How do you fill an array with strings in C?

Question

I'm trying to fill an array with names from a file:

Andrew
Andy
Bob
Dan
Derek
Joe
Pete
Richy
Steve
Tyler

Here is the function I wrote...but the program crashes when I run it:

#include <stdio.h>

main(){
  int i=0, size=10;
  char fname[15];
  char** data;
  char* name;
  FILE* fp;

  printf("Enter a filename to read names:\n");
  scanf("%s", fname);

  fp = fopen(fname, "r");
  if(fp == NULL)
    exit();

  data = (char**)malloc(sizeof(char**)*size);

  while(!feof(fp)){
    fscanf(fp, "%s", name);
    data[i] = (char*)malloc(sizeof(name));
    data[i] = name;
    i++;
  }

  fclose(fp);

  printf("\n\n");

  for(i=0; i<size; i++)
    printf("%s ", data[i]);

  free(data);
}

Anyone know what I'm doing wrong? Thanks

Ziezi · Accepted Answer · 2017-06-15 17:30:28Z

8

You have a couple of errors here:

1) You never allocate memory where name will get stored. You could get around this simply by using:

char name[128];

Then, when you use fscanf, you'd have:

fscanf(fp, "%127s", name); // stores this in name correctly, now...

2) You're inappropriately allocating space for data[i]:

data[i] = (char*)malloc(sizeof(name));

This will allocate space to hold one pointer to char (char*), since name is a char*. You need to be doing:

data[i] = (char*)malloc(sizeof(char) * (strlen(name) + 1 ) );

This will allocate enough space for the data plus a single terminating character.

3) You aren't assigning data[i] correctly. You can't just use = here, but need to use strcpy:

strcpy(data[i], name);

4) You're not freeing the individual pointers in the data[..] elements. You should add, after your printf, a free:

for(i=0; i<size; i++)
{
    printf("%s ", data[i]);
    free(data[i]);  // Free each pointer you allocate
}

Every malloc call should eventually have a matching free call.

edited Jun 15, 2017 at 17:30

Ziezi

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answered Feb 20, 2010 at 0:58

Reed Copsey

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3 Comments

user124493 Over a year ago

Reed, I'm not going to downvote because you answered succintly and correctly, but you aren't helping him when you don't make him think about the process.

Andrew Over a year ago

Great thanks a lot, it is working now. And yes I now understand what I did wrong.

Reed Copsey Over a year ago

@San Jacinto: I don't know - I felt like this was clear,but not just rewriting all of his code. Hard to help without pointing out why things are flaws... but I do see your point. @Andrew: I'm glad it's working, and you understand it now.

Hogan · Accepted Answer · 2010-02-20 00:55:59Z

5

You never allocate space for name. But you put stuff in on this line:

 fscanf(fp, "%s", name);

change

 char name[100];

and

fscanf(fp, "%99s", name);

you should limit the imput to filename too:

scanf("%14s", fname);

also you never free the data for each element in the array, if this were a sub-routine in a larger system it would have a memory leak.

edited Feb 20, 2010 at 0:55

answered Feb 20, 2010 at 0:50

Hogan

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9 Comments

Andrew Over a year ago

Oh...but how do I know how much space to allocate if I don't know how long the name is going to be? Do I have to scan characters on every line and count?

kibitzer Over a year ago

you should have a line length limit; create a buffer and scan into it - the rest will be truncated. If you want an endlessly long lines possible, you should dynamically reallocate the buffer until everything is read. Then allocate exactly the number of bytes in the name (+1) and strncpy the name into it.

Hogan Over a year ago

I alway just made a really big buffer. But you can scan first and count if you want. For a homework assignment you can just make a big buffer IMHO

Andrew Over a year ago

Ok, now the program doesn't crash...but when I print it prints Tyler Tyler Tyler...10 times.

Hogan Over a year ago

right you have to allocate space for name in data and do a strcpy not an assignment. see this answer: stackoverflow.com/questions/2241834/…

|

R Samuel Klatchko · Accepted Answer · 2010-02-20 01:01:05Z

0

There are quite a few problems. Other answer talk about name not being initialized, so I won't repeat that.

A second problem is that you are not handling EOF correctly. feof will only return true once you've tried to read just past the end of the file. So you will get an empty 11'th name.

A final problem is that the data array can only hold 10 names. Because of the second problem you'll overflow the buffer with the blank 11'th name. Furthermore, your code depends on the details of the input file - give it a different input file with more names and you'll still crash even after fixing problem 2.

answered Feb 20, 2010 at 1:01

R Samuel Klatchko

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