Context for this is that the program is basically reading through a filestream, 4K chunks at a time, looking for a certain pattern. It starts by reading in 4k, and if doesn't find the pattern there, it starts a loop which reads in the next 4k chunk (rinse and repeat until EOF or pattern is found).
On many files the code is working properly, but some files are getting errors.
The code below is obviously highly redacted, which I know might be annoying, but it includes ALL lines that reference the file descriptor or the file itself. I know you don't want to take my word for it, since I'm the one with the problem...
Having done a LITTLE homework before crying for help, I've found:
The file descriptor happens to always = 6 (it's also 6 for the files that are working), and that number isn't getting changed through the life of execution. Don't know if that's useful info or not.
By inserting print statements after every operation that accesses the file descriptor, I've also found that successful files go through the following cycle "open-read-close-close" (i.e. the pattern was found in the first 4K)
Unsuccessful files go "open-read-read ERROR (Bad File Descriptor)-close." So no premature close, and it's getting in the first read successfully, but the second read causes the Bad File Descriptor error.
.
int function(char *file)
{
int len, fd, go = 0;
char buf[4096];
if((fd = open(file, O_RDONLY)) <= 0)
{
my_error("Error opening file %s: %s", file, strerror(errno));
return NULL;
}
//first read
if((len = read(fd, buf, 4096)) <= 0)
{
my_error("Error reading from file %s: %s", file, strerror(errno));
close(fd); return NULL;
}
//pattern-searching
if(/*conditions*/)
{
/* we found it, no need to keep looking*/
close(fd);
}
else
{
//reading loop
while(!go)
{
if(/*conditions*/)
{
my_error("cannot locate pattern in file %s", file);
close(fd); return NULL;
}
//next read
if((len = read(fd, buf, 4096)) <= 0) /**** FAILS HERE *****/
{
my_error("Error reading from file, possible bad message %s: %s",
file, strerror(errno));
close(fd); return NULL;
}
if(/*conditions*/)
{
close(fd);
break;
}
//pattern searching
if(/*conditions*/)
{
/* found the pattern */
go++; //break us out of the while loop
//stuff
close(fd);
}
else
{
//stuff, and we will loop again for the next chunk
}
} /*end while loop*/
}/*end else statement*/
close(fd);
}
.
Try not to worry about the pattern-reading logic - all operations are done on the char buffer, not on the file, so it ought to have no impact on this problem.
EOF returns 0 (falls into if ... <= 0), but does not set errno, which may have an out of date code in it.
Try testing for 0 and negative (error, -1) values seperately.
Regarding "strace": I've used it a little at home, and in previous jobs. Unfortunately, it's not installed in my current work environment. It is a useful tool, when it's available. Here, I took the "let's read the fine manual" (man read) approach with the questioner :-)
Related
EDIT 3
THE FILE CONTAINS BYTES - I guess I have to sort the bytes, the task doesn't say more - it says that I pass an argument - the name of a binary file that contains bytes - that's it. And I am trying to work with low-level funcs.
I am trying to sort a binary file using qsort but I got stuck - I dont know how to write the content of a file to a buffer so I could pass it to qsort
What I have done:
int main(int argc, char*argv[]){
int fd1;
if((fd1=open(argv[1], O_RDONLY))==-1){
printf("Error occurred while opening the file");
exit(-1);
}
int size;
char c;
while(read(fd1, &c, 1)){
size=size+1;
}
size=size+1;
close(fd1);
fd1=open(argv[1], O_RDONLY);
if(fd1==-1){
printf("Error occured while opening the file");
}
char*buffer;
buffer=malloc(size);
setbuf(fd1, buffer);
//EDIT I TRIED THIS AND IT STILL DOES NOT WORK
int i=0;
while(read(fd1, &c, 1)){
buffer[i]=c;
i++;
}
for(int i=0; i<size;i++){
printf("lele %s", buffer[i]);
}
//EDIT 2: after making buffer[i]=c I get this error Segmentation fault
}
SetBuf does not work this way.. How to make it work? Also, I am trying to use func like open, close, read, write, etc.
Your algorithm for reading a file into a buffer is good:
Open the file
Count bytes in file
Close the file
Allocate the buffer
Open the file
Read the file
Close the file
A bit inefficient, because you read the file twice, but that's fine. You just have to implement it properly; any small mistake will make it look like it doesn't work. Use a debugger to check each step.
