System calls in C (File Descriptor) - c

follwing code has written to open a file and write data to terminal using sysyem calls in linux.
To read the value of the file descriptor (fd) it should assign a value. As we know in if else statement, from if part else part or else if part one part will implement at a time. So according to following code fd will have a value only at else if line. But when I pass a file name and run this program it opens the file. File opening is happen in while loop from read(() system call. But while loop is in else part and since file descriptor can't have any value theoretically. So how does the read function get recognize the file exactly? This is confusing me.
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#define SIZE 10
int main(int argc, char *argv[])
{
int fd,n;
char buff[SIZE];
if(argc != 2)
{
printf("USAGE : %s\n",argv[0] );
exit(1);
}
else if ((fd = open(argv[1],0)) == -1)
{
perror("STATUS");
exit(1);
}
else
{
while((n = read(fd,buff,SIZE)) > 0)
{
write(1,buff,SIZE);
}
close(fd);
}
}

Following happens here:
Let's suppose the program is started with xyz.txt on the command line and let's suppose the xyz.txt file does exist:
if(argc != 2)
{
// we don't get here because argc == 2
printf("USAGE : %s\n",argv[0] );
exit(1);
}
else if ((fd = open(argv[1],0)) == -1) // the statement in the if clause will therefore
// be executed, fd will be something different
// from -1 because open succeeded
{
perror("STATUS"); // therefore we dont ge here either
exit(1);
}
else
{ // instead we get here and
while((n = read(fd,buff,SIZE)) > 0) // everything works as expected
{
write(1,buff,SIZE);
}
close(fd);
}

Related

C - Declare a file in function parameters

so here is my problem :
int isopen()
{
int fd;
fd = open("myfile", O_RDONLY);
if (fd == 0)
printf("file opening error");
if (fd > 0)
printf("file opening success");
return(0);
}
int main(void)
{
isopen();
return(0);
}
Is use this code to check if this the open command worked, as i'm just starting to lurn how to use it.
Basically this code is working just fine, but I would like to declare the file I would like to open directly in the parameters of my function isopen.
I saw some other posts using main's argc and argv, but I really need to declare my file in the parameters of my function isopen, not using argc & argv.
Is it even possible ?
Thank you for your help, I'm quite lost here.
Your question is unclear, but maybe you want this:
int isopen(const char *filename)
{
int fd;
fd = open(filename, O_RDONLY);
if (fd < 0) //BTW <<<<<<<<<<<< fd < 0 here !!
printf("file opening error");
else // else here
printf("file opening success");
return(0);
}
int main(void)
{
isopen("myfile");
return(0);
}
BTW, the isopen function as it stands here is still pretty useless as it just opens the file and throwing away fd.
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int isOpen(char *filename)
{
return open(filename, O_RDONLY);
}
int main()
{
printf("%d\n", isOpen("/home/viswesn/file1.txt"));
printf("%d\n", isOpen("file2.txt"));
return 0;
}
Output
viswesn#viswesn:~$ cat /home/viswesn/file1.txt
hello
viswesn#viswesn:~$
viswesn#viswesn:~$ cat /home/viswesn/file2.txt
cat: /home/viswesn/file2.txt: No such file or directory
viswesn#viswesn:~$
viswesn#viswesn:~$ ./a.out
3 <---------- File exist and it give file descriptor number '3'
STDIN-0, STDOUT-1, STDERR-2 are reserved and
next file opened will start with 3 and it keeps going
-1 <--------- File not found; so open gives -1 as error

Why is stat() returning EFAULT?

