How feof() works in C - c

Does feof() checks for eof for the current position of filepointer or checks for the position next to current filepointer?
Thanks for your help !

Every FILE stream has an internal flag that indicates whether the caller has tried to read past the end of the file already. feof returns that flag. The flag does not indicate whether the current file position is as the end of the file, only whether a previous read has tried to read past the end of the file.
As an example, let's walk through what happens, when reading through a file containing two bytes.
f = fopen(filename, "r"); // file is opened
assert(!feof(f)); // eof flag is not set
c1 = getc(f); // read first byte, one byte remaining
assert(!feof(f)); // eof flag is not set
c2 = getc(f); // read second byte, no bytes remaining
assert(!feof(f)); // eof flag is not set
c3 = getc(f); // try to read past end of the file
assert(feof(f)); // now, eof flag is set
This is why the following is the wrong way to use eof when reading through a file:
f = fopen(filename, "r");
while (!feof(f)) {
c = getc(f);
putchar(c);
}
Because of the way feof works, the end-of-file flag is only set once getc
tries to read past the end of the file. getc will then return EOF, which is
not a character, and the loop construction causes putchar to try to write it
out, resulting in an error or garbage output.
Every C standard library input method returns an indication of success or
failure: getc returns the special value EOF if it tried to read past the
end of the file, or if there was an error while reading. The special value is
the same for end-of-file and error, and this is where the proper way to use
feof comes in: you can use it to distinguish between end-of-file and error
situations.
f = fopen(filename, "r");
c = getc(f);
if (c == EOF) {
if (feof(f))
printf("it was end-of-file\n");
else
printf("it was error\n");
}
There is another internal flag for FILE objects for error situations:
ferror. It is often clearer to test for errors instead of "not end of file".
An idiomatic way to read through a file in C is like this:
f = fopen(filename, "r");
while ((c = getc(f)) != EOF) {
putchar(c);
}
if (ferror(f)) {
perror(filename):
exit(EXIT_FAILURE);
}
fclose(f);
(Some error checking has been elided from examples here, for brevity.)
The feof function is fairly rarely useful.

You can get a better understanding of how feof works, by knowing how it's implemented. Here is a simplified version of how the 7th Edition Unix stdio library implements feof. Modern libraries are very similar, adding code offering thread-safety, increased efficiency, and a cleaner implementation.
extern struct _iobuf {
char *_ptr;
int _cnt;
char *_base;
char _flag;
char _file;
} _iob[_NFILE];
#define _IOEOF 020
#define feof(p) (((p)->_flag&_IOEOF)!=0)
#define getc(p) (--(p)->_cnt>=0? *(p)->_ptr++&0377:_filbuf(p))
int
_filbuf(FILE *iop)
{
iop->_ptr = iop->_base;
iop->_cnt = read(fileno(iop), iop->_ptr, BUFSIZ);
if (iop->_cnt == 0) {
iop->_flag |= _IOEOF;
return(EOF);
}
return(*iop->_ptr++ & 0377);
}
The stdio library maintains with each file a structure containing an internal buffer pointed by _base. The current character in the buffer is pointed by _ptr and the number of characters available is contained in _cnt. The getc macro, which is the base for a lot of higher-level functionality, like scanf, tries to return a character from the buffer. If the buffer is empty, it will call _filbuf to fill it. _filbuf in turn will call read. If read returns 0, which means that no more data is available, _filbuf will set the _IOEOF flag, which feof checks each time you call it to return true.
As you can understand from the above, feof will return true the first time you try to read a character past the end of the file (or a library function tries in your behalf). This has subtle implications on the behavior of various functions. Consider a file containing a single character: the digit 1. After you read that character with getc, feof will return false, because the _IOEOF flag is unset; nobody has yet tried to read past the end of the file. Calling getc again will result in a call to read, the setting of the _IOEOF flag, and this will cause feof to return true. However, after reading the number from the same file using fscanf("%d", &n), feof will immediately return true, because fscanf will have tried to read additional digits of the integer.

Related

What is clearerr used for?