Here is my try. I didn't debug, to not deny you the "fun" of debugging. I put comments instead.
int main(int argc, char*argv[])
{
// 1. Open the file
int fd1;
if((fd1=open(argv[1], O_RDONLY))==-1){
printf("Error occurred while opening the file");
exit(-1);
}
// 2. Count bytes in file
int size = 0;
char c;
while(read(fd1, &c, 1))
size=size+1;
// To check that this part is good, print the size here!
// 3. Close the file
close(fd1);
// Allocate the buffer
char *buffer;
buffer = malloc(size);
// Might want to print the buffer here, to make sure it's not NULL
// 5. Open the file
fd1=open(argv[1], O_RDONLY);
if(fd1==-1){
printf("Error occurred while opening the file");
}
// 6. Read the file
for (int index = 0; index < size; ++index)
read(fd1, &buffer[index], 1);
// Might want to print what "read" returns in each iteration, to make sure it's successful
// 7. Close the file
close(fd1);
}
As noted by Eric Postpischil, the algorithm is actually not good.
The size of the file at one time does not guarantee the size at another time.
If you want to do that correctly, you must read the file only once. This will make the allocation harder: you cannot calculate the required buffer size, so you have to "guess" an initial size and use realloc.
However, in this small example, this is clearly not the requirement - you can probably ignore the possibility of the file changing asynchronously.
There is another possible problem - I/O error on the file when you read it the second time. This is easy to check, so maybe you should add it.
In the code below, I am trying to read from a socket and store the results in a file.
What actually happens, is that my client sends a GET request to my server for a file.html. My server finds the file and writes the contents of it to the socket. Lastly my client reads the content from thread_fd and recreates the file.
For some reason the recreated file has less content than the original. I have located the problem to be some lines in the end, that are missing. When I use printf("%s", buffer) inside the while loop everything seems fine in STDOUT but my fprintf misses somewhat 3.000 bytes for a file of 81.000 bytes size.
#define MAXSIZE 1000
int bytes_read, thread_fd;
char buffer[MAXSIZE];
FILE* new_file;
memset(buffer, 0, MAXSIZE);
if((new_file = fopen(path, "wb+")) == NULL)
{
printf("can not open file \n");
exit(EXIT_FAILURE);
}
while ((bytes_read = read(thread_fd, buffer, MAXSIZE)) > 0)
{
fprintf(new_file, "%s", buffer);
if(bytes_read < MAXSIZE)
break;
memset(buffer, 0, MAXSIZE);
}
You read binary data from the socket that may or may not contain a \0 byte. When you then fprintf that data the fprintf will stop at the first \0 it encounters. In your case that is 3000 bytes short of the full file. If your file contains no \0 byte the fprintf will simply continue printing the ram contents until it segfaults.
Use write() to write the data back to the file and check for errors. Don't forget to close() the file and check that for errors too.
Your code should/could look like:
int readfile(int thread_fd, char *path)
{
unsigned int bytes_read;
char buffer[MAXSIZE];
int new_file;
if ((new_file = open(path, _O_CREAT|_O_BINARY,_S_IWRITE)) == -1) return -1;
while ((bytes_read = read(thread_fd, buffer, MAXSIZE)) > 0)
{
if (write(new_file, buffer, bytes_read)!= bytes_read) {
close(new_file);
return -2;
}
}
close(new_file);
return 0;
}
There are a few issues with your code that can cause this.
The most likely cause is this :
if(bytes_read < MAXSIZE)
break;
This ends the loop when read returns less than the requested amount of bytes. This is however perfectly normal behavior, and can happen eg. when not enough bytes are available at the time of the read call (it's reading from a network socket after all). Just let the loop continue as long as read returns a value > 0 (assuming the socket is a blocking socket - if not, you'll also have to check for EAGAIN and EWOULDBLOCK).
Additionally, if the file you're receiving contains binary data, then it's not a good idea to use fprintf with "%s" to write to the target file. This will stop writing as soon as it finds a '\0' byte (which is not uncommon in binary data). Use fwrite instead.
Even if you're receiving text (suggested by the html file extension), it's still not a good idea to use fprintf with "%s", since the received data won't be '\0' terminated.
