How to create an array of strings given an array of pointers?

Question

I am given a text file and I need to put it in a buffer and use get_lines to make an array of pointers after converting each line to a string. I am having trouble with just the get_lines function as I am getting a seg fault when run it.

Here's my code:

#include <stdlib.h>
#include <stdio.h>

int readfile(FILE *fp, char **cbuf);       
char **get_lines(char *cbuf, int bufsize, int word);
int readword(FILE*tmp);

int main(int argc, char *argv[])
{
  int i,bufsize, num_word;
  char *cbuf;
  char **lines;
  FILE *fp;
  FILE *tmp;

  if( (fp=fopen(argv[1],"r")) == NULL)
    {
      perror("ERROR: bad/no filename");
      exit(0);
    }
  tmp = fopen(argv[1],"r");
  bufsize = readfile(fp,&cbuf);
  num_word = readword(tmp);
  lines = get_lines(cbuf, bufsize, num_word) ; 
  i=0;
  while( lines[i] != NULL) { 
    printf("%i\t%s\n",i,lines[i]); 
    i++;
    }
  return 0;
}

int readfile(FILE *fp,char**cbuf)
{
  int i;
  char c;

  fseek(fp, 0L, SEEK_END);
  int bufsize = ftell(fp);
  fseek(fp, 0L, SEEK_SET);

  *cbuf = (char *)malloc(sizeof(char) * bufsize);

  for (i = 0; i < bufsize; i++)
    {
      c = fgetc(fp);
      (*cbuf)[i] = c;
    }
  return bufsize;
}

int readword(FILE*tmp)
{
  int word = 0;
  char c;

  while((c = fgetc(tmp)) != EOF )
    {
      if (c == '\n')
      word++;
    }
  return word;
}

char **get_lines(char *cbuf, int bufsize, int word)
{
  int i = 0, j = 0, counter = 0;
  char (*lines)[bufsize];
  lines = (char**)malloc(sizeof(char*)*bufsize);

  counter = cbuf;  
  for (i = 0; i < bufsize; i++)
    {
      if(cbuf[i] == '\n')
    {
      cbuf[i] == '\0';
      counter = cbuf[i + 1];
      j++;
    }else
    {
      *lines[j] = &counter;
    }
    }

  lines[word] == NULL;
  return lines;
}

The violation causing the fault is not immediately obvious to me, can someone tell me what might be wrong in get_lines()?

I've improved the wording of your question for clarity, and added the C tag to it so folks will be more likely to notice and answer it. Good luck! — user50049
– user50049, Commented Mar 11, 2015 at 0:24
char (*lines)[bufsize]; you are declaring a pointer, pointing to a char array, which is reasonable, but lines = (char**)malloc(sizeof(char*)*bufsize); makes a lot of mistakes. you should have lines = malloc(sizeof *lines);. — Jason Hu
– Jason Hu, Commented Mar 11, 2015 at 1:04
too much inconsistency in your code, you should explain what get_lines does in plain English in details. — Jason Hu
– Jason Hu, Commented Mar 11, 2015 at 1:06

M.M · Accepted Answer · 2015-03-11 01:32:50Z

1

This code is wrong:

char (*lines)[bufsize];
lines = (char**)malloc(sizeof(char*)*bufsize);

It allocates a pointer to an array of char. Then you malloc the wrong amount of space, cast to the wrong type, and write *lines[j] = &counter; which tries to store a pointer in a char.

You should get many compiler errors/warnings for the get_lines function. It's important to pay attention to such messages as they are telling you that something is wrong with your code. There's no point even starting to investigate a segfault until you have fixed all the errors and warnings.

See here for a great guide on how to debug your code; I suspect you would fail the rubber duckie test on the get_lines function.

Here is a fixed version (untested):

// Precondition: cbuf[bufsize] == '\0'
//
char **get_lines(char *cbuf, size_t bufsize, size_t num_lines)
{
    // +1 for the NULL termination of the list
    char **lines = malloc((num_lines + 1) * sizeof *lines);

    size_t line = 0;
    while ( line < num_lines )
    {
        lines[line++] = cbuf;
        cbuf = strchr(cbuf, '\n');

        if ( !cbuf )
            break;

        *cbuf++ = '\0';
    }

    lines[line] = NULL;
    return lines;
}

In your existing code there is no room to write the null terminator for the last line; my advice is to make readfile actually malloc one extra byte and make sure that is set to 0.

edited Mar 11, 2015 at 1:32

answered Mar 11, 2015 at 1:14

M.M

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4 Comments

Jimmy Krew Over a year ago

Hey thanks for all the help! When I debugged the program using your get_lines My output consisted of the first word repeating itself for the three words in the text file. i got a warning that said there was an 'implicit declaration of built in function' for the strchr and i'm thinking that might be it. Do you know what's wrong?

M.M Over a year ago

@JamesKang you need #include <string.h>

Jimmy Krew Over a year ago

Thanks it got rid of the warning! but the output still only repeats the first line of the text for some reason.

M.M Over a year ago

@JamesKang I copy-pasted your code, replacing your get_lines with mine; and updating the get_lines prototype and doing #include <string.h> and it works almost perfectly (there is a bug in readword but not important at this stage). Check that your code actually matches what is posted on this thread, and that the text file contains what you think it does. If there is still a problem then post a new question that contains your exact code and also the input file you are using.

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