – S.S. Anne Sep 04 '19 at 23:39
Also, man is short for manual. There's no need to make it all caps. – S.S. Anne Sep 04 '19 at 23:40
@JL2210, regarding MAN That is just so it stands out in the comments The command `manual` results in a message: `manual: command not found` – user3629249 Sep 04 '19 at 23:46
in the end, both `__off_t` and `off_t` cleanly compile, so it makes no difference – user3629249 Sep 04 '19 at 23:52
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`__off_t` is not portable, so I suggest `off_t`. – S.S. Anne Sep 05 '19 at 00:11

reset the file location using lseek() system call

2 Answers2