I am working on a school project in which we have to do some operations (select, min, max) on a table saved in .txt file.
The problem is that we can't use common functions such as fopen, fscanf, fclose.
The program will be launched from command line like this: .\project.exe select parameters <table.txt
Do you have some ideas how to get content of the .txt file to stdin without using fopen?
Thanks.
You do not need to open the file - the operating environment will do it for you.
When your program is called with <table.txt, your standard input is switched to read from that file instead of the keyboard. You can use scanf to read the data, and do not worry about opening and closing the file.
Same goes for the output of your program and the >table_out.txt redirection: rather than printing to the screen, printfs in your program would be writing to a file, which would be automatically closed upon your program's exit. Of course if you need to print something to the screen when your output is redirected, you can do so by printing to stderr (e.g. fprintf(stderr, "Invalid table format\n").
There are few ways to acomplish this.
Reading STDIN
I guess the teacher wants this method in particular. The idea is reading standard input rather than particular file.
In C++ you can simply read the stdin object. Here's an example:
#include <stdio.h>
#include <string.h>
int main(void)
{
char str[80];
int i;
printf("Enter a string: ");
fgets(str, 10, stdin);
/* remove newline, if present */
i = strlen(str)-1;
if( str[ i ] == '\n')
str[i] = '\0';
printf("This is your string: %s", str);
return 0;
}
Source: http://www.java2s.com/Code/C/Console/Usefgetstoreadstringfromstandardinput.htm
Using system utils
You can call "type" util # Windows (not sure about it) or "cat" util in Linux as a subprocess to read some partticular file. But this is rather a "hack", so I do not recommend using this one.
Related
I have this command line argument -
cat file_name | ./a.out
The problem is not reading from the cat command inside the C program as we can do that with read(), fgets(), fgetc() but the actual problem I am facing is after reading the data from cat I am not able to take input from user using fgets.
Here is my sample code
while(fgets(buffer, BUFSIZ, stdin ) != NULL )
puts( buffer ); // Here I have tried strtok( buffer, "\n" ) too.
memset( buffer, 0, BUFSIZ );`
The problem is after this line, it is not asking for the input like the below is not working-
puts("Name: ");
fgets( buffer, BUFSIZ, stdin );
Help me with what's wrong happening here?
When you do cat file_name | ./a.out the standard input of your program is tied to a pipe linking it to the output of cat. Your program will never get to see the user input - the very stream from where it would arrive has been replaced by the aforementioned pipe.
Mind you, I suspect that with some horrible POSIX-specific trickery you may be able to reopen it going straight for the tty device, but it's just bad design. If you need to both read from a file and accept interactive user input just accept the file as a command line argument and use stdin to interact with the user.
Edit
This is an example of the Unix-specific kludges that one can attempt, assuming that the process still has a controlling terminal. After reading all the original stdin, I'm opening /dev/tty (which is the controlling terminal of the process) and re-linking stdin to it.
Disclaimer: this is for entertainment purposes only, don't do this for real.
#include <stdio.h>
#include <stdlib.h>
void die(const char *msg) {
fprintf(stderr, "%s\n", msg);
fputs(msg, stderr);
exit(1);
}
int main() {
/* Read all of stdin and count the bytes read (just to do something with it) */
int ch;
unsigned long count = 0;
while((ch = getchar())!=EOF) {
count++;
}
printf("Read %lu bytes from stdin\n", count);
/* Open the controlling terminal and re-link it to the relevant C library FILE *
* Notice that the UNIX fd for stdin is still the old one (it's
* surprisingly complex to "reset" stdio stdin to a new UNIX fd) */
if(freopen("/dev/tty", "r", stdin) == NULL) {
die("Failed freopen");
}
/* Do something with this newly gained console */
puts("How old are you?");
fflush(stdout);
int age = -1;
if(scanf("%d", &age)!=1) {
die("Bad input");
}
printf("You are %d years old\n", age);
return 0;
}
(previously I had a solution that checked if stderr or stdout were still consoles, which was even more of a kludge; thanks #rici for reminding me of the fact that POSIX has the concept of "controlling terminal", which is accessible through /dev/tty)
If you need to use stdin for user interaction, then you need to use a different file descriptor for reading the input stream.