I'm trying to understand when the stdio function clearerr() should be used.
For example, if I fread() or fwrite() on a valid FILE* and get a short count and ferror is true, what can I do?
From what I've read so far, fread() and fwrite() are robust and will block and/or retry (if there are locks and/or interrupts which could happen in lower level functions) so there never seems any point in using clearerr() because fread or fwrite errors will be so catastrophic there is no point in trying to recover.
Additionally, ferror() only tells me that there is an error, not what the error is.
#define SZ 1024
FILE* fp = fopen( "foo", "r" );
if ( fp ) {
char b[SZ];
int ch_count = fread( b, sizeof(char), SZ, fp );
if ( ch_count != SZ && ferror( fp ) ) {
// how would clearerr() be used. I don't know?
// ....
// should I drop through here to fclose? (when I've got an ferror)
}
fclose( fp );
}
There is at least one real world use case for clearerr: when you want to mimic tail -f on a file that is not opened in exclusive mode. That means that another (or many other) process(es) write at the end of a file, and one process repeatedly reads even after having reached the end of file in order to look whether new data has arrived. In that case, could would look like:
for (;;) {
if (NULL == fgets(line, sizeof(line), fd)) {
sleep(n);
clearerr(fd); // reset EOF condition
}
else {
fputs(line, fdout);
}
}
Functions that set the error status of a FILE (as reported by ferror) do not clear it even if later called successfully. Likewise if you encounter the end of file while reading, it will not be cleared automatically even if the file later has more data available.
Basically this means that if you are using ferror to check for an error state and you have some way of recovering from it, the ferror will keep indicating an error until you use clearerr.
In your example, if you just use the return value of fread as the condition for terminating the read (i.e., EOF and any type of error are considered final), there is no need to clearerr: just fall through to fclose (and perhaps use ferror to determine whether to print an error message).
On the other hand, if the FILE is in fact a stream on which read can later succeed, and you detect (or assume) that specific condition and retry, you should clearerr before retrying or you will keep seeing the old error condition on future attempts.
Likewise, as pointed out in comments, clearerr also clears the end of file state, so this also applies when using feof to check for the end of file. (Note, however, that you generally shouldn't use !feof(file) as the loop condition when reading.)
clearerr() clears the error and EOF flags from a stream.
Say FILE were like this:
typedef struct {
int fd;
char *buf;
int error;
int eof;
} FILE;
FILE *file;
This would set file->error and file->eof to 0.
Some reasons for doing this include file I/O, such as when a file gives EOF, but then another program (or another thread, etc.) appends to it. If you clear the error after doing this, you can have your program act as sort of a tail -f-substitute.
clearerr() clears both the error and end-of-file flags.
A pedantic use of clearerr():
// Return -1 on end-of-file
// Return -2 on rare file error
// Else return the unsigned char value
int my_get_quandry() {
// At this point, the file end-of-file flag may be set.
// At this point, the file file error flag may be set.
// They may both be set.
// Attempt to read another
int ch = fgetc();
if (ch != EOF) {
return ch;
}
// Now was the EOF due to a end-of file or error?
// feof() is true if end-of-file just occurred OR if end-of-file was set earlier
// ferror() is true if error just occurred OR if error was set earlier
// If only one feof() or ferror() is true, we know why EOF just occurred,
// Yet if both set, we do not know.
...?
}
Use clearerr()
// Return -1 on end-of-file
// Return -2 on rare file error
// Else return the unsigned char value
int my_get_crystal() {
clearerr(stdin);
// Attempt to read another
int ch = fgetc();
if (ch != EOF) {
return ch;
}
// Now EOF due to at most one reason
if (feof(stdin)) return -1;
if (ferror(stdin)) return -2;
// if code reaches this point, it is due to the odd-ball platform of `char` having the
// same range as `int`. But let us leave that platform for another day.
return ch;
}

