I/O in C Errors - c

I'm trying for hours to find the answer for this question i've got in university. I tried running this with writing a file with two lines of :
hello
world
and it reads the file perfectly, So i cant find the answer. I would appreciate your help !
A student wrote the next function for reading a text file and printing it exactly as it is.
void ReadFile(FILE *fIn)
{
char nextLine[MAX_LINE_LENGTH];
while(!feof(fIn))
{
fscanf(fIn,"%s",nextLine);
printf("%s\n",nextLine);
}
}
What are the two errors in this function?
You can assume that each line in the file is not longer than MAX_LINE_LENGTH characters, and that it is a text file that contains only alphabet characters, and that each line is terminated by '\n'.
Thanks.

It discards white space. Try adding multiple spaces and tabs.
It may evaluate a stream more than once, and If there is a read error, the loop never terminates.
See: Why is “while ( !feof (file) )” always wrong?
Reading strings via scanf is dangerous. There is no bounds checking. You may read past you MAX_LINE_LENGTH.(and boom! Segfault)

The main error is that fsacnf( fIn, "%s", nextLine ) doesn't scan a complete line.
From man page:
s
Matches a sequence of non-white-space characters; the next pointer must be a pointer to character array that is long enough to hold the input sequence and the terminating null byte ('\0'), which is added automatically. The input string stops at white space or at the maximum field width, whichever occurs first.
Thus if you have a line "a b" the first fscanf() will scan just "a" and the second one "b" and both are printed in two different lines. You can use fgets() to read a whole line.
The second one is maybe that it's stated "each line in the file is not longer than MAX_LINE_LENGTH characters" but nextLine can contain atmost MAX_LINE_LENGTH-1 characters (+ '\0'). That problem becomes even more important if you replace fscanf() by fgets() because than nextLine must have also capacity to store '\n' or '\r\n' (depending on the platform you're on)

A correct way of doing that is:
void ReadFile(FILE *fIn)
{
char nextLine[MAX_LINE_LENGTH];
while(fgets(nextLine, MAX_LINE_LENGTH, fIn)) {
printf("%s", nextLine);
}
}
As some have posted using feof to control a loop is not a good idea nor using fscanf to read lines.

Related

Skipping oversized inputs when using fgets(3)

this is probably quite easy to figure out, maybe i'm just looking in the wrong places, but how does one test if fgets has read an oversized input? In the code below, i'm trying to skip further processing for empty lines and oversized ones and go straight to the next line, for empty lines it works just fine.
Printing the strlen(buffer) when using line lengths < maxsize and it gives me expected values.
However when i enter lines that exceed the maxsize, it prints a value over 9000, which should still exceed the maxsize, and therefore enter the if-clause, but this doesn't happen. I've tried casting the return value of strlen into an int, didn't work.
What am i missing here? Thanks for any replies :)
char buffer[102];
while (fgets(buffer,100,stdin)!=NULL){
size_t maxsize = 102;
printf("%ld",strlen(buffer));
if(strcmp(buffer,"\n")==0||strlen(buffer)>maxsize){
continue;
}
//further processing
}
I
in the code:
char buffer[102];
while (fgets(buffer,100,stdin)!=NULL){
You don't need to give two more characters to buffer. The parameter size of fgets just can be the application of the sizeof operator, as in:
char buffer[102];
while (fgets(buffer, sizeof buffer, stdin) != NULL) {
That will give you space for lines of up to 101 characters (to leave space to the string terminator) including (or not, see below) the new line character.
But, answering your question, I understand that you want to know what happens if your input in one line is indeed bigger that the buffer size you provided, what happens then to the input, and how fgets deal with this:
Fgets() reads as many characters as it finds a \n in the input, or the buffer fills completely (this is, after including the \0 character that it must append to the string to terminate it) So, fgets() will fill as many characters in the buffer as the buffer has, minus one, reserved for the null string terminator, and the rest of the line will be read in the next fgets() (or another call to any of the functions of the stdio package).
So, basically, lines longer than one less than the buffer size are split in pieces, in which all except the last don't actually end in a new line, and the last will have the new line included, and will be shorter, all with a length of the length you specified minus one, but the last piece, in which the length is what it requires (again, always less than or equal than the length specified minus one)

