Question

I’m testing this function that’s supposed to read input from the user but it throws me a segmentation fault

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

#define MAX_STRING_LENGTH 10

int 
readinput(char *input)
{
    int c;
    int i=0;
    while((c=getchar()) != EOF && c != '\n')  //Here is where the warning occurs.
    {
        input[i]=c;
        i++;
    }
    input[i]=0;
    if(strlen(input)>0)
    {
        if(isalpha(input[0]) && input[1]=='-' && isalpha(input[2]) && strlen(input)==3)
            return 0;
        else if(!strcmp(input, "quit"))
            return 1;
        else if(!strncmp(input, "save ", 5))
            return 2;
        else if(!strcmp(input, "undo"))
            return 3;
    }
    return -1;
}

int main()
{
   char *str;
   printf("write a string\n");
   int nr=readinput(str);
   printf("%d\n", nr);
   printf("%s\n", str);
   return 0;
}

I did notice the stupid error I made, but still, segmentation fault, why?

Answer 1

The segmentation fault arises because you’ve passed an uninitialized pointer to the function readinput(). You need to do something like:

char str[4096];

int nr = readinput(str);

You should pass in a length of the array so that the called code can verify that it does not overflow its boundaries. Or you can live dangerously and decide that 4096 is big enough, which it probably will be until someone is trying to break your program deliberately.

---

The original compiler warning was because the multi-character constant '/n' has a value (of type int) which is outside the range of values that can be stored in a char, so when c is promoted to int, the != comparison with the (implementation-defined) value of '/n' is bound to be true. Hence the warning:

Warning: comparison is always true due to limited range of data type

All multi-character character constants have implementation-defined values. There are no portable multi-character character constants.

ISO/IEC 9899:2011 §6.4.4.4 Character constants

¶10 … The value of an integer character constant containing more than one character (e.g.,
'ab'), or containing a character or escape sequence that does not map to a single-byte
execution character, is implementation-defined. …