Usage of ferror with a regular file in c

In this related question How to use feof and ferror for fgets (minishell in C), the answers aren't really clear for my problem.
What I'm trying to do is: I want to read all of the characters from a plain text file on my disk. It's not a link, devicefile, socket etc. just a regular text file. Once I've read all the characters I want to see if everything succeeded. What I'm seeing now is, in my Debug builds everything goes successfully, but in my Release builds ferror() always indicates there is a an error. While the Debug build returns the 0 in the end. In both cases I can see that the content of the file has been obtained in a debugger.
my_function(File* f) {
int c;
while ((c = fgetc(f)) != EOF) {
char character = (char) c;
// store character in a dynamic growing buffer
}
// append '0' byte to buffer (to terminate the string).
if (ferror(f)) {
// return error
return 1;
}
return 0; // no error.
}
Rationale of having a already opened file as function argument, is to make it relatively easy to read from a file, without the need to bother with a platform dependent encoding. This is inside a private part of my library, the public functions handle the case that f == NULL.
Do I need to call clearerr(file) first, because the error bit is not initialized in a Release builds?

fscanf is scanning an extra character at the end

it is always printing an extra character at the end. here is the code:
#include <stdio.h>
int main ()
{
char bit;
FILE *fp_read,*fp_write;
fp_read = fopen("test.txt","r");
if(fp_read==NULL) {
printf("Error!! Unable to open the file!!");
return 1;
}
while(!feof(fp_read)) {
fscanf(fp_read,"%c",&bit);
printf("%c",bit);
}
fclose(fp_read);
return 0;
}
if test.txt contains 010101 it prints 0101011 . if 00110 it prints 001100. if it contains abc it prints abcc . that means it always repeats the last character.
What is the problem ? can anybody explain ?
I am not able to reproduce the error.
Refer to David Bowling's first comment in the original post for a neat explanation.
The cppreference page for feof has a shorter version.
The eof function only reports the stream state as reported by the most recent I/O operation, it does not examine the associated data source. For example, if the most recent I/O was a fgetc, which returned the last byte of a file, feof returns zero. The next fgetc fails and changes the stream state to end-of-file. Only then feof returns non-zero.
In typical usage, input stream processing stops on any error; feof and ferror are then used to distinguish between different error conditions.
This means that the use of feof in the while loop may not be appropriate. The last character from the file may be junk and will be different in different systems.
Try doing this instead.
while(fscanf(fp_read,"%c",&bit) != EOF) {
printf("%c",bit);
}