You could use a specific pre-opened file descriptor and document that (e.g. "the input stream should be connected to fd 3"), but the usual approach is to accept a file name as a command-line argument. You can then provide a named pipe as the argument; shells such as Bash provide process substitution to make that easy:
./a.out <(cat file_name)
When that is run interactively like that, stdin is still connected to the terminal, and can be used at the same time as the stream from the connected command.
(Obviously, if the command actually is cat with a single argument, then you could just provide the filename itself as the argument, but I'm assuming that's a placeholder for a more involved pipeline).
My program is supposed to let the user edit a line of a file. The user edits the line and sends it back by pressing enter. Therefore I would like to print the current line which is about to be edited, but kind of print it on stdin instead of stdout.
The only problem I don't know how to solve is how I can prefill the stdin. I've already tried this:
char cprefill[] = {"You may edit this line"};
char cbuffer[100];
fprintf(stdin, cprefill);
fgets(cbuffer, 100, stdin);
This seems to be the simplest solution, but is probably too simple to work. The fprintf doesn't print anything to stdin. What is the correct way?
Edit:
This is how it is supposed to look like. Please mind the cursor which can be moved.
The C language has no notion of terminal nor of line edition, so it cannot be done in a portable way. You can either rely on a library like [n]curses to get an almost portable solution, or if you only need that on one single OS use low level OS primitives.
For exemple on Windows, you could feed the input buffer by simulating key strokes into the appropriate window (for example by sending WM_CHAR messages) just before reading, but that would be highly non portable - and in the end is no longer a C but a Windows solution...
First you need the libreadline developer package. (You might also need the libreadline if it's not already available on your system)
On Debian / Ubuntu that's apt install libreadline-dev (plus libreadline6 if you need the binaries also - 6 might be different on your platform)
Then you can add an history to readline, like this
#include <stdio.h>
#include <readline/readline.h>
#include <readline/history.h>
...
char cprefill[] = {"You may edit this line"};
add_history(cprefill);
char *buf = readline("Line: ");
printf("Edited line is %s\n", buf);
// free the line allocated by readline
free(buf);
User is prompted "Line: ", and has to do UP ARROW to get and edit the history, i.e. the cprefill line.
Note that you have to compile/link with -lreadline
readline prints the prompt given as argument, then waits for user interaction, allowing line edition, and arrows to load lines stored in the history.
The char * returned by readline has then to be freed (since that function allocates a buffer with malloc()).
You could use GNU Readline. It calls the function that rl_startup_hook points to when starting, where we use rl_insert_text to put our text in the line buffer.
#include <stdio.h>
#include <stdlib.h>
#include <readline/readline.h>
int prefill(void)
{
rl_insert_text("You may edit this line");
return 0;
}
int main(void)
{
char *cbuffer;
puts("Please edit the following line");
rl_startup_hook = prefill;
if ((cbuffer = readline(NULL)) == NULL) /* if the user sends EOF, readline will return NULL */
return 1;
printf("You entered: %s\n", cbuffer);
free(cbuffer);
return 0;
}
For more information, see the GNU Readline manual.
A friend of mine needs to use MATLAB for one of his classes, so he called me up (a Computer Science Major) and asked if I could teach him C. I am familiar with C++, so I am also familiar with the general syntax, but had to read up on the IO library for C.
I was creating some simple IO programs to show my friend, but my third program is causing me trouble. When I run the program on my machine using Eclipse (with the CDT) Eclipse's console produces a glitchy output where instead of prompting me for the data, it gets the input and then prints it all at once with FAILURE.
The program is supposed to get a filename from user, create the file, and write to it until the user enters a blank line.
When I compile/run it on my machine via console (g++ files2.c) I am prompted for the data properly, but FAILURE shows up, and there is no output file.
I think the error lies with how I am using the char arrays, since using scanf to get the filename will create a functional file (probably since it ignores whitespace), but not enter the while loop.
