Strange behavior with function strrchr

Question

I get some strange result with strrchr. please help me.

Here is my code:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main()
{

    char szString[32] = "hello-world-and-everyone";
    char* ptemp = NULL;
    ptemp = strrchr(szString, '-');
    if (ptemp == NULL)
    {
        printf("Invalid string\n");
        return 1;
    }
    printf("%s\n", szString);
    printf("%s\n", ptemp );
    *ptemp = 0; 
    printf("%s\n", szString);
    return 0;
}

Expected result:

hello-world-and-everyone
-everyone
hello-world-and-everyone

Acutal result:

hello-world-and-everyone
-everyone
hello-world-and

score 4 · Answer 1 · answered Jan 17 '14 at 03:49

The actual results are correct (which should come as no surprise, really).

The first two lines of output agree with your expectations, so I assume that you understand them well. Here is what's happening in the last two lines: the first line (the assignment) null-terminates the szString at the position of the last dash:

*ptemp = 0; // Replaces the last dash '-' with '\0'

so the string becomes

h e l l o - w o r l d - a n d \0 e v e r y o n e \0

When you print it now, printf finds the first terminator, and stops there. That is why you see the truncated string in the output, rather than the original one.

Brian Roach · Answer 2 · 2014-01-17T04:01:58.440

3

strrchr returns a pointer to the last occurrence of - in szString.

Here:

*ptemp = 0;

You set the value it points at to 0 (NULL), replacing the -. So when you print szString it now ends there because strings are terminated using 0.

Change it to:

*ptemp = 65; // ASCII 'A'

And your output would be:

hello-world-and-everyone
-everyone
hello-world-andAeveryone

edited Jan 17 '14 at 04:01

answered Jan 17 '14 at 03:48

Brian Roach

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score 1 · Answer 3 · answered Jan 17 '14 at 03:48

strrchr doesn't allocate and copy into a new string. It returns a pointer into the existing string where the given character is. So, when you did this:

*ptemp = 0;

You set the character at that location in your original string to NULL (0). and ended up with:

hello-world-and

score 1 · Answer 4 · answered Jan 17 '14 at 03:48

Strings in C are zero terminated, meaning a character with a value of 0 marks the end of the string. When you did *ptemp = 0 you inserted such a character at the start of the found string, which is now the end of the original string.

Perhaps you meant to null the pointer instead by doing ptemp = 0 (no *)?

Strange behavior with function strrchr

4 Answers4