Trouble reading a line using fscanf()

Question

I'm trying to read a line using the following code:

while(fscanf(f, "%[^\n\r]s", cLine) != EOF )
{
    /* do something with cLine */
}

But somehow I get only the first line every time. Is this a bad way to read a line? What should I fix to make it work as expected?

Answer 1

It looks to me like you're trying to use regex operators in your fscanf string. The string [^\n\r] doesn't mean anything to fscanf, which is why your code doesn't work as expected.

Furthermore, fscanf() doesn't return EOF if the item doesn't match. Rather, it returns an integer that indicates the number of matches--which in your case is probably zero. EOF is only returned at the end of the stream or in case of an error. So what's happening in your case is that the first call to fscanf() reads all the way to the end of the file looking for a matching string, then returns 0 to let you know that no match was found. The second call then returns EOF because the entire file has been read.

Finally, note that the %s scanf format operator only captures to the next whitespace character, so you don't need to exclude \n or \r in any case.

Consult the fscanf documentation for more information: http://www.cplusplus.com/reference/clibrary/cstdio/fscanf/

Answer 2

It's almost always a bad idea to use the fscanf() function as it can leave your file pointer in an unknown location on failure.

I prefer to use fgets() to get each line in and then sscanf() that. You can then continue to examine the line read in as you see fit. Something like:

#define LINESZ 1024
char buff[LINESZ];
FILE *fin = fopen ("infile.txt", "r");
if (fin != NULL) {
    while (fgets (buff, LINESZ, fin)) {
        /* Process buff here. */
    }
    fclose (fin);
}

fgets() appears to be what you're trying to do, reading in a string until you encounter a newline character.

Answer 3

Your loop has several issues. You wrote:

while( fscanf( f, "%[^\n\r]s", cLine ) != EOF ) 
    /* do something */;

Some things to consider:

1. fscanf() returns the number of items stored. It can return EOF if it reads past the end of file or if the file handle has an error. You need to distinguish a valid return of zero in which case there is no new content in the buffer cLine from a successfully read.

2. You do a have a problem when a failure to match occurs because it is difficult to predict where the file handle is now pointing in the stream. This makes recovery from a failed match harder to do than might be expected.

3. The pattern you wrote probably doesn't do what you intended. It is matching any number of characters that are not CR or LF, and then expecting to find a literal s.

4. You haven't protected your buffer from an overflow. Any number of characters may be read from the file and written to the buffer, regardless of the size allocated to that buffer. This is an unfortunately common error, that in many cases can be exploited by an attacker to run arbitrary code of the attackers choosing.

5. Unless you specifically requested that f be opened in binary mode, line ending translation will happen in the library and you will generally never see CR characters, and usually not in text files.

You probably want a loop more like the following:

while(fgets(cLine, N_CLINE, f)) {
    /* do something */ ;
}

where N_CLINE is the number of bytes available in the buffer starting a cLine.

The fgets() function is a much preferred way to read a line from a file. Its second parameter is the size of the buffer, and it reads up to 1 less than that size bytes from the file into the buffer. It always terminates the buffer with a nul character so that it can be safely passed to other C string functions.

It stops on the first of end of file, newline, or buffer_size-1 bytes read.

It leaves the newline character in the buffer, and that fact allows you to distinguish a single line longer than your buffer from a line shorter than the buffer.

It returns NULL if no bytes were copied due to end of file or an error, and the pointer to the buffer otherwise. You might want to use feof() and/or ferror() to distinguish those cases.

Answer 4

If you try while( fscanf( f, "%27[^\n\r]", cLine ) == 1 ) you might have a little more luck. The three changes from your original:

• length-limit what gets read in - I've used 27 here as an example, and unfortunately the scanf() family require the field width literally in the format string and can't use the * mechanism that the printf() can for passing the value in
• get rid of the s in the format string - %[ is the format specifier for "all characters matching or not matching a set", and the set is terminated by a ] on its own
• compare the return value against the number of conversions you expect to happen (and for ease of management, ensure that number is 1)

That said, you'll get the same result with less pain by using fgets() to read in as much of a line as will fit in your buffer.

Answer 5

Using fscanf to read/tokenise a file always results in fragile code or pain and suffering. Reading a line, and tokenising or scanning that line is safe, and effective. It needs more lines of code - which means it takes longer to THINK about what you want to do (and you need to handle a finite input buffer size) - but after that life just stinks less.

Don't fight fscanf. Just don't use it. Ever.