#include <stdio.h>
#define name_length 20
#define line_size 80
int main() {
FILE * write_file; // pointer to file you will write to
char filename[name_length]; // variable to hold the name of file
char string_buffer[line_size]; // buffer to hold your text
printf("Filename: "); // prompt for filename
fgets(filename, name_length, stdin); // get filename from user
if (filename[name_length-1] == '\n') // if last char in stream is newline,
{filename[name_length-1] = '\0';} // remove it
write_file = fopen(filename, "w"); // create/overwrite file user named
if (!write_file) {printf("FAILURE");} // failed to create FILE *
// inform user how to exit
printf("To exit, enter a blank line (no spaces)\n");
// while getting input, print to file
while (fgets(string_buffer, line_size, stdin) != NULL) {
fputs(string_buffer, write_file);
if (string_buffer[0] == '\n') {break;}
}
fclose(write_file);
return 0;
}
How should I go about fixing the program? I have found next to nothing on user-terminated input being written to file.
Now if you will excuse me, I have a couple of files to delete off of my University's UNIX server, and I cannot specify them by name since they were created with convoluted filenames...
EDIT------
Like I said, I was able to use
scanf("%s", filename);
to get a working filename (without the newline char). But regardless of if I use scanf or fgets for my while loop, if I use them in conjunction with scanf for the filename, I am not able to write anything to file, as it does not enter the while loop.
How should I restructure my writing to file and my while loop?
Your check for the newline is wrong; you're looking at the last character in filename but it may be before that if the user enters a filename that's shorter than the maximum. You're then trying to open a file that has a newline in it's name.
These lines seem to be incorrect:
if (filename[name_length-1] == '\n') // if last char in stream is newline,
{filename[name_length-1] = '\0';} // remove it
You verify the name_length - 1 character,, which is 19 in your case without any regard of the introduced filename's length. So if your file name's length is less then 18 you won't replace the '\n' character at the end of your string. Obviously the file name can't contain '\n' character.
You need to get the size of you file name first with strlen() as an example.
if (filename[strlen(filename) - 1] == '\n')
{
filename[strlen(filename) - 1] = '\0';
}
(Don't forget to include the string.h header)
I hope I was able to help with my weak english.
I am trying to get a basic understanding on how to use fputc in C. I have read some documentation that is out there and believed I had it right. But every time I try to use the script I wrote by executing ./fputc > test.txt where text.txt is a text file with one line of text.
This is my script:
int
main(int argc, char **argv){
int ch;
FILE *input;
input = fopen("text.txt", "w+");
while ((ch = getchar()) != EOF){
fputc(ch, input);
}
fclose(input);
return 0;
}
I get no errors on compilation and for some reason the script does not reach EOF at the end of the text file. Shouldn't the getchar return EOF when it reached the end of the text file?
The text (text.txt) file does not appear to be edited, although it is created. So somewhere in my while loop something is going wrong.
I am new to C programming (if you couldn't tell) and this little script has me befuddled.
Any help would be appreciated, or any links to sites with further detail would also be great.
Cheers,
S.
What you in essence say is:
Console: Run my_program and write anything it outputs to test.txt.
Program: Open text.txt and write any input to stdin to that file.
Your console normally have three standard streams stdin, stdout and stderr. These streams you can redirect. If you are on Windows also look at i.e. redirection.
When you say ./my_prog > test.txt, what you tell your console, (not my_prog), is to write anything my_prog writes to stdout to the file test.txt.
If you in your code say i.e. printf("Hello");, then Hello would be written to the file test.txt.
If you had turned your redirection around by saying ./my_prog < test.txt instead, would be; stream the file test.txt to my_prog. Which, in turn, if there was any text in test.txt would result in a copy of test.txt to text.txt.
Now in your code you say:
int main(void)
{
int ch;
FILE *input;
/* Here you open a handle to the file text.txt for reading and writing */
input = fopen("text.txt", "w+");
while ((ch = getchar()) != EOF) { /* get next char from stdin */
fputc(ch, input); /* write that char to the handle input */
}
fclose(input); /* close the handle */
return 0;
}
So what happens, the way you run it, is:
In your code:
Open text.txt
Wait for input (data entered to stdin) - typically user entering text to console, passed to program when Enter is pressed.
In console:
Redirect anything from my_prog to test.txt.
You say:
the script does not reach EOF
Well, as it reads from stdin it will only (not without exception) get EOF under two conditions.
If you redirect a file to your program. I.e. ./my_prog < foo.txt (notice <, not >).
- What would happen then is that my_prog would read the data from the file foo.txt and when that file ends your program would receive a EOF. And, hence quit.
