I can't understand an error in a function

Question

This function make a strange error after using it several times and I really can't understand the reason behind it.

    char *get_range(char *str,int min,int max){
    char *_res=(char *)malloc(sizeof(str));
    int cur=0;
    while (min<max){
        _res[cur]=str[min];
        min++;
        cur++;
        }
    return _res;
    }

The problem is that after using this function several times, the output comes with additional chars and I don't understand why.

Notice: The additional chars are allway used returned by the function beffor

`malloc(sizeof(str));` reserves enough memory for a string of 7 characters at most. You probably should change this to `malloc(strlen(str)+1);`. — r3mainer, Jul 14 '17 at 22:11
1) `sizeof(str)` is pointer (`char *`) size. 2) `_res` doesn't null-terminated. — BLUEPIXY, Jul 14 '17 at 22:11
https://stackoverflow.com/questions/49046/different-sizeof-results — Retired Ninja, Jul 14 '17 at 22:13
@squeamishossifrage although correct its very misleading. you dont say why 7. (or 3). I know why but I had to think about it a bit. — pm100, Jul 14 '17 at 22:26
It sounds like you want to return a substring. See [this](https://ideone.com/xkOapz), it may help. — Bob__, Jul 14 '17 at 22:56

pm100 · Answer 1 · 2017-07-15T00:13:34.670

2

  char *_res=(char *)malloc(sizeof(str));

is wrong. sizeof(str) is measuring the size of a char pointer. This is either 4 or 8 (typically) depending on your system (32 or 64 bit).

You need

char *_res=(char *)malloc(strlen(str) + 1);

strlen returns the number of characters in the string, and you need to add 1 for the terminating 0;

Second you have to add a terminating zero at the end, do:

_res[cur] = '\0';

before returning

edited Jul 15 '17 at 00:13

answered Jul 14 '17 at 22:28

pm100

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Really thank you ....i have another question if i am working on a array of strings......I should add the terminating char manually to every string is this true ?! – Aly Mohamed Jul 15 '17 at 10:48
every c string must have a '0' at the end, thats the definition of a string in C – pm100 Jul 16 '17 at 22:36

I can't understand an error in a function

1 Answers1