fgets error and strings declaration style on c

Question

I'm curious about one thing, the following code works perfectly.

#include <stdio.h>
int main()
{
  FILE * fp = fopen("test.txt", "r");
  char line[100];

  while( fgets(line, sizeof(line), fp) != NULL )
    fputs(line, stdout);
  fclose(fp);
  return 0;
}

But why it is not possible to use instead:

char *line;

Without causing fgets to crash?

"works perfectly" is an interesting claim for code which can easily be made to segfault. — William Pursell, May 27 '14 at 15:36
If 'test.txt' does not exist or is not readable, fopen will fail and fgets will segfault. — William Pursell, May 27 '14 at 17:01

score 3 · Answer 1 · answered May 27 '14 at 14:41

3

It is possible. Just point it to some valid memory.

char line[100];
char* linep = line ;
while( fgets(linep, sizeof(line), fp) != NULL )...

If you don't assing line to linep, then linep points to random memory, and you cannot use that.

answered May 27 '14 at 14:41

this

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score 3 · Accepted Answer · edited May 23 '17 at 11:52

fgets crashes because it is writing the result into unallocated memory.

Without any further memory allocation, char *line points to "nothing". It points to something, but not to any valid memory location -- null if your compiler sets uninitialized values to 0, or any random memory location anywhere on or not on your system.

Even if fgets does not crash, you cannot predict what it does as writing into unallocated memory results in undefined behavior (What is undefined behavior?).

Suvarna Pattayil · Answer 3 · 2014-05-27T15:41:37.300

1

char *line and char line[100] don't mean the same. The first is a pointer to a char, we don't know if it will be a single char or an array of chars it is pointing to. The second on the other hand is an array.

fgets needs a valid location to write to. In this case, char *line doesn't have any valid location to point at, whereas char line[100] is a valid char array location

edited May 27 '14 at 15:41

answered May 27 '14 at 14:45

Suvarna Pattayil

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You seem to be confusing "stream" and "buffer". – Medinoc May 27 '14 at 15:04

fgets error and strings declaration style on c

3 Answers3