how to scan line in c program not from file

How to scan total line from user input with c program?
I tried scanf("%99[^\n]",st), but it is not working when I scan something before this scan statment.It worked if this is the first scan statement.
How to scan total line from user input with c program?
There are many ways to read a line of input, and your usage of the word scan suggests you're already focused on the scanf() function for the job. This is unfortunate, because, although you can (to some extent) achieve what you want with scanf(), it's definitely not the best tool for reading a line.
As already stated in the comments, your scanf() format string will stop at a newline, so the next scanf() will first find that newline and it can't match [^\n] (which means anything except newline). As a newline is just another whitespace character, adding a blank in front of your conversion will silently eat it up ;)
But now for the better solution: Assuming you only want to use standard C functions, there's already one function for exactly the job of reading a line: fgets(). The following code snippet should explain its usage:
char line[1024];
char *str = fgets(line, 1024, stdin); // read from the standard input
if (!str)
{
// couldn't read input for some reason, handle error here
exit(1); // <- for example
}
// fgets includes the newline character that ends the line, but if the line
// is longer than 1022 characters, it will stop early here (it will never
// write more bytes than the second parameter you pass). Often you don't
// want that newline character, and the following line overwrites it with
// 0 (which is "end of string") **only** if it was there:
line[strcspn(line, "\n")] = 0;
Note that you might want to check for the newline character with strchr() instead, so you actually know whether you have the whole line or maybe your input buffer was to small. In the latter case, you might want to call fgets() again.
How to scan total line from user input with c program?
scanf("%99[^\n]",st) reads a line, almost.
With the C Standard Library a line is
A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. C11dr §7.21.2 2
scanf("%99[^\n]",st) fails to read the end of the line, the '\n'.
That is why on the 2nd call, the '\n' remains in stdin to be read and scanf("%99[^\n]",st) will not read it.
There are ways to use scanf("%99[^\n]",st);, or a variation of it as a step in reading user input, yet they suffer from 1) Not handling a blank line "\n" correctly 2) Missing rare input errors 3) Long line issues and other nuances.
The preferred portable solution is to use fgets(). Loop example:
#define LINE_MAX_LENGTH 200
char buf[LINE_MAX_LENGTH + 1 + 1]; // +1 for long lines detection, +1 for \0
while (fgets(buf, sizeof buf, stdin)) {
size_t eol = strcspn(buf, "\n"); **
buf[eol] = '\0'; // trim potential \n
if (eol >= LINE_MAX_LENGTH) {
// IMO, user input exceeding a sane generous threshold is a potential hack
fprintf(stderr, "Line too long\n");
// TBD : Handle excessive long line
}
// Use `buf[[]`
}
Many platforms support getline() to read a line.
Short-comings: Non C-standard and allow a hacker to overwhelm system resources with insanely long lines.
In C, there is not a great solution. What is best depends on the various coding goals.
** I prefer size_t eol = strcspn(buf, "\n\r"); to read lines in a *nix environment that may end with "\r\n".
scanf() should never be used for user input. The best way to get input from the user is with fgets().
Read more: http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html
char str[1024];
char *alline = fgets(str, 1024, stdin);
scanf("%[^'\n']s",alline);
I think the correct solution should be like this. It is worked for me.
Hope it helps.

Reading the string with defined number of characters from the input

So I am trying to read a defined number of characters from the input. Let's say that I want to read 30 characters and put them in to a string. I managed to do this with a for loop, and I cleaned the buffer as shown below.
for(i=0;i<30;i++){
string[i]=getchar();
}
string[30]='\0';
while(c!='\n'){
c=getchar(); // c is some defined variable type char
}
And this is working for me, but I was wondering if there is another way to do this. I was researching and some of them are using sprintf() for this problem, but I didn't understand that solution. Then I found that you can use scanf with %s. And some of them use %3s when they want to read 3 characters. I tried this myself, but this command only reads the string till the first empty space. This is the code that I used:
scanf("%30s",string);
And when I run my program with this line, if I for example write: "Today is a beatiful day. It is raining, but it's okay i like rain." I thought that the first 30 characters would be saved in to the string. But when i try to read this string with puts(string); it only shows "Today".
If I use scanf("%s",string) or gets(string) that would rewrite some parts of my memory if the number of characters on input is greater than 30.
You can use scanf("%30[^\n]",s)
Actually, this is how you can set which characters to input. Here, carat sign '^' denotes negation, ie. this will input all characters except \n. %30 asks to input 30 characters. So, there you are.
The API you're looking for is fgets(). The man page describes
char *fgets(char *s, int size, FILE *stream);
fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte ('\0') is stored after the last character in the buffer.

