C/ String data from file to array. Segmentation Fault

Question

I am writing a program that streams words from a text file and puts them into an array. I've also added a display of array data to see if everything is ok but I get a segmentation fault after compiling.

I am new to system programming so I am aware I might have done some basic mistakes. Please find the code below and tell me where have I gone wrong.

#include <stdio.h>

#define BUFFER 100

int main() {
    char tab[BUFFER];
    FILE *wp;
    wp = fopen("tab_file_b_words.txt", "r");
    if(wp == NULL)
    {
        printf("error/n");
        return -1;
    }
    int i;
    for(i=0; i<sizeof(wp); i++) {
        if(wp != NULL)
            fscanf(wp, "%s", &tab[i]);
        else 
            break;
        }
    printf("Content: \n");
    for(i = 0; i < BUFFER; i++) {
        if(tab[i] != NULL)
            printf("%s\n", tab[i]);
        else 
            break;
        }
    fclose(wp);
    return 0;
}

When you trace the program's executing using a debugger, at what point does the program first behave differently than you expected? — Scott Hunter, May 24 '19 at 12:10

kiran Biradar · Answer 1 · 2019-05-24T13:36:47.537

2

As of now you are having array of char

what you need is

   char tab[BUFFER]; --> char tab[BUFFER][SOME_LENGTH];

And

   fscanf(wp, "%s", &tab[i]); --> fscanf(wp, "%s", tab[i]);

%s expects null terminated char * but you are passing char.
```
    printf("%s\n", tab[i]);
```

sizeof(wp) is size of pointer.

for(i=0; i<sizeof(wp); i++)

What you need is

  while (1 == fscanf(wp, "%s", tab[i]))
  {
         ...
  }

edited May 24 '19 at 13:36

answered May 24 '19 at 12:10

kiran Biradar

12,700
3
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ryyker · Accepted Answer · 2019-05-24T17:26:16.213

This section:

for(i=0; i<sizeof(wp); i++) {
    if(wp != NULL)
        fscanf(wp, "%s", &tab[i]);
    else 
        break;
    }

Is problematic.

First, there are a couple of issues with the line: fscanf(wp, "%s", &tab[i]); It should be written as: fscanf(wp, "%s", tab); //removed & and array notation.
Unlike int or float variable types, the name of your char array (i.e. tab) is already a pointer pointing to the address of the variable, making it unnecessary (and incorrect) to use the address of operator (&).

Related to above... (and likely cause of segmentation fault.)
Because the definition of tab is for a simple array of char ( char tab[BUFFER]; ), the notation tab[i] refers only to the ith byte (or char) of the array, not the entire array. Because of the "%s" format specifier used, the function fscanf() expects a char * not a char, making tab the correct argument to use.

If you want to use an array of lines the variable must be created as a 2D array of char:

#define NUM_LINES 100
#define LINE_LEN 80

int main() {
    char tab[NUM_LINES][LINE_LEN] = {{0}}; // zero initialized array of 
                                           // NUM_LINE strings
                                           // each of LINE_LEN-1 capacity

In the statement for(i=0; i<sizeof(wp); i++) {

sizeof(wp)

will be equal to the number of bytes of the pointer wp, either 32 or 64 depending on the target addressing of your application. This is probably not what you intended. (or want.)

Consider a different approach:
Given you are working with text files, try using a combination of while() and fgets() to read lines from the file. Then you can process each line based on its known syntax.
(Following example uses a single dimension char array for simplified illustration.)

char line[some_len];

wp = fopen(some_file_name, "r");
if(wp)
{
    while(fgets(line, some_len, wp))
    {
        // use line.  In this case, just print to stdout
        printf("%s\n", line);
    }
    fclose(wp);
}

score 0 · Answer 3 · answered May 24 '19 at 12:10

0

sizeof(wp) is the problem. sizeof returns the length of a type in bytes and not the length of the file.

If you want to get the size of a file this may help you.

answered May 24 '19 at 12:10

dan1st

12,568
8
34
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C/ String data from file to array. Segmentation Fault

3 Answers3