c - Values always do not tally -

i'm relatively new c programming , trying code program reads in user's marks , assign grade mark inputted. have used getline() function acquire user input. below code.

# include <stdio.h> # include <stdlib.h> # include <string.h> # include <ctype.h>  int getmark(); void display(char grade); char convert(int mark);  int getmark() {     {         int marks;         char * buffer;         char characters;         size_t bufsize = 16;          buffer = (char *)malloc(bufsize * sizeof(char));         if( buffer == null) {         perror("unable allocate buffer");         exit(1);         }          printf("enter score (0 - 100)\nor press enter key quit ==> ");         characters = getline(&buffer, & bufsize, stdin);         int i;         long check;         (i = 0; < sizeof(buffer); i++) {             if (buffer[i] == '\n' && != 0) {                 buffer[i] = '\0';             }              if (isdigit(buffer[i]) || buffer[i] == '\0') {                 check++;             }         }          if (buffer[0] == '\n') {             return -1;         } else if (check == strlen(buffer) - 1) {             marks = atoi(buffer);         } else {             printf("please enter integer.\n");             continue;         }          if (marks >= 0 && marks <= 100) {             return marks;         } else {             printf("sorry, marks can between 0 100.\n");             continue;         }      } while (1); }  void display(char grade) {     printf("the grade input score %c \n", grade); }  char convert(int mark) {     mark /= 10;     char grade;      switch(mark) {         case 0:         case 1:         case 2:         case 3:         grade = 'f';         break;         case 4:         grade = 'e';         break;         case 5:         grade = 'd';         break;         case 6:         grade = 'c';         break;         case 7:         grade = 'b';         break;         case 8:         case 9:         case 10:         grade = 'a';     }      return grade; }  int main() {     int marks;     (;;) {         marks = getmark();         if (marks == -1) {             break;         }         display(convert(marks));     }      return 0; } 

when press enter, program exits, expected. however, when integer entered, example 12, output please enter integer.

can me on this? thanks!

char * buffer; ... (i = 0; < sizeof(buffer); i++) {     if (buffer[i] == '\n' && != 0) { 

sizeof(buffer) returns size of pointer char (4 or 8 bytes depending on architecture), not length of string, in consequence, reading outside of bounds of array when 12 entered.

change strlen() or better yet, use result of getline:

on success, getline() , getdelim() return number of characters read, including delimiter character, not including terminating null byte ('\0').