CODEWITHSUNDEEP
c
mathjacks
mathjacks

Reputation: 335

small fopen/fwrite issue. Suggestions please

Can anyone tell me what's wrong with this code? I'm getting a seg fault. I'm trying to read just the first line of a file into a newly-created file.

char *buffer;

int main(int argc, char *argv[])
{
  FILE *source = fopen(argv[0], "r");
  FILE *destination = fopen("destination", "w");

  fgets(buffer, 500, source);
  fwrite(buffer, 1, sizeof(buffer), destination);

}

Upvotes: 2

Views: 238

Answers (3)

Sangeeth Saravanaraj
Sangeeth Saravanaraj

Reputation: 16597

You should allocate memory to store the data. It can be either statically allocated using arrays or dynamically allocated using malloc()

#define BUFLEN 50

/* static allocation */    
char buffer_array[BUFLEN];

/* dynamic allocation */
char * buffer_ptr = NULL;
if ((buffer_ptr = (char *)malloc((int)sizeof(char) * BUFLEN)) == NULL) {
    printf("ERROR: unable to allocate memory \n");
    return 0;
}

and here you can pass either buffer_array or buffer_ptr to fgets() and fwrite()

But all the dynamically allocated memory must be free()'ed as follows.

free(buffer_ptr);

Upvotes: 0

Mysticial
Mysticial

Reputation: 471229

You haven't allocated anything for buffer.

Change:

char *buffer;

to

char buffer[500];

As your code is right now, buffer is just an uninitialized pointer. Attempting to dereference it will cause undefined behavior. (and seg-fault in your case)

Alternatively, you can dynamically allocate memory for buffer:

buffer = (char*)malloc(500 * sizeof(char));

but you should remember to free the memory later on:

free(buffer);

If you go with this latter method, the code will look like this:

char *buffer;

int main(int argc, char *argv[])
{
  FILE *source = fopen(argv[0], "r");
  FILE *destination = fopen("destination", "w");

  //  Allocate
  buffer = (char*)malloc(500 * sizeof(char));

  fgets(buffer, 500, source);
  fwrite(buffer, 1, 500 * sizeof(char), destination); //  Fixed here

  //  Free memory
  free(buffer);

  //  Don't forget return value
  return 0;
}

Upvotes: 6

Adrian Cornish
Adrian Cornish

Reputation: 23868

Your buffer has no size - its a pointer to char array. You need to malloc some space to read into

Upvotes: 4

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