CODEWITHSUNDEEP
cstring
Alex
Alex

Reputation: 2299

Advancing one character past strrchr in C

I feel like this should be an easy thing to do but I keep getting segmentation fault errors. I'm trying to find the last occurrence of a character, '/' and then print one past that character.

This is mainly just for printing out a file, so that this:

directory1/directory2/hello.txt

should print out like this:

hello.txt

I'm trying to use strrchr to do this, but since it returns the last occurence of a character, It prints out this:

/hello.txt

I thought I could just advance the character pointer one past the slash, and then print that, but I get a segfault doing this:

void print_path(char *dir_name)
{
    char *c_ptr = strrchr(dir_name, '/');
    *c_ptr = *c_ptr + 1;
    printf("%s\n", c_ptr);
}

I also looked around and tried doing a method where I copy c_ptr to a temp variable, add one to the new temp string and then add a null character to the end, but that also caused a fault.

What should I do?

Upvotes: 0

Views: 152

Answers (2)

huderlem
huderlem

Reputation: 251

The problem is with this line:

*c_ptr = *c_ptr + 1;

You're dereferencing c_ptr. That means you're getting the value of '/' + 1, whatever that might be. You want this instead:

void print_path(char *dir_name)
{
    char *c_ptr = strrchr(dir_name, '/');
    c_ptr = c_ptr + 1;
    printf("%s\n", c_ptr);
}

Upvotes: 0

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53016

You should do pointer arithmetic, this is wrong:

*c_ptr = *c_ptr + 1; // incorrect

it should be

c_ptr = c_ptr + 1;

or

c_ptr++;

or

c_ptr += 1;

in *c_ptr = *c_ptr + 1 you are dereferencing the pointer and adding to the previous value at the begening of the string, a 1.

After strrchr c_ptr will point to '/' character, i.e. *c_ptr == '/' so

*c_ptr = *c_ptr + 1;

is equivalent to

c_ptr[0] = c_ptr[0] + 1;

i.e.

c_ptr[0] = '/' + 1;

and after that, c_ptr[0] contains the character '0', so the printf() must be printing 0hello.txt

If the substring wasn't found in the source string, strrchr() will return NULL, so you should check for that too.

A fixed version of your program would be

void print_path(char *dir_name)
{
    char *c_ptr = strrchr(dir_name, '/');
    if (c_ptr != NULL)
    {
        c_ptr = c_ptr + 1;
        printf("%s\n", c_ptr);
    }
}

Upvotes: 4

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