stat system call reports "No such file or directory" - c

I'm working on a homework assignment and I ran into a little snag.
I'm trying to read a filename from standard input and then stat the file to get the size (as per the assignment's requirements):
#define BUFFSIZE 4096
int
main(void) {
int n;
char buffer[BUFFSIZE];
struct stat buf;
while ((n = read(STDIN_FILENO, buffer, BUFFSIZE)) > 0) {
stat(buffer, &buf);
perror("stat");
}
}
Here's the output when ran (I entered the filename file):
file
stat: No such file or directory
But if I try something like this:
#define BUFFSIZE 4096
int
main(void) {
int n;
char buffer[BUFFSIZE] = "file";
struct stat buf;
stat(buffer, &buf);
perror("stat");
}
I get:
stat: Success
The file named file is in the directory that I'm running the program from.
How come there is a difference between reading in the string "file" and just initializing the array to the string "file"?

Before calling stat() print the value of buffer to standard output:
printf("[%s]\n", buffer);
It will not be what you expect as read() will not NULL terminate buffer for you.
Initialise buffer before read().
Not sure why you looping on the read() as you should acquire the complete name of the file before calling stat(). If you have not been forced to use read() consider using fgets().

Did you try printing the buffer? Most likely your read call returned a newline on the end of the string "file," and there is no file "file\n" in your directory. I would recommend using fgets instead to read the filename from the console. Use standard C input/output when you can, and only delegate to platform-specific code when there is a measurable benefit (e.g. there is no cross-platform stat function in the C standard library, and sometimes Unix I/O can measurably improve performance).

There is a '\n' in the buffer in the 1st snippet. Take it out
buffer[strlen(buffer) - 1] = 0;

The problem is that the read leaves the newline in the buffer, so you try to stat "file\n".

read() may be reading too much (or not enough) on the first loop. Try printing out what it's read just before the stat() call:
printf("Read %d characters (%s)\n",n,buf);
read() is probably a bit low level for this task - I'd recommend using scanf() instead.
while ( scanf ("%s",buffer) == 1) {
For safe code, you'll need to specify the maximum number of characters to read, which you can do like this:
while ( scanf ("%4096s",buffer) == 1) {
However, if you want to use the BUFFSIZE macro, you'll need to do a bit of mucking about:
#define XLIM_PERCENT_S(x) "%" #x "s"
#define LIM_PERCENT_S(x) XLIM_PERCENT_S(x)
....
while ( scanf ( LIM_PERCENT_S( BUFFSIZE ) , buffer) == 1) {

Related

Reading text from stdin (without fopen)

I'm trying to write some code to read my text file from stdin char by char, line by line, without using fopen. I need 42 lines with a maximum of 100 chars.
Help me to get this working and explain to me how it works. Thank you!
int main()
{
char str[100];
fgets(str, sizeof(str), stdin);
fputs(str, stdout);
return 0;}
without using fopen
If open is tolerated. Why not use it with read ? This solution will allow you to read char by char.
int main(void)
{
int file_descriptor = open("./path", O_RDWR); /* Open with read/write */
char text_block[10]; // Random size
read(file_descriptor, text_block, 1); /* This will read 1 characters
from the file */
/* Then do what you want with the char readed. */
}
Considering this solution, here is a process which will work :
Use stat() function to know the size of the file (read the manual)
Use the same process as above but instead of read 1 char read the size of the file
Each line is separated by '\n' exept the last one. Just use this to separate your fully readed file into lines
I guess it's for homework... This code have an error and won't compile until you found it. Which mean search the manual on the net and read it.