This worked!
ps: I don't know if I should be doing this, since I am new here, but really there is no reason for negativity. Any question is a good question. Just answer it if you know it. Do not judge it.
#define MAXSIZE 1000
int bytes_read, thread_fd, new_file;
char buffer[MAXSIZE];
memset(buffer, 0, MAXSIZE);
if((new_file = open(path, O_RDONLY | O_WRONLY | O_CREAT)) < 0)
{
printf("can not open file \n");
exit(EXIT_FAILURE);
}
while ((bytes_read = read(thread_fd, buffer, MAXSIZE)) > 0)
write(new_file, buffer, bytes_read);
close(new_file);
I have a utility that is supposed to optimize files by transforming them into an alternate file-format. If it cannot make the files smaller, I would like the original file returned.
The design is to use stdin in and stdout for input and output. This is for a case where the processed size is larger than the original file size. All other branches are tested as working.
char readbuffer[65536];
ssize_t readinbytes;
while ((readinbytes = fread(readbuffer, sizeof(char), insize, stdin)) > 0) {
if (fwrite(readbuffer, sizeof(char), readnbytes, stdout) != readnbytes) {
fatal("can't write to stdout, please smash and burn the computer\n");
}
}
Problem This is resulting in a file with size 0
Right this question has a strange answer. Essentially I had to read stdin into a buffer (inbuf), then output the contents of that buffer. The overarching reason I was getting no output was multi-faceted.
Firstly I'd failed to spot a branch which already determined if the input buffer was smaller than the output buffer
if((readinbytes < outbuffersize) || force) {
// inside this is where the code was...
It looks like (because stdout was being used to write to) there was a section that contained a log statement that was not output in the matching else block. The code inherited was terribly formatted so it was never picked up on.
As outputting error messages is not fulfilling the purpose of the utility (always output a valid output file if a valid input file is provided)
solution (stdin is read into inbuf at the start of the program)
set_filemode_binary(stdout);
if (fwrite(inbuf, 1, readinbytes, stdout) != insize) {
fprintf(stderr, "error writing to stdout\n");
free(inbuf);
exit(3);
}
errata (reading in stdin)
unsigned char * inbuf = NULL;
size_t readinbytes;
long insize = 0;
// elsewhere...
// die if no stdin
insize = getFileSize(stdin);
if (insize < 0) {
fprintf(stderr, "no input to stdin\n");
exit(2);
}
// read stdin to buffer
inbuf = createBuffer(insize); // wrapper around malloc handling OOM
if ((readinbytes = fread(inbuf, sizeof(char), insize, stdin)) < 0) {
fprintf(stderr, "error reading from stdin\n");
free(inbuf);
exit(3);
}
Also don't forget to free(inbuf).
if(inbuf){ free(inbuf); }
I Hope this helps someone.
sorry if this is a dumb question, but I'm kind of inexperienced with C. I'm trying to create a simple TCP client/server connection. This is how it works: the server stores several files, including .txt and .jpg. When the client wants one of them, he sends the name of the file to the server, reading and writing it to a certain location.
Everything works fine, except for the fact that the server's response is "RES status size data" . I'm only interested in the data to create the new file, but I'm not being able to deny the writing of RES status size into the output file. I tried to use strtok but am getting a segfault because of it, dunno why. This is the section of code where I receive the response from the server and start writing it to a new file fp.
while(success == 0)
{
while(f_block_sz = recvfrom(fd2, buffer, sizeof(buffer), 0, (struct sockaddr*)&serveraddr2, &addrlen))
{
if(f_block_sz < 0)
{
printf("Receive file error.\n");
break;
}
int write_sz = fwrite(buffer, sizeof(char), f_block_sz, fp);
if(write_sz < f_block_sz)
{
printf("File write failed.\n");
break;
}
bzero(buffer, LENGTH);
}
printf("ok!\n");
success = 1;
fclose(fp);
}
I don't even know what's best. Should I remove those "useless" words before writing to the newfile, or should I edit the file after it's finished?
Thanks in advance.
I not sure but i just thought if your "RES status size" is of fixed size, say 'x' bytes then while writing to the output file just skip x bytes of received data...
int write_sz = fwrite(buffer + x, sizeof(char), f_block_sz, fp);
I am not sure this is good practice or even correct, i just gave in my thought.
