Value of string changes upon return from function

Question

I'm writing a function that removes the last segment of the path to a file, but I'm not getting the right string back from my function. I give the function the input sample/four.txt/one.txt" as a sample parameter, and I want to get sample/four.txt back. I've tried the code on my own machine, and getOldPath always returns sample/f. Inside the getOldPath function, curPath is sample/four.txt. Once I exit and assign it to newPath in the main, newPath is sample/f. However, when I run the program on my friend's machine, he gets the expected sample/four.txt. What is causing this issue and how can I fix it? Here is the code that I am working with:

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


char *getOldPath(char *curPath)
{
    // Loop to find the count of characters

    int count;
    count = 0;

    int end;
    end = strlen(curPath);
    char tempChar;
    tempChar = curPath[end-1];

    while(tempChar != '/')
    {
            count++;
            end--;
            printf("End is: %i\n",end);
            tempChar = curPath[end];
    }


    char temp[256];
    int numChar;
    numChar = strlen(curPath) - count;
    strncpy(temp,curPath,numChar);
    curPath = temp;

    printf("The path is: %s\n",curPath);

    return curPath;
}


int main(int argc, char **argv)
{
    char *path = "sample/four.txt/one.txt";
    char *newPath = getOldPath(path);

    printf("Do we get the new path back: %s\n",newPath);

}

cnicutar · Accepted Answer · 2014-03-08 04:49:17Z

3

You are returning a pointer to an object - temp - that has automatic storage duration. What this means is that temp only exists during the lifetime of the invocation of that function. Using it after the function has returned is undefined behavior and will produce erratic results.

If you want it to persist beyond the end of the function, you can use malloc or refactor your code altogether to use caller-provided memory. For example, a drop-in replacement could be:

char *temp = malloc(256);
/* The caller must remember to free this. */

answered Mar 8, 2014 at 4:49

cnicutar

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Comments

Jonathan Leffler · Accepted Answer · 2014-03-08 06:48:07Z

1

You are returning the temp address which is a local variable in the function so it will not work. Modified your function to contain the truncated path in the input string and also used the strrchr() function. Check — it might be useful to you.

void getOldPath(char *curPath)
{
    char *ptr;

    ptr = strrchr(curPath,'/');
    if(ptr != NULL)
    {  
        *ptr = '\0';      
    }

    printf("The path is: %s\n",curPath);
}

edited Mar 8, 2014 at 6:48

Jonathan Leffler

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answered Mar 8, 2014 at 5:10

M Thotiger

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2 Comments

rafafan2010 Over a year ago

So once I replace the last backslash with the null character, what happens to the rest of the characters that appear after it?

M Thotiger Over a year ago

The characters after null character will still be in the "curPath", the string functions strlen, strcpy etc will parse the string till null character encountered so the null is replaced.

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