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

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()?

Answer 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.