Please see below code.
#include < stdio.h >
#include < fcntl.h >
#include < stdlib.h >
#include < string.h >
int main(int argc,char **argv,char **envp)
{
int fd;
size_t sz;
char filebuffer[1024];
int loop;
fd=open("sample",O_RDONLY);
if(fd==-1)
{
perror("");
exit(1);
}
loop=0;
while(++loop<300)
{
lseek(fd,0,SEEK_SET);
memset(filebuffer,0,1024);
sz=read(fd,filebuffer,1024);
printf("%d.sz=%zd\t%s\n",loop,sz,filebuffer);
sleep(1);
}
close(fd);
return 0;
}
In this code, I am able to read file. But when I am changing file (reading file "sample") at the same time during reading. Then I am not able to read the changed file. I tried O_SYNC flag too. but still, it is not working, but O_DIRECT is undefined error is coming up. How can I ensure that I am able to read changes? Second thing, but I observed, if I close and open the file reading, then I am able to read changed file.
Question:
How can I read changed file without closing and opening?
I think that you're asking the following question:
I have a program which opens a file called sample and repeatedly reads the first block of that file. That works fine. However, if I edit the file sample, for example with a text editor, then my program does not see the changes, although it will if it closes and reopens the file. How can I see the changes without closing and reopening the file?
If that's your question, then the answer is:
Sorry, you cannot, because the text editor does not modify the file. It creates a new file with the old name.
In Unix, once you open a file, it will not actually get deleted, even if its name is unlinked. If another program "deletes" the file and then creates a new file with the same name, the file you have open is no longer accessible to any other program, but it is still the same file and it will not get deleted until you close it.
Most Unix text utilities, even the ones which claim to work "in-place" (such as sed -i) really do not modify files. That includes text editors. So your program doesn't see changes in the file because the file is not changing; the name has been given to a new file.
So the only way to deal with this is to close and reopen the file. When you reopen, you will be opening the new file with the old name.
The reason for not getting updated data in file could be sync time in filesystem.
I suggest fflush() after writing in to file. This makes your cache data to be written in file.
Related discussions.
Small file not committed to disk for over a minute
Is fwrite non-blocking?
This is an adaptation of your code. It forks to create two processes. The child contains your code, substantially unchanged (different error message, file name variable, and more care with printing the filebuffer which is not null terminated). The parent writes characters (the same character, over and over) to the file.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
static char const filename[] = "sample";
int main(void)
{
int fd;
size_t sz;
char filebuffer[1024];
int loop;
switch (fork())
{
case -1:
fprintf(stderr, "Failed to fork\n");
break;
case 0:
sleep(1);
fd = open(filename, O_RDONLY);
if (fd == -1)
{
fprintf(stderr, "Failed to open file %s for reading\n", filename);
exit(1);
}
loop = 0;
while (++loop < 300)
{
lseek(fd, 0, SEEK_SET);
memset(filebuffer, 0, 1024);
sz = read(fd, filebuffer, 1024);
printf("%d.sz=%zd\t%.*s\n", loop, sz, (int)sz, filebuffer);
sleep(1);
}
close(fd);
break;
default:
fd = open(filename, O_WRONLY|O_CREAT, 0644);
if (fd == -1)
{
fprintf(stderr, "Failed to create file %s for writing\n", filename);
exit(1);
}
for (loop = 0; loop < 256; loop++)
{
memset(filebuffer, (loop % 64) + 33, sizeof(filebuffer));
lseek(fd, 0L, SEEK_SET);
write(fd, filebuffer, sizeof(filebuffer));
sleep(1);
}
close(fd);
break;
}
return 0;
}
Example output:
1.sz=1024 """"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
2.sz=1024 ################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################
3.sz=1024 ################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################################
4.sz=1024 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
5.sz=1024 &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
6.sz=1024 ''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
7.sz=1024 ((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((
8.sz=1024 ((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((
9.sz=1024 ****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
As you can see, the processes run asynchronously, so the data in the file does not always change between reads, but the process is seeing the changes.
Try running this on your computer. It should work. (Sample output from Mac OS X 10.8.5.) If it doesn't, you'll need to identify which file system type you have, but I don't think it'll be a problem.