Reading input from a file. Program crashes

**inu11byte** · 11-10-2012

Okay, so I'll start off by telling you what I am trying to do.
I have a text file that has words on each line. E.g.

Code:

this
is
a
test
forMyProgram

I want my program to scan through the file and store each word on its own line in my variable 'fullSub'. I want to then be able to use that variable as a string. Here's what I have so far:

Code:

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

int main(int argc, char *argv[])
{
    FILE *fp;
    static const char filename[] = "subdomains.txt";
    char *subs;
    char *fullSub = malloc(sizeof(char));

    fp = fopen(filename, "r");

    if (fp != NULL)
    {

        while((subs = fgetc(fp)) != EOF && subs != '\n' && subs != ' ')
        {
            realloc(fullSub, fullSub+1);
            strcat(fullSub, subs);
        }

    }else{
        fprintf(stderr, "Failed to open file '%s'!\n", filename);
        return 1;
    }


free(fullSub);

    system("pause");// just here for testing purposes... I will use getchar() at a later date.
    return 0;
}

**Salem** · 11-10-2012

It's no wonder it crashes, with so many problems.

Code:

$ gcc -Wall -Wextra foo.c
foo.c: In function ‘main’:
foo.c:17:21: warning: assignment makes pointer from integer without a cast [enabled by default]
foo.c:17:34: warning: comparison between pointer and integer [enabled by default]
foo.c:17:49: warning: comparison between pointer and integer [enabled by default]
foo.c:17:65: warning: comparison between pointer and integer [enabled by default]
foo.c:19:13: warning: passing argument 2 of ‘realloc’ makes integer from pointer without a cast [enabled by default]
/usr/include/stdlib.h:485:14: note: expected ‘size_t’ but argument is of type ‘char *’
foo.c:5:14: warning: unused parameter ‘argc’ [-Wunused-parameter]
foo.c:5:26: warning: unused parameter ‘argv’ [-Wunused-parameter]
foo.c:19:20: warning: ignoring return value of ‘realloc’, declared with attribute warn_unused_result [-Wunused-result]

Think about what kind of 'array' of strings you need.
You can have one of 4 choices.

Code:

char strings[10][100];  // fixed number of strings, fixed maximum length
char *strings[10]; // fixed number of strings, variable length of each
char (*strings)[100]; // variable number of strings, fixed maximum length
char **strings;  // variable number of strings, with variable length

I suggest you start with the first one, to allow you to focus on your 'read a single word' from the file to begin with.
When that works, you can move onto how to allocate and store the string in your chosen array.

**grumpy** · 11-10-2012

Yet another example (you're not the first today) of someone hacking together code without any real thought, and then wondering why it doesn't work.

The program is not crashing. Your compiler would have complained bitterly (plenty of error messages) and refused to compile your code.

It would help if you had bothered to read the documentation for all the functions you're using. After reading the documentation, it would then help if you used the function in a manner that was even slightly consistent with the documentation.

fgetc() does not return a pointer to char. strcat() works with nul terminated strings, not pointers to single characters. realloc(x,newlen) does not change x (it returns a value if it succeeds).

A pointer to character (eg subs) cannot be compared with a single character ('\n').

Thread: Reading input from a file. Program crashes

Thread Tools

Search Thread

Display

Reading input from a file. Program crashes

Similar Threads

CodeBlocks - my program crashes on file import (debugging).

Program not reading file input :?

Utter newb?: Program crashes after input

program crashes when copying binary file

Program crashes on reading a line from a file