seg fault in program

[Brian Tong]

I have the following program:

#include "ten.h"

int main(int argc, char **argv)
{
    off_t offset;
    int counter, bytes;
    char c;

    if (argc != 3) {
        fprintf(stderr,"Usage: %s [file1] [file2]\n", argv[0]);
        exit(1);
    }

    if ( (openfiles(argv[1], argv[2])) == -1)
        exit(1);

    if ( (offset = lseek(files->fdin, offset, SEEK_END)) == -1) {
        perror("lseek in main 1");
        exit(1);
    }

    printf("%d\n", (int)offset);
    /*counter holds each printable digit. 
    for (counter = 0; counter < MAX; ++counter) {
    */

    return 0;
}

#include "ten.h"

filedes *files;

int
openfiles(char *infile, char *outfile)
{
    if ( (files->fdin = open(infile, O_RDONLY)) == -1){
        perror("open in openfiles 1");
        return -1;
    }

    if ( (files->fdout = open(outfile, O_WRONLY)) == -1) {
        perror("open in openfiles 2");
        return -1;
    }

    return 1;
}

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>

#define MAX 10

int openfiles(char *, char *);

typedef struct {
    int fdin;
    int fdout;
} filedes;

typedef enum {
    OUT, IN
} iorout;

extern filedes* files;

I think there's a problem with the line: fd->fdin but I'm not sure how to fix it.

[laserlight]

You should avoid global variables as they make it difficult to reason about and maintain your program, i.e., change files into a local variable, e.g., one that is local to the main function.

[Brian Tong]

I'd still like to know why this program fails.

[WoodSTokk]

I have the following program:

…
    off_t offset;
…
    if ( (offset = lseek(files->fdin, offset, SEEK_END)) == -1) {
        perror("lseek in main 1");
        exit(1);
    }
…

offset is uninitialized and you seek to an bad position.

[laserlight]

I'd still like to know why this program fails.

Avoiding the use of global variables can help you come closer to understanding that. Also, pay attention to compiler warnings, e.g., your compiler would probably have warned you that you used offset before setting its value here:

if ( (offset = lseek(files->fdin, offset, SEEK_END)) == -1) {

Furthermore, it can help if you elaborated on the problem, e.g., post the test input, expected output and actual output (or in this case, the place where the problem was detected).

[Brian Tong]

I ran this in a debugger and it's failing in the opefiles() function. I tried printing the value of files->fdin and it says It can't access the memory

[laserlight]

I ran this in a debugger and it's failing in the opefiles() function. I tried printing the value of files->fdin and it says It can't access the memory

files is a pointer. What does files point to? Again, files should not have been a global variable in the first place.

[Brian Tong]

I'm not sure...I thought to access the fdin in the filedes struct you'd use:
files->fdin;

[christop]

files is null, which means it doesn't point to a valid object. Where do you want it to point?

[laserlight]

I'm not sure...I thought to access the fdin in the filedes struct you'd use:
files->fdin;

As far as I can tell, files does not point to a filedes object. Since it is global, it was initialised to be a null pointer, so files->fdin dereferences a null pointer. This is unequivocally a bug, albeit one that is not so easy to spot at a glance because files is a global variable. You should avoid global variables.

[Brian Tong]

I want to use fdin as a member of the filedes struct. Should I allocate the size of an int w/ malloc to do this?

[Brian Tong]

You should avoid global variables.

I'll keep this in mind but want to know where i went wrong w/ this program. I'll switch the global to a local variable later

[laserlight]

I want to use fdin as a member of the filedes struct. Should I allocate the size of an int w/ malloc to do this?

Why an int? files is a pointer to filedes, therefore you need a filedes object, not an int. For example, perhaps you could declare a filedes variable in main, then declare files in main and initialise it to the address of that filedes variable.

[Brian Tong]

I tried using this in the openfiles function which is a sepearate file:
filedes *files;
files = malloc(sizeof(filedes));

it still won't compile

[laserlight]

I tried using this in the openfiles function which is a sepearate file:
filedes *files;
files = malloc(sizeof(filedes));

it still won't compile

What is the error message? Note that files would likely need to be used in the main function, so declaring it in openfiles is not likely to be the right approach. Rather, you should add a parameter:

int openfiles(filedes *files, char *infile, char *outfile)

The openfiles function then assumes that files already points to a valid filedes object.

Have you ever used standard C I/O with FILE, fopen, etc? The idea would be similiar.