I'm writing a program that when run from two separate bash sessions as two separate processes, opens a named pipe between the two to allow strings to be sent from one to the other.
When the process is first executed from one terminal, it checks stat(fname, buf) == -1 to see if a file at path fname exists and if not, creates it. The process then assumes that since it was the one to make the FIFO, it is the one that will be sending messages through it and continues accordingly.
After that occurs, the program can then be run from another terminal that should determine that it will be the receiver of messages through the pipe by checking stat(fname, buf) == -1. The condition should return false now, and stat(fname, buf) itself should return 0 because there exists a file at fname now.
But for reasons I am unable to discern, when the second process is run, stat(fname, buf) still returns -1. The variable errno is set to EFAULT. The man page for stat() only decribes EFAULT as "Bad address." Any help determining why the error occurs or what is meant by "Bad address." would be greaty appreciated.
I've verified that the file is indeed created by the first process as intended. The first process waits at the line pipe = open(fname, O_WRONLY); because it can't continue until the other end of pipe is opened.
Edit: The following is a self-contained implementation of my code. I have confirmed that it compiles and experiences the problem I described here.
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#define MAX_LINE 80
#define oops(m,x) { perror(m); exit(x); }
int main(int argc, char const *argv[]) {
char line[MAX_LINE];
int pipe, pitcher, catcher, initPitcher, quit;
struct stat* buf;
char* fname = "/tmp/absFIFOO";
initPitcher = catcher = pitcher = quit = 0;
while (!quit) {
if (((!pitcher && !catcher && stat(fname, buf) == -1) || pitcher) && !quit) {
// Then file does not exist
if (errno == ENOENT) {
// printf("We're in the file does not exist part\n");
if (!pitcher && !catcher) {
// Then this must be the first time we're running the program. This process will take care of the unlink().
initPitcher = 1;
int stat;
if (stat = mkfifo(fname, 0600) < 0)
oops("Cannot make FIFO", stat);
}
pitcher = 1;
// open a named pipe
pipe = open(fname, O_WRONLY);
printf("Enter line: ");
fgets(line, MAX_LINE, stdin);
if (!strcmp(line, "quit\n")) {
quit = 1;
}
// actually write out the data and close the pipe
write(pipe, line, strlen(line));
close(pipe);
}
} else if (((!pitcher && !catcher) || catcher) && !quit) {
// The first condition is just a check to see if this is the first time we've run the program. We could check if stat(...) == 0, but that would be unnecessary
catcher = 1;
pipe = open("/tmp/absFIFO", O_RDONLY);
// set the mode to blocking (note '~')
int flags;
flags &= ~O_NONBLOCK;
fcntl(pipe, F_SETFL, flags); //what does this do?
// read the data from the pipe
read(pipe, line, MAX_LINE);
if (!strcmp(line, "quit\n")) {
quit = 1;
}
printf("Received line: %s\n", line);
// close the pipe
close(pipe);
}
}
if (initPitcher)
unlink(fname);
return 0;
}
You have this piece of code:
struct stat* buf;
...
if (((!pitcher && !catcher && stat(fname, buf) == -1)
When you call stat(), buf isn't initalized and there's no telling what it points to.
You must allocate some storage for it, so stat() has a valid place to store the result.
The easiest thing is to just allocate it on the stack:
struct stat buf;
...
if (((!pitcher && !catcher && stat(fname, &buf) == -1)
You have not shown your code, but EFAULT means 'bad address'. This indicates that you have not properly allocated (or passed) your buffer for stat or the filename (fname).
buf isn't initialised anywhere. What exactly do you expect to happen?

beginner in C under linux. write function doesn't work properly

I'm trying to write a C program, that make user able to write stuff in a file. My Problem is that after making and running the program the file stay empty ?? any idea how can I solve this.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
// the user should give a file to write the file
int main (int argc , char**argv)
{
int fd; // file descriptor
char ret; // the character
int offset;
if(argc != 2) {
printf("You have to give the name or the path of the file to work with \n");
printf("Exiting the program \n")
return -1;
}
fd = open (argv[1], O_WRONLY/*write*/|O_CREAT/*create if not found */, S_IRUSR|S_IWUSR/*user can read and write*/);
if (fd == -1) {
printf("can'T open the file ");
return -1;
}
printf("At wich position you want to start ");
scanf("%d",&offset);
lseek(fd,offset,SEEK_SET);
while(1) {
ret = getchar();
if(ret == '1') {
printf("closing the file");
close (fd);
return 1;
}
else
write (fd,red, sizeof(char));
}
return 0;
}
thanks in advance for you help.
I have made some changes,this should work:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
int main (int argc , char**argv)
{
int fd; // file descriptor
char ret; // the character
int offset;
if(argc != 2){
printf("You have to give the name or the path of the file to work with \n");
printf("Exiting the program \n"); **//There was ';' missing here**
return -1;
}
fd = open (argv[1], O_WRONLY|O_CREAT,S_IRUSR|S_IWUSR);
if (fd == -1) {
printf("can'T open the file ");
return -1;
}
printf("At wich position you want to start ");
scanf("%d",&offset);
lseek(fd,offset,SEEK_SET);
while(1){
ret = getchar();
if(ret == '1'){
printf("closing the file");
close (fd);
return 1;
}
else
write (fd,&ret, sizeof(char)); **//red has been changed to &ret**
}
return 0;
}
One error I can notice, the call of write function:
write (fd,red, sizeof(char));
should be:
write (fd, &red, sizeof(char));
You forgot & before red, write need address.
syntax of write: int write( int handle, void *buffer, int nbyte );
This will cause an undefined behavior in your code at run time
Edit: in write function you are using red that is not defined, I think it should be ret variable in your code. correct it as write (fd, &ret, sizeof(char));
second, you forgot ; after printf("Exiting the program \n") in if, but I also think its mistake while posting question as you says you are getting run time error.
side note: If you are using gcc compiler then you can use gcc -Wall -pedantic to generate warnings
It should be:
write (fd,&ret, sizeof(char));
write takes the pointer to the memory position, and since ret is a single char, you need to pass a pointer to it.