Why system call read() does not work when using user input

It`s a file copying program.
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <stdlib.h>
int main()
{
int fd1,fd2, ndata;
char data[128];
char *openname[1], *creatname[1];
write(1, "Write open file name\n", 24); //user input
read(0, openname, 30 );
write(1, "Write creat file name\n", 25); //user input
read(0, creatname,30);
if((fd1 = open(openname, 0666))<0)
{
perror("cannot open the file");
exit(1);
}
if((fd2 = creat(creatname,0666))<0)
{
perror("cannot create the file");
exit(1);
}
while((ndata = read(fd1, data, 128))>0)
{
if((write(fd2, data, ndata))!=ndata)
{
perror("cannot write the file");
exit(1);
}
}
close(fd1);
close(fd2);
write(1, "File copy is done.",19);
return 0;
}
This code ain`t work. This code print the error message:
cannot open the file.
but if i change the code to this :
if((fd1 = open("copy.c", 0666))<0)
and this :
if((fd2 = creat("real.c",0666))<0)
worked well.
Why this error happend? Please answer.
Your declarations of openname and creatname are incorrect. They should be:
char openname[31], creatname[31];
read() does not add a null terminator to the input, you need to add it. read() returns the number of bytes read. So it should be:
int nread = read(0, openname, sizeof openname -1);
openname[nread-1] = '\0'; // subtract 1 to overwrite the newline
The type of openname and creatname is wrong, and gcc -Wall -g would have warned you. Declare e.g. char openname[256];
And you should use fgets(openname, sizeof(openname), stdin); to read it.
If you insist on using read, take care of the newline (if any) and add a zero terminating byte.
Learn also to use the gdb debugger.
read is very low level. In this case, it reads 30 bytes, including your enter key and also without a terminating null-byte. So the filename won't be what you think you've entered, it will contain additional garbage (and could even make your program crash due to the missing null-termination). You want to use fgets or readline instead.
In a nutshell, by using read() to input the file names, you are making this unnecessarily hard for yourself: it does not terminate the input with NUL, is not guaranteed to read the number of characters you expect, etc.
My advice would be to stick with scanf() or fgets().

How do I read file into a command line?

Basically what I want to do is have a program with int main(argc, *argv[]) and instead of writing chars into command line, I want to have my program read those words from a file. How could I accomplish this? Is there a special command in Linux for that?
You can use standard redirect operations in a *nix shell to pass files as input:
./myprogram < inputfile.txt
This statement executes your program (myprogram) and pumps the data inside of inputfile.txt to your program
You can also redirect the output of program to a file in a similar fashion:
./myprogram > outputfile.txt
Instead of doing
for(int i = 1; i < argc; i++)
{
insert(&trie, argv[i]);
}
you could doing something like
FILE *input;
char *line;
....
while (fscanf(input, "%ms", &line) != EOF) {
insert(&trie, line);
/* If you make a copy of line in `insert()`, you should
* free `line` at here; if you do not, free it later. */
free(line);
}
Use redirection
yourprogram < youtextfile
will offer the content of yourtextfile as standard input (stdin) to yourprogram. Likewise
yourprogram > yourothertextfile
will send everything the program writes to standard output (stdout) to yourothertextfile
You'll notice when reading man pages that most system calls have a version that works directly with stdin or stdout
For example consider the printf family:
printf ("hello world\n");
is a shorter version of
fprintf (stdout,"hello world\n");
and the same goes for scanf and stdin.
This is only the most basic usage of redirection, which in my opinion is one of the key aspects of "the unix way of doing things". As such, you'll find lots of articles and tutorials that show examples that are a lot more advanced than what I wrote here. Have a look at this Linux Documentation Project page on redirection to get started.
EDIT: getting fed input via redirection ior interactively "looks" the same to the program, so it will react the same to redirected input as it does to console input. This means that if your program expects data line-wise (eg because it uses gets() to read lines), the input text file should be organized in lines.
By default, every program you execute on POSIX-compliant systems has three file descriptors open (see <unistd.h> for the macros' definition): the standard input (STDOUT_FILENO), the standard output (STDOUT_FILENO), and the error output (STDERR_FILENO), which is tied to the console.
Since you said you want read lines, I believe the ssize_t getline(char **lineptr, size_t *n, FILE *stream) function can do the job. It takes a stream (FILE pointer) as a third argument, so you must either use fopen(3) to open a file, or a combination of open(2) and fdopen(3).
Getting inspiration from man 3 getline, here is a program demonstrating what you want:
#define _GNU_SOURCE
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
FILE *fp;
size_t len;
char *line;
ssize_t bytes_read;
len = 0;
line = NULL;
if (argc > 1)
{
fp = fopen(argv[1], "r");
if (fp == NULL)
{
perror(*argv);
exit(EXIT_FAILURE);
}
}
else
fp = stdin;
while ((bytes_read = getline(&line, &len, fp)) != -1)
printf("[%2zi] %s", bytes_read, line);
free(line);
exit(EXIT_SUCCESS);
}
Without arguments, this program reads lines from the standard input: you can either feed it lines like echo "This is a line of 31 characters" | ./a.out or execute it directly and write your input from there (finish with ^D).
With a file as an argument, it will output every line from the file, and then exit.
You can have your executable read its arguments on the command line and use xargs, the special Linux command for passing the contents of a file to a command as arguments.
An alternative to xargs is parallel.

