How to copy char array to a double pointer in C

Question

Im writing a fairly simple program to read a file line by line and store it into an array of lines, my program compiles fine but it crashes everytime I run it. This is my code:

#include <stdio.h> 
#include <string.h>
#define LINESIZE 512

typedef struct { 
char **data; 
size_t nused; 
} lines_t;  

lines_t readlines(FILE *fp); 

int main(int argc,char* argv[]) { 
    FILE *fp; 
   (void)argc; 
   if((fp = fopen(argv[1],"r+")) == 0) { 
      perror("fopen"); 
   }
   readlines(fp); 
   return 0; 
}

lines_t readlines(FILE *fp) {
    lines_t line_data;
    char line[LINESIZE]; 
    char temp[20];  
    int num_lines = 0; 
    (*line_data.data) = (char *)malloc(LINESIZE); 
    while(fgets(line,LINESIZE,fp)) { 
        sscanf(line,"%s\n",temp); 
        strcpy(line_data.data[num_lines], temp); /* Program crashes here */
        num_lines++;  
    } 
    return line_data; 
}

The line where I try to copy my array is giving me trouble, So my question is, How do I copy my character array temp into the char **data inside struct lines_t if I am not doing it right?

Where is char **data is being allocated memory? you need to malloc it first inside readlines — Ishmeet
– Ishmeet, Commented Jul 13, 2015 at 3:51

dreamlax · Accepted Answer · 2015-07-13 04:01:00Z

3

You are dereferencing an invalid pointer here:

(*line_data.data) = (char *)malloc(15);

line_data.data is a char **. You are trying to deference it but it is not yet set to any meaningful value. You need to allocate memory for line_data.data before you allocate memory for *line_data.data.

(char *)malloc(15) is particularly suspicious also. Where does 15 come from and what are you actually allocating memory for? Casting the result of malloc is generally considered bad practice, and in your case, rightly so, because malloc is declared in stdlib.h and you aren't including that header. If you want to allocate enough space to hold 15 char *, then use malloc(15 * sizeof(char *)) or alternatively, malloc(15 * sizeof(*line_data.data)) (here it is safe to use *line_data.data even if it doesn't point to anything, because sizeof does not evaluate its operand).

answered Jul 13, 2015 at 4:01

dreamlax

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1 Comment

Manish Mallavarapu Over a year ago

Thanks! allocating for both line_data.data and *line_data.data worked, yes I forgot to add the <stdlib.h> for malloc when I edited it

Ishmeet · Accepted Answer · 2015-07-13 03:55:28Z

0

You can try malloc line_data.data before strcpy.

lines_t readlines(FILE *fp) {
    lines_t line_data;
    line_data.data = malloc(LINESIZE);
    *line_data.data = malloc(LINESIZE);

    return line_data; 
}

answered Jul 13, 2015 at 3:55

Ishmeet

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1 Comment

M.M Over a year ago

LINESIZE is surely not the right size to use in the first malloc

Srinath Mandava · Accepted Answer · 2015-07-13 04:30:42Z

0

You first need to allocate memory.
You need to allocate both for the array of strings and string array. Here you need to have an upper limit while calling a malloc.

line_data = malloc(10* sizeof(char*));  // for example here the upper limit is 10
while(fgets(line,LINESIZE,fp)) { 
        sscanf(line,"%s\n",temp); 
        line_data.data[num_lines] = malloc(sizeof(char) * (strlen(temp)+1));
        strcpy(line_data.data[num_lines], temp); /* Program crashes here */
        num_lines++;  
    }

edited Jul 13, 2015 at 4:30

answered Jul 13, 2015 at 3:59

Srinath Mandava

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1 Comment

M.M Over a year ago

You forgot about strings having null terminators

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