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

Answer 1

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

Answer 2

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 *)?

Answer 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

Answer 4

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.