I can't understand an error in a function [closed]

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
– r3mainer, Commented Jul 14, 2017 at 22:11
1) sizeof(str) is pointer (char *) size. 2) _res doesn't null-terminated. — BLUEPIXY
– BLUEPIXY, Commented Jul 14, 2017 at 22:11
@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
– pm100, Commented Jul 14, 2017 at 22:26
It sounds like you want to return a substring. See this, it may help. — Bob__
– Bob__, Commented Jul 14, 2017 at 22:56

pm100 · Accepted Answer · 2017-07-15 00:13:34Z

 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

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 ?!
every c string must have a '0' at the end, thats the definition of a string in C

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I can't understand an error in a function [closed]

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