compiler says:cannot convert int to FILE*

While doing filing im stuck here.The condition of the while loop is not working.The compiler says cannot convert int to FILE*.
while(pFile!=EOF);
Should i typecase the pFile to int?I tried that but it did not worked.Thanks in advance.
The complete code is:
int main()
{
char ch;
char name[20];
FILE *pFile;
int score;
pFile=fopen("database.txt","r");
if(pFile!=NULL)
{
while(pFile!=EOF);
{
fscanf(pFile,"%c",ch);
}
}
else
printf("Cant open the file.......");
fclose(pFile);
return 0;
}
First, you do not want to use while (!feof(pFile)) -- ever! Doing so will almost inevitably lead to an error where the last data you read from the file appears to be read twice. It's possible to make it work correctly, but only by adding another check in the middle of the loop to exit when EOF is reached -- in which case, the loop condition itself will never be used (i.e., the other check is the one that will actually do the job of exiting the loop).
What you normally do want to do is check for EOF as you read the data. Different functions indicate EOF in different ways. fgets signals failure (including EOF) by returning NULL. Most others (getc, fgetc, etc.) do return EOF, so you typically end up with something like this:
int ch; // Note, this should be int, NOT char
while (EOF != (ch=getc(pFile)))
process(ch);
or:
char buffer[MAX_LINE_SIZE];
while (fgets(buffer, sizeof(buffer), pFile))
process(buffer);
With scanf, checking for success is a little more complex -- it returns the number of successful conversions, so you want to make sure that matches what you expected. For example:
while (1 == fscanf(fPfile, "%d", &input_number))
process(input_number);
In this case I've used 1 because I specified 1 conversion in the format string. It's also possible, however, for conversion to fail for reasons other than EOF, so if this failes, you'll frequently want to check feof(pFile). If it returns false, do something like reading the remainder of the line, showing it to the user in a warning message, and then continuing to read the rest of the file.
It depends what pFile and EOF are defined as, but I will asssume that pFile is a *FILE, and EOF is from stdio.h. Then I guess you should do something like:
#include <stdlib.h>
#include <stdio.h>
#define FILENAME "file.txt"
int main(void) {
FILE *pFile;
int ch;
pFile = fopen(FILENAME,"r");
if (pFile) {
while ((ch = getc(pFile)) != EOF) {
printf("Read one character: %c\n", ch);
}
close(pFile);
return EXIT_SUCCESS;
} else {
printf("Unable to open file: '%s'\n", FILENAME);
return EXIT_FAILURE;
}
}
which yields
$ echo "abc" > file.txt
$ /tmp/fileread
Read one character: a
Read one character: b
Read one character: c
Read one character:
# last character being a linefeed
Assuming pFile is your file handle, this doesn't change as you read from the file. EOF is returned by e.g. fgetc(). See e.g. http://www.drpaulcarter.com/cs/common-c-errors.php#4.2 for common ways to solve this.
here is correct way:
c = getc(pFile);
while (c != EOF) {
/* Echo the file to stdout */
putchar(c);
c = getc(pFile);
}
if (feof(pFile))
puts("End of file was reached.");
else if (ferror(pFile))
puts("There was an error reading from the stream.");
else
/*NOTREACHED*/
puts("getc() failed in a non-conforming way.");
fclose(pFile);
pFile is a pointer to a file. EOF is usually defined as -1, a signed integer.
What you should do is fopen, make sure pFile != NULL, then call some function on the file handle until that function returns EOF. A pointer will (or rather, should) never be EOF. But a function acting on that pointer may return EOF.
I'm guessing you want to keep looping while you haven't hit end-of-file. In that case, you are looking for this:
while (!feof(pFile))
{
...
}
That said, this is still not quite correct. feof will only return true once it tries to read beyond the end of the file. This means feof can return false and yet there is no more data to read. You should really try your operation and only check for end of file if it fails:
char buffer[SIZE];
while (fgets(buffer, sizeof(buffer), pFile))
{
...
}
if (!feof(pFile))
{
// fgets failed for some reason *other* then end-of-file
}

feof() in C file handling

I am reading a binary file byte-by-byte,i need determine that whether or not eof has reached.
feof() doesn't works as "eof is set only when a read request for non-existent byte is made". So, I can have my custom check_eof like:
if ( fread(&byte,sizeof(byte),1,fp) != 1) {
if(feof(fp))
return true;
}
return false;
But the problem is, in case when eof is not reached, my file pointer is moved a byte ahead.
So a solution might be to use ftell() and then fseek() to get it to correct position.
Another solution might be to buffer the byte ahead in some temporary storage.
Any better solutions?
If you're reading a byte at a time, the idiomatic way to do this is with fgetc:
int c;
while ((c = fgetc(fp)) != EOF) {
// Do something.
}
and then you shouldn't need to deal with feof.
I typically do something like this:
int get_next_char(FILE* fp, char *ch)
{
return fread(ch, sizeof(char),1, fp) == 1;
}
// main loop
char ch;
while (get_next_char(fp, &ch))
process_char(ch);
if (!feof(fp))
handle_unexpected_input_error(fp);
It's best to structure your code so that you try to read some data and if the read does not succeed due to reaching the end-of-file, you deal with it there (i.e. see Kristopher Johnson's answer).
If you absolutely hate this, you can use ungetc to return a single character back to the stream and it will be available in the next read call:
int c = fgetc(fp);
if (c == EOF)
{
// handle eof / error
}
else
{
ungetc(c, fp);
// the next read call is guaranteed to return at least one byte
}
I'm not clear, but if you're interested in knowing before you read the byte if EOF has been reached, put your feof() test before the fread() rather than after it.
actually, if I read correctly, you don't even want to do the fread so:
return feof(fp) ? true : false;
I would suggest using:
fseek(myFile, 0, SEEK_END);
fileLen = ftell(myFile);
fseek(myfile, 0, SEEK_SET);
When you first open the file to determine the length. Then set up your read-loop to never read off the end. You can use ftell to figure out where you are, and compare to fileLen to figure out how much further you have to go.

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