how to rewind to file beginning immediately after writing...

[Rajorshi Biswas]

Hi all,
I'm aware that this might not be a "C-specific" question, if so,
please let me know which group is the most appropriate for this kind of
question.

This is a question on unix filehandling in C, all I have to do is
rewind to the beginning of the file immediately after "write"ing some
bytes. My code looks something like this -
/* create a file and write something into it */
int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);
char b[100] = "this is some stupid text!\n";
write(fd, b, sizeof(b));

p = lseek(fd, 0, SEEK_SET);
printf("lseek returned = %d\n",p); /* returns 0 */

/* char block[BUFSIZ]; */
/* char tmp[] = "/tmp/fileXXXXXX"; */

newfd = mkstemp(tmp);

//copy contents of fd to newfd
while((nread = read(fd, block, sizeof(block))) 0) /* READ FAILS
HERE <<< */
write(newfd, block, nread);

I am clueless as to why after a write to a new file, and an lseek to
position zero, a subsequent read fails. Even if I do an fsync() after
the write, it fails. I'm on RHEL3.
Thanks in advance.
Raj

[Rajorshi Biswas]

Sorry folks, fixed it - a simple case of changing O_WRONLY to O_RDWR ..
duh :(
Rajorshi Biswas wrote:

Hi all,
I'm aware that this might not be a "C-specific" question, if so,
please let me know which group is the most appropriate for this kind of
question.

This is a question on unix filehandling in C, all I have to do is
rewind to the beginning of the file immediately after "write"ing some
bytes. My code looks something like this -
/* create a file and write something into it */
int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);
char b[100] = "this is some stupid text!\n";
write(fd, b, sizeof(b));

p = lseek(fd, 0, SEEK_SET);
printf("lseek returned = %d\n",p); /* returns 0 */

/* char block[BUFSIZ]; */
/* char tmp[] = "/tmp/fileXXXXXX"; */

newfd = mkstemp(tmp);

//copy contents of fd to newfd
while((nread = read(fd, block, sizeof(block))) 0) /* READ FAILS
HERE <<< */
write(newfd, block, nread);

I am clueless as to why after a write to a new file, and an lseek to
position zero, a subsequent read fails. Even if I do an fsync() after
the write, it fails. I'm on RHEL3.
Thanks in advance.
Raj

[Flash Gordon]

Rajorshi Biswas wrote:

Hi all,
I'm aware that this might not be a "C-specific" question, if so,
please let me know which group is the most appropriate for this kind of
question.

This is a question on unix filehandling in C, all I have to do is
rewind to the beginning of the file immediately after "write"ing some
bytes. My code looks something like this -

/* create a file and write something into it */
int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);

open and friends are not part of standard C so you would do better
asking on comp.unix.programmer, however I would point out that if
O_WRONLY has the obvious meaning of opening the file for writing only
you will not be able to read from it.

<snip>

while((nread = read(fd, block, sizeof(block))) 0) /* READ FAILS
HERE <<< */

<snip>

If you want further information on using open and friends, please ask in
comp.unix.programmer
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

[Rajorshi Biswas]

Thanks for pointing me to the appropriate group, Gordon!

~Rajorshi

Flash Gordon wrote:

Rajorshi Biswas wrote:

Hi all,
I'm aware that this might not be a "C-specific" question, if so,
please let me know which group is the most appropriate for this kind of
question.

This is a question on unix filehandling in C, all I have to do is
rewind to the beginning of the file immediately after "write"ing some
bytes. My code looks something like this -

/* create a file and write something into it */
int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);

open and friends are not part of standard C so you would do better
asking on comp.unix.programmer, however I would point out that if
O_WRONLY has the obvious meaning of opening the file for writing only
you will not be able to read from it.

<snip>

while((nread = read(fd, block, sizeof(block))) 0) /* READ FAILS
HERE <<< */

<snip>

If you want further information on using open and friends, please ask in
comp.unix.programmer
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc