C program crasher or gives unexpected output

Asked Sep 08 '20 at 19:43

Active Sep 08 '20 at 19:43

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I was given this code to look at and find the problem of why its crashing or giving unexpected output. I can not seem to find it so a little hint or help would be great.

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

char* strdup(const char *s)
{
    size_t len = strlen(s);
    char d[len+1];
    return strncpy(d, s, len+1);
}

int main(int argc, char *argv[])
{
    int i;
    for(i = 1; i < argc; i++){
        puts(strdup(argv[i]));
    }

    return 0;
}

asked Sep 08 '20 at 19:43

Shahin Mamedov

6

Hint: Your local `strdup()` function returns a `char *`. To what does that pointer point? – John Bollinger Sep 08 '20 at 19:46
1

@EugeneSh. The one I found is actual C++ ... but it's the same problem and the top answer there is equally relevant to C, IMHO. – Adrian Mole Sep 08 '20 at 19:59
@JohnBollinger it tries to return copied character which is const char *s – Shahin Mamedov Sep 08 '20 at 20:49
No, @ShahinMamedov, it does not. Pointers point to *objects*, not to *values*. To what object does the pointer point? – John Bollinger Sep 08 '20 at 20:55
@JohnBollinger to char *argv? – Shahin Mamedov Sep 08 '20 at 20:58
Now you're just guessing. Your function returns the return value of its `strncpy()` call, but what's that? It's understandable if you don't know off the top of your head what `strncpy()` returns, but you need to know how to *look that up*. Try Googling "man strncpy". – John Bollinger Sep 08 '20 at 21:04
@JohnBollinger I know what strncpy is, I know that strdup returns d, copied from s with length. – Shahin Mamedov Sep 08 '20 at 21:08
And `d` is a *local* array. Here ends the hint. – John Bollinger Sep 08 '20 at 21:43

C program crasher or gives unexpected output

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