If you manually enter EOF to stdin.
- On Linux and OSX Ctrl-D, on Windows Ctrl-Z
Now, if you test this by typing text to console remember that write actions like fputc()is buffered. What this mean is that the data is not written to the file right away, but only when a given amount of data is in buffer, fflush() is called, stream is closed, you turn off buffering, etc.
Also; if you run your program. Enter text, enter some more text, and then hit Ctrl-C to abort the program it is a big chance you end with no data in your text.txt.
The reason for this is that the program is killed and thereby fclose() never called, and hence no flush to file.
On your further endeavors in programming it would be a very good idea to make a habit of not presuming anything. I.e. do not presume fopen() is OK.
FILE *fh;
char *outfile = "foo.txt";
if ((fh = fopen(outfile, "w")) == NULL) {
fprintf(stderr,
"Unable to open file %s\n --",
outfile);
perror(" fopen() ");
return 1;
}
Most functions has a way to check if operation was a success. I.e:
if (fputc(ch, fh) != ch) { err ...
This will make your code a lot safer, give you hints on where it fails etc.
Some links:
Look at redirection links at top of post.
Look at the functions in stdio.h (good overview), stdio.h (examples etc.). I.e.:
stdin
stdout
stderr
fopen()
fflush()
setvbuf()
setbuf()
...
getchar returns the next character from the standard input (stdin).
It is equivalent to getc with stdin as its argument.
Hence, your code reads from standard input instead of FILE* input.
Use fgetc here.
fgetc returns the character currently pointed by the internal file position indicator of the specified stream. The internal file position indicator is then advanced by one character to point to the next character.
So, Use fgetc to read from a file:
while ((ch = fgetc(input)) != EOF)
your program and the shell are both writing the same file. you should remove the output redirection > test.txt from your command line
I've searched high and low, but can not find the answer to what I would've thought to be a rather simple question. I'm rather new to C, and due to the restrictions placed on me during this project I'm having a bit of trouble figuring out how to do this.
I am trying to read in text from a text file, and store that data in an array. Simple enough. HOWEVER, I'm forced to do so by using the command line operator '<' and to redirect the stdin to the text file.
The only way I can seem to figure out how to properly open a file and perform the subsequent operations is the following:
#include <stdio.h>
FILE *fr;
main()
{
fr = fopen ("mytext.txt", "r"); /* open the file for reading */
The problem with that is that I can't seem to get the first parameter of fopen() to be the filename provided by the stdin '<'. It only works if I explicitly type a string in for the parameter. For example, if I were to run
myprog.o < mytxt.txt
how could I pass the name of the text file provided by that stdin redirection to the fopen function? Or is there a different/better way to do what I'm trying to do?
Thanks in advance.
You need to read from stdin instead of trying to open the file directly.
This is because of how redirection works - think of it a bit like this:
The file is opened (for purposes of demonstration, let's say fopen is used for this).
The existing stdin is closed. It no longer refers to a terminal or similar construct.
stdin is replaced with the open file in step 1.
Any reads from stdin now work directly from the input file.
By using input redirection you can permit your user to either redirect a file or directly type content into your program.
As for your actual problem, you might be better off passing the filename as an argument to your program. You would use argv and call your program like so:
myprog.o mytxt.txt
In this case, argv[1] will be mytxt.txt.
A C program never sees the redirection because it is handled by the shell. The C program should look at argc and read from stdin if no args are given or from the given files otherwise.
There is a standard FILE* handle declared within <stdio.h> that points to the standard input for the executing process. Such file handle is called stdin.
If all you ever want this program to do is read from standard input, then you don't need to open any files. The OS and C libraries will handle opening and closing the file for you.
So to read a file in from standard input, and write it back out, try something as simple as
#include <stdio.h>
int main( int argc, char ** argv ) {
int ch = getchar();
while ( ch != EOF ) {
putchar( ch );
ch = getchar();
}
}
As you can see, no opening or closing of files. putchar and getchar write to stdin and stdout relatively.
If you want to be more explicit, you can use the predefined file handles.
int ch = fgetc( stdin );
while ( ch != EOF ) {
fputc( ch, stdout );
ch = fgetc( stdin );
}
You should look up printf() and fprintf(), scanf() and fscanf(), and all the other wonderful stdio functions.