`read` in C hangs the program when string of length more than 16 entered

Question

I made following program that uses read (system call in C) to get a string from user (of length lesser than 100).

#include<stdio.h>
#include <unistd.h>
#include <string.h>
int main() {
char *s;
int a = read(0, s, 100);
s[a-1] = '\0';
printf("\"%s\" \n read returned: %i; NUL at: %u", s, a, strlen(s));
return 0;
}

What I expected here is, it will get characters until user enters a new-line character. It will then replace that '\n' character by '\0', and print it.

The program works well until I enter 15 or less characters in stdin, but stops working when there are more than 16 characters.

My inputs are as follows:

E:\My Files\Codes>a.exe
1234567890123456
"1234567890123456"
 returned = 17; length = 16
E:\My Files\Codes>a.exe
12345678901234567
[My program hanged on this input.]

Why does it only hangs on 16? What is special in this 2^2? Post script: I used the string.h just to get the length of the string. Once my program starts working good, I will remove it.

`s` is a pointer that doesn't point to anything. Try `char s[100];` or use `malloc` to get some memory for `s`, i.e. `char *s = malloc(100);` — user3386109, Apr 05 '18 at 21:39
FYI - this is "Undefined Behavior." in the c standards world — pm100, Apr 05 '18 at 21:40
Using an uninitialized pointer causes undefined behavior. It's just coincidence that the hang happens on a certain size of input. — Barmar, Apr 05 '18 at 21:40
_sigh_ the 113444223th time: a pointer is not an array! Why do you use a pointer, not an array? — too honest for this site, Apr 05 '18 at 21:40
Sorry, community. Now it's working when I've initialized the `s` (either by making it an array, or by `malloc`). Thanks, community. — Suraj, Apr 05 '18 at 21:43
Possible duplicate of [How to use sigsuspend](https://stackoverflow.com/questions/16041754/how-to-use-sigsuspend) — Scott Stensland, Apr 05 '18 at 21:43

JuMoGar · Accepted Answer · 2018-04-05T22:48:34.043

I have been testing your code. The faults is: You have a pointer which points nowhere. I solve it reserving and allocating memory for your string (char array). I will post the working code:

#include <stdlib.h> // It is needed for malloc, free, etc...
#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main() {
    char *s = malloc(100*sizeof(char)); // Allocate memory with malloc
    int a = read(0, s, 100);
    s[a-1] = '\0';
    printf("\"%s\" \n read returned: %i; NUL at: %u", s, a, strlen(s));
    free(s); // You need liberate memory before exit
    return 0;
}

Also, other way to solve this, without dynamic memory is:

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

int main() {
    char s[100]; // s is a char array of 100 elements
    int a = read(0, s, 100);
    s[a-1] = '\0';
    printf("\"%s\" \n read returned: %i; NUL at: %u", s, a, strlen(s));
    return 0;
}

`read` in C hangs the program when string of length more than 16 entered

1 Answers1