c - unexpected results on simple string reverse algorithm -

i'm relatively new c. wanted lern language bit solving coderbyte challenges.

but i'm stucked @ first. supposed simple string reverse algorithm.

when input things "asdf" or "1234567" output correct ("fdsa", "7654321"). when type "12345678" or "thisiscool" "87654321▒@"/"loocsisiht@" result. don't know @ comming from.

this code:

#include <stdio.h> #include <string.h>  void firstreverse(char str[]) {    int len = strlen(str);   char nstr[len];   int i;    for(i = 0; < len; i++) {       nstr[i] = *(str+len-1-i);   }    printf("%s\n", nstr);  } int main(void) {    char str[100];    firstreverse(gets(str));   return 0;  } 

can please tell me can find error? in advance :)

as other answers have mentioned, you're missing terminator. should noted it's bad practice allocate strings way did. array should have fixed size if create way.

you should instead do:

char * nstr = malloc(sizeof(char) * (len+1)); 

thereby allocating size of each character (1 byte) times lenght. note +1 because need room string terminator.

when call printf(, string); , it's gonna start first letter , print terminator. since have no terminator here, prints random characters, such @.

what you're gonna wanna fix that, adding:

nstr[i] = '\0';   

after loop.

also remember free allocated memory.