How to scan [ and ] with fscanf?

[crazyfuturamanoob]

I wrote some C code:


if ( fscanf( fp, "[%s]", buf ) != 1 )
{
snprintf( error_msg, sizeof(error_msg), "%s", strerror(errno) );
return 0;
}

It always fails, with error message "Success", because [ and ] have special meaning. It should parse text like this:


[globals]

How to make fscanf read [ and ] just like any normal character?

[gmargo]

There's nothing special about brackets in a scanf string, unless the opening bracket is preceded by a percent sign(%). You have some other problem with your input stream. You can prove this to yourself by changing the bracket to some other character, like 'A'.

BTW, your scanf won't work right anyway since the %s matches up to the next whitespace; it will not stop at the closing bracket.

[dwhitney67]

How to make fscanf read [ and ] just like any normal character?
As indicated already, the [] brackets are normal characters.

I would suggest you read in the entire line, including the brackets, into a temporary buffer. Then parse the buffer using strtok().

For example:


char buf[80] = {0};
char* tag = 0;

if (fscanf(fp, "%s", buf) == 1)
{
tag = strtok(buf, "[]");

if (!tag)
{
/* error; no tag found */
}
}
else
{
/* error w/ fscanf() */
}

...


P.S. If you know the length of the string within the [] ahead of time, then you can specify how many characters within the fscanf() format statement, but IMO, this is crude.


fscanf(fp, "[%6s]", buf);

[crazyfuturamanoob]

Before I read dwhitney's post I wrote this:


void read_until( FILE *fp, int x )
{
int ch;
while( (ch=fgetc(fp)) != EOF && ch != x );
}


static int read_section_name( FILE *fp, iniSection *section, char *filename )
{
char buf[128];
long start, end, length;
char too_long = 0;

read_until( fp, '[' );
start = ftell( fp );
read_until( fp, ']' );
end = ftell( fp ) - 1;

length = end - start;

if ( length >= sizeof(buf) )
{
length = sizeof( buf );
too_long = 1;
}

fseek( fp, start, SEEK_SET );
fread( buf, length, 1, fp );
buf[length] = '\0';

section->name = strdup( buf );

if ( section->name == NULL )
{
snprintf( error_msg, sizeof(error_msg), "%s", strerror(errno) );
return 0;
}

if ( too_long )
{
fprintf( stderr, "Warning: too long section name in %s: \"%s\"...\n", filename, section->name );
fprintf( stderr, "Maximum section name length: %lu\n", sizeof(buf) - 1 );
}

read_until( fp, '\n' );

return 1;
}


Edit: Sometimes that function doesn't work. It thinks the string length is >127 when it isn't and prints the warning messages? I will try a mix of above and scanf.

[soltanis]

// scanf("%[^[][%[^]]]%s", before, inside, after); NO
ret = scanf("[%[^]]]", inside); // YES


From man 3 scanf:




%[ Matches a non-empty sequence of characters from the specified
set of accepted characters; the next pointer must be a pointer
to char, and there must be enough room for all the characters in
the string, plus a terminating null byte. The usual skip of
leading white space is suppressed. The string is to be made up
of characters in (or not in) a particular set; the set is
defined by the characters between the open bracket [ character
and a close bracket ] character. The set excludes those charac‐
ters if the first character after the open bracket is a circum‐
flex (^). To include a close bracket in the set, make it the
first character after the open bracket or the circumflex; any
other position will end the set. The hyphen character - is also
special; when placed between two other characters, it adds all
intervening characters to the set. To include a hyphen, make it
the last character before the final close bracket. For
instance, [^]0-9-] means the set "everything except close
bracket, zero through nine, and hyphen". The string ends with
the appearance of a character not in the (or, with a circumflex,
in) set or when the field width runs out.


No need to use strtok or any ugliness like that.

[dwhitney67]

scanf("%[^[][%[^]]]%s", before, inside, after);


From what I understand from the man-pages and other sources, you are correct about being able to have scanf (or fscanf) disregard certain chosen character(s), and yet accept others. So yes, this would be better than doing the strtok() approach.

Now, if we could only get the code you have presented above to actually work, then the OP would be in business. Is there perhaps a typo in the format statement?

[dwhitney67]

Now, if we could only get the code you have presented above to actually work, then the OP would be in business. Is there perhaps a typo in the format statement?

Got it...


char dontCare[2] = {0};
char tag[80] = {0};

int ret = fscanf(fp, "%[[]%[^]]", dontCare, tag);

assert(ret == 2);


P.S. The code above assumes the string "[globals]" is read from the file.

[soltanis]

Actually, you're right, but you need not use a dummy variable or a format conversion at all. Use the format string, Luke:



ret == fscanf(fp, "[%[^]]", tag);
assert(ret == 1);


I've tested that and it does work on extracting any string from inside some []'s. Of course, it assumes that the next string is enclosed in brackets and will abort if it isn't (but that's a good reason to test the return value of the function).

[crazyfuturamanoob]

I solved it in a different way:


static int read_section_name( FILE *fp, iniSection *section, char *filename )
{
char buf[128];

fgetc( fp ); // [
fgets( buf, sizeof(buf), fp );
strchr( buf, ']' )[0] = '\0';

section->name = strdup( buf );
if ( section->name == NULL )
{
SET_ERROR_FROM_ERRNO( errno );
return 0;
}

return 1;
}


I tested it and it works. PS. The next string is not enclosed in brackets so the above solution does not work.

Thanks for the replies. :)