Thread: strspn problems with '#' character.

I have noticed a problem that I can't explain.

When using `strspn`, it won't recognize the pound symbol unless it's the first character in the string.

Create 2 files:

Code:

touch "#here" "there#"

Compile this code:

Code:

#include <stdio.h>
#include <string.h>

int main (int argc, char *argv[])
{
    printf("Location: %lu\n", (argv[1], "#"));
    return(0);
}

When you run `program \#here` it lists the location as 1, which is correct. When you run `program there#` it says the position is 0, which is wrong.

I'm using `strspn` to search for a bunch of special characters, and every other character seems to work with it, except the pound symbol. I understand I won't be able to change this behaviour, but why is it happening?

You left out strspn in your example code, right?

However, what shell do you use to call your program?

Oh, right. However, # is also a character with special meaning in shell languages. It introduces comments for example in bash or PowerShell.

Originally Posted by aGerman

You left out strspn in your example code, right?

However, what shell do you use to call your program?

Sorry, must have cut the function name instead of copying it.

Code:

#include <stdio.h>
#include <string.h>
 
int main (int argc, char *argv[])
{
    printf("Location: %lu\n", strspn(argv[1], "#"));

    return(0);

}

I use Bash shell, and I am starting to think this may be the problem. I just don't know why it's happening with filenames.

Ok, this must be related to `strspn`:

Code:

#include <stdio.h>
#include <string.h>

int main (int argc, char *argv[])
{
    printf("Location: %lu\n", strspn(argv[1], "#"));

    for(size_t x = 0; x < strlen(argv[1]); x++)
        putchar(argv[1][x]);

    return(0);
}

The loop shows that the pound symbol is still a pound symbol inside the program. I guess I won't use `strspn`, I'll have to step through each individual character.

Originally Posted by john.c

strcspn does what you want. The 'c' stands for compliment, and it counts how many leading characters of your string are not any of the given characters. It returns the length of the string if none of the given characters are in the string.

Code:

    size_t n = strcspn(argv[1], "#");
    if (n < strlen(argv[1]))
        printf("First # is at position %zu\n", n);  // %zu is correct for size_t
    else
        printf("# is not in the string\n");

You could also use strchr and subtract the original string start from the result (if not NULL).

Ty, I will look into these.

Ok, `strcspn` doesn't work. `strcspn` does the exact opposite of my first problem. Yes, it solves the problem of the location with `there#`, but not `#here`. And `strchr` won't work without extra effort. Here is the full scope of what I am trying to do.

I want to remove special characters from filenames. If it is marked for tv, the special character is encoded, to keep the filename safe for processing. It's nice to have an '!' displayed in a filename, without the filename having an '!'. And if it isn't for tv, the character is removed by replacing it with a '.'.

Code:

#define SPECIAL_CHARS      "':,\"?&!()$/#;_<>*%|^@"

void fix_file (char *file, int tv_flag)
{
// Declare variables.
    char new_filename[FILENAME_SIZE] = {0};

// Loop through all characters.
    for(size_t x = 0; x < strlen(file); x++)
// If it is a special character.
        if(strspn(&file[x], SPECIAL_CHARS) == 1)
// If it needs to be encoded.
            if(tv_flag == FLAG_YES)
                add_character(file[x], new_filename);
            else
            {
// Remove character.
                new_filename[strlen(new_filename) + 1] = '\0';
                new_filename[strlen(new_filename)] = '.';
            }
        else
        {
// Keep the character.
            new_filename[strlen(new_filename) + 1] = '\0';
            new_filename[strlen(new_filename)] = file[x];
        }
}

If I go the `strchr` route, which would work, I would need a bunch of tests for all the special characters I'm searching for. I was hoping to use this function, because I liked the ability of skipping directory entires '.' and '..'

Code:

// Skip '.' and '..' entries.
        if(dir->d_name[strspn(dir->d_name, ".")] == '\0')
            continue;

I guess I could make one test for '#' with `strchr` outside of the other search.

Thanks for your input guys.

Originally Posted by john.c

What do you mean by with `#here` strcspn says the position is 0, so it works.

Your right. My local copy of `man strcspn` does not list what errors, or what happens if it is not found. I read somewhere that `strspn` returns 0 when not found. But, I wrongly assumed that 0 in `strcspn` had the same meaning. Ty for clearing this up, this will work.

Originally Posted by john.c

If file[x] is a single character (I don't know what the & is for)

The & is because both functions require a string to search. This is why I was testing for 1, it means the first character is special. If the character is not special, I have to copy the character as is.

Originally Posted by john.c

then you could use strchr backwards, strchr(SPECIAL_CHARS, file[x])

That fixes that problem. Thanks.

Originally Posted by Yonut

The & is because both functions require a string to search. This is why I was testing for 1, it means the first character is special. If the character is not special, I have to copy the character as is.

But strspn returns 1 only when the first character is special and the next character is not special. So strspn("##foo", SPECIAL_CHARS) will return 2.

strspn and strcspn are the wrong tools for this job, as you've found.

strspn() and strcspn() counts how many chars of a set are or aren't in the original string since it's beginning:

Code:

  size_t sz = strspn( "#!#abc", "#!" ); // will return 3 because '#' and '!' are the 3 first chars.

strcspn() is complementary to this span count. It will count how many chars from the set aren't in the original string, since its beginning:

Code:

  size_t sz = strcspn( "abc#!#", "#!" ); // will return 3 because 'a', 'b' and 'c' aren't '#' or '!'

That's it. If you use:

Code:

  size_t sz = strspn ( "abc#", "#!" ); // will return 0, because 'a' isn't '#' or '!'.

Originally Posted by christop

But strspn returns 1 only when the first character is special and the next character is not special. So strspn("##foo", SPECIAL_CHARS) will return 2.

strspn and strcspn are the wrong tools for this job, as you've found.

Actually, following the course of the conversation, you would notice that I changed my use of `strspn` to `strcsnp`, which does the following:

1) If there are no special characters, it returns the length of the original string.
2) If it does start with a special character, it will return 0.

That seems to be what I need. Can you expand on your comment:

Originally Posted by christop

strspn and strcspn are the wrong tools for this job

This is what I test to see if I need to change the filename:

Code:

if(strlen(filename) != strcspn(filename, SPECIAL_CHARS))

And this is what I use while changing the filename.

Code:

for(size_t x = 0; x < strlen(filename); x++)
    if(strchr(SPECIAL_CHARS, filename[x]) != NULL)

I'm going character by character. If the next 2 characters are special, that's great, but I'm still only dealing with the first character. The second one can wait till next round.