How do i read a file backwards using read() in c? [duplicate]

This question already has answers here:
Reading a text file backwards in C
(5 answers)
Closed 9 years ago.
I am supposed to create a program that takes a given file and creates a file with reversed txt. I wanted to know is there a way i can start the read() from the end of the file and copy it to the first byte in the created file if I dont know the exact size of the file?
Also i have googled this and came across many examples with fread, fopen, etc. However i cant use those for this project i can only use read, open, lseek, write, and close.
here is my code so far its not much but just for reference:
#include<stdio.h>
#include<unistd.h>
int main (int argc, char *argv[])
{
if(argc != 2)/*argc should be 2 for correct execution*/
{
printf("usage: %s filename",argv[0[]);}
}
else
{
int file1 = open(argv[1], O_RDWR);
if(file1 == -1){
printf("\nfailed to open file.");
return 1;
}
int reversefile = open(argv[2], O_RDWR | O_CREAT);
int size = lseek(argv[1], 0, SEEK_END);
char *file2[size+1];
int count=size;
int i = 0
while(read(file1, file2[count], 0) != 0)
{
file2[i]=*read(file1, file2[count], 0);
write(reversefile, file2[i], size+1);
count--;
i++;
lseek(argv[2], i, SEEK_SET);
}
I doubt that most filesystems are designed to support this operation effectively. Chances are, you'd have to read the whole file to get to the end. For the same reasons, most languages probably don't include any special feature for reading a file backwards.
Just come up with something. Try to read the whole file in memory. If it is too big, dump the beginning, reversed, into a temporary file and keep reading... In the end combine all temporary files into one. Also, you could probably do something smart with manual low-level manipulation of disk sectors, or at least with low-level programming directly against the file system. Looks like this is not what you are after, though.
Why don't you try fseek to navigate inside the file? This function is contained in stdio.h, just like fopen and fclose.
Another idea would be to implement a simple stack...
This has no error checking == really bad
get file size using stat
create a buffer with malloc
fread the file into the buffer
set a pointer to the end of the file
print each character going backwards thru the buffer.
If you get creative with google you can get several examples just like this.
IMO the assistance you are getting so far is not really even good hints.
This appears to be schoolwork, so beware of copying. Do some reading about the calls used here. stat (fstat) fread (read)
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
int main(int argc, char **argv)
{
struct stat st;
char *buf;
char *p;
FILE *in=fopen(argv[1],"r");
fstat(fileno(in), &st); // get file size in bytes
buf=malloc(st.st_size +2); // buffer for file
memset(buf, 0x0, st.st_size +2 );
fread(buf, st.st_size, 1, in); // fill the buffer
p=buf;
for(p+=st.st_size;p>=buf; p--) // print traversing backwards
printf("%c", *p);
fclose(in);
return 0;
}

I/O issues writing on file

I'm having a hard time trying to figure out why this piece of code doesn't work as it should. I am learning the basics of I/O operations and I have to come up with a C program that writes on a 'log.txt' file what is given from standard input and as the 'stop' word is entered, the program must halt.
So my code is:
#include "main.h"
#define SIZE 1024
int main(int argc, char *argv[])
{
int fd;
int readBytes;
int writBytes;
char *buffer;
if ((fd = open("log.txt", O_WRONLY|O_APPEND)) < 0)
{
perror("open");
}
buffer = (char *) calloc (SIZE, sizeof(char));
while ((readBytes = read(0, buffer, SIZE) < SIZE)&&(strncmp(buffer, "stop", 4) != 0));
if ((writBytes = write(fd, buffer, SIZE)) < 0)
{
perror("write");
}
if ((close(fd)) < 0)
{
perror("close");
}
}
If I enter:
this is just a text
stop
The output is
stop
is just a text
If I enter more than a sentence:
this is just a text
this is more text
and text again
stop
This is what is logged:
stop
ext again
xt
t
And on top of that if I try to edit the log.txt file from vim or just a text editor I can see '\00's. I guess \00 stands for all the bytes left empty from the 1024 available, right? How can I prevent that from happening?
It looks like you're expecting
readBytes = read(0, buffer, SIZE) < SIZE)
to somehow accumulate things in buffer. It doesn't. Every subsequent read will put whatever it read at the start of the buffer, overwriting what the previous read has read.
You need to put your write in the while block - one write for every read, and only write as much as you read, otherwise you'll write garbage (zeros from the calloc and/or leftovers from the previous read) in your log file.
Also note that while your technique will probably work most of the time for a line-buffered input stream, it will not do what you expect if you redirect from a file or a pipe. You should be using formatted input functions (like getline if you your implementation has that, scanf, or fgets).

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