Detecting file that isn't there

So this is one of the first programs I've ever used some self created error checks however for some reason when I compile this and run it using:
./file test1.txt test2.txt 10
I get absolutely an error suggesting that the output file exists and I've checked the file and it doesn't even when I change the name of the output file (second argument) I get nothing. Anyone who can help? I've been racking my brain for ages now. This is a UNIX homework assignment I'm compiling and running in Gentoo. I have it running in a VB and have a linked folder between my windows and linux OS's.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#define BUFFT 25
int main (int argc, char *argv[])
{
int count;
int readin;
int writeout;
printf ("This program was called \"%s\".\n",argv[0]);
if (argc > 1)
{
for (count = 1; count < argc; count++)
{
printf("argv[%d] = %s\n", count, argv[count]);
}
}
else
{
perror("The command had no arguments.\n");
exit(-1);
}
// check correct number of arguments parsed //
if (argc == 4)
{
printf("There are the correct number of arguments(4)\n");
}
else
{
perror("Not enough arguments! please try again \n");
exit(-1);
}
//Check original file is there//
int openFD = open(argv[1], O_RDWR);
if (openFD <0)
{
perror("Error unable to read file \n");
exit(-1);
}
//Check existence of output file, if it doesn't exist create it//
int CheckFile = open(argv[2], O_RDONLY);
if (CheckFile < 0)
{
perror("Error output file already exists \n");
exit(-1);
}
else
{
int CheckFile = open(argv[2], O_CREAT);
printf("The file has successfully been created \n");
}
//Create buffer
int bufsize = atoi(argv[3]);
char *calbuf;
calbuf = calloc(bufsize, sizeof(char));
//Read text from original file and print to output//
readin = read(openFD, calbuf, BUFFT);
if (readin < 0){
perror("File read error");
exit(-1);
}
writeout = write(openFD,bufsize,readin);
if (writeout <0){
perror("File write error");
exit(-1);
}
return 0;
}
The open call for HANDLE CheckFile is printing Error File Exists. This is your problem. You are printing out the wrong statement when Output File Is Not Found and moreover you are exiting which prevents the code to create any.
int CheckFile = open(argv[2], O_RDONLY);
if (CheckFile < 0)
{
//Means the file doesn't exist
int CheckFile = open(argv[2], O_CREAT);
// Check for errors here
}
And why are you trying to do this::
writeout = write(openFD,bufsize,readin);
when your HANDLE TO OUTPUT FILE IS CheckFile
int CheckFile = open(argv[2], O_RDONLY);
if (CheckFile < 0)
{
perror("Error output file already exists \n");
A negative return from open means that the file could not be opened, most likely because it does not exist ... it does not mean that the file already exists. Failing to open the input file certainly does not mean that the output file already exists. Please check your code more carefully for obvious errors, e.g.,
int CheckFile = open(argv[2], O_CREAT);
printf("The file has successfully been created \n");
Here you don't check the return code.
Take a look at this fragment of your code:
int CheckFile = open(argv[2], O_RDONLY);
if (CheckFile < 0)
{
perror("Error output file already exists \n");
exit(-1);
}
You are saying trying to open a file in READ ONLY MODE. From what I read in your question, it's not an error if the file doesn't exist, but in the code you are validating the opposite, if the file doesn't exists, throw an error (in fact your message error is incorrect here).
Double check your logic and you will find your solution.

lseek function problem in a copy file program!

Got to use lseek function in this program below... Program is simply copying file (that already exist). I wanned to copy the existing file with the chars from the end of file
for example: Sorce_File.txt contains:"1 2 3" after copy Target_File.txt contains:"3 2 1"
I'm pretty sure it's simple problem but couldn't find out since 2 days how to do it
#include <fcntl.h>
#include <stdio.h>
#define MAX 512
int main(int argc, char* argv[]){
char buf[MAX];
int desc_sorc, desc_targ;
int lbajt;
if (argc<3){
argv[0];
exit(1);
}
desc_sorc = open(argv[1], O_RDONLY);
if (desc_sorc == -1){
}
desc_targ = creat(argv[2], 0640);
if (desc_targ == -1){
exit(1);
}
while((lbajt = read(desc_sorc, buf, MAX)) > 0){
if (lbajt == -1) {
perror("position error");
exit(1);}
if (write(desc_targ, buf, lbajt) == -1)
{
exit(1);
}
}
if (lbajt == -1){
exit(1);
}
if (close(desc_sorc) == -1 || close(desc_targ) == -1){
exit(1);
}
exit(0);
}
int desc_sorc, desc_targ;
You don't actually initialize these to anything. Anywhere.
EDIT: Now that you've fixed that, have you actually tested it again?
You are missing the equivalent of strrev(...) in there to reverse the string you write out as well starting from the end of the source file and reading backwards or writing from the end of the target file back to the beginning.
The actual implementation is left as an exercise to the reader.

Resources