Reading line by line in C

Currently to read a file line by line in C I am using:
char buffer[1024];
while(fgets(buffer, sizeof(buffer), file) != NULL) {
//do something with each line that is now stored in buffer
}
However there is no guarantee in the file that the line will be shorter than 1024. What will happen if a line is longer than 1024? Will the rest of the line be read in the next iteration of the while loop?
And how can I read line by line without a maximum length?
Yes, the rest of the line will be read in the next iteration.
You can detect whether or not you read a whole line by inspecting the last character of the string (i.e. the one before the null terminator) to see if it is '\n' or not -- fgets passes '\n' through to you.
There is no Standard C function which will read a line whilst dynamically allocating enough memory for it, however there is a POSIX function getline() which does that. You could write your own that uses fgets or otherwise to do the reading, in a loop with realloc, of course.
From the standards §7.19.7.2,
char *fgets(char * restrict s, int n, FILE * restrict stream);
The fgets function reads at most one less than the number of
characters specified by n from the stream pointed to by stream into the
array pointed to by s. No additional characters are read after a
new-line character (which is retained) or after end-of-file. A null
character is written immediately after the last character read into
the array.
From MSDN,
fgets reads characters from the current stream position to and including the first newline character, to the end of the stream, or until the number of characters read is equal to n – 1, whichever comes first. The newline character, if read, is included in the string.
So, yes fgets will read the rest of the line in next iteration if the it doesn't encounters the newline character within sizeof(buffer)-1 range.
If you want to read the whole line in one shot, then it is better to go with malloc and, if needed, reallocing the memory as per your needs.

C: fscanf - infinite loop when first character matches

I am attempting to parse a text (CSS) file using fscanf and pull out all statements that match this pattern:
#import "some/file/somewhere.css";
To do this, I have the following loop set up:
FILE *file = fopen(pathToSomeFile, "r");
char *buffer = (char *)malloc(sizeof(char) * 9000);
while(!feof(file))
{
// %*[^#] : Read and discard all characters up to a '#'
// %8999[^;] : Read up to 8999 characters starting at '#' to a ';'.
if(fscanf(file, "%*[^#] %8999[^;]", buffer) == 1)
{
// Do stuff with the matching characters here.
// This code is long and not relevant to the question.
}
}
This works perfectly SO LONG AS the VERY FIRST character in the file is not a '#'. (Literally, a single space before the first '#' character in the CSS file will make the code run fine.)
But if the very first character in the CSS file is a '#', then what I see in the debugger is an infinite loop -- execution enters the while loop, hits the fscanf statement, but does not enter the 'if' statement (fscanf fails), and then continues through the loop forever.
I believe my fscanf formatters may need some tweaking, but am unsure how to proceed. Any suggestions or explanations for why this is happening?
Thank you.
I'm not an expert on scanf pattern syntax, but my interpretation of yours is:
Match a non-empty sequence of non-'#' characters, then
Match a non-empty sequence of up to 8999 non-';' characters
So yes, if your string starts with a '#', then the first part will fail.
I think if you start your format string with some whitespace, then fscanf will eat any leading whitespace in your data string, i.e. simply " %8999[^;]".
Oli already said why fscanf failed. And since failure is a normal state for fscanf your busy loop is not the consequence of the fscanf failure but of the missing handling for it.
You have to handle a fscanf failure even if your format would be correct (in your special case), because you cannot be sure that the input always is matchable by the format. Actually you can be sure that much more nonmatching input exists than matching input.
Your format string does the following actions:
Read (and discard) 1 or more non-# characters
Read (and discard) 0 or more whitespace characters (due to the space in the format string)
Read and store 1 to 8999 non-; characters
Unfortunately, there is no format specifier for reading "zero or more" characters from a user-defined set.
If you don't care about multiple #include statements on a line, you could change your code to read a single line (with fgets), and then extract the #include statement from that (if the first character does not equal #, you can use your current format string with sscanf, otherwise, you could use sscanf(line, "%8999[^;]", buffer)).
If multiple #include statemens on a line should be handled correctly, you could inspect the next character to be read with getc and then put it back with ungetc.

Resources