Program received signal: “EXC_BAD_ACCESS”?

I am making a command line program in C using XCode. When running the program, it initially does what it is supposed to do (asks me for a file path). However, when I type in a valid and existing file path, it gives me the following error:

Program received signal:  “EXC_BAD_ACCESS”. sharedlibrary apply-load-rules all (gdb)

I have two warnings in my program, both of which have to do with the function strcat. The warnings are:

warning: implicit declaration of function 'strcat'

and

warning: incompatible implicit declaration of built-in function 'strcat'

I am wondering why my program is not executing properly.

Thanks, Mike

My code is posted below:

#include "stdlib.h"
int main (void)
{
   char *string1;
   printf("Type in your file path: ");
   scanf("%s", string1);
   char *string2 = "tar czvf YourNewFile.tar.gz ";
   strcat(string2, string1);
   system(string2);
}

Maybe it has to do with allocating the chars?

Answers


Examine this line:

char *string1

and this line:

scanf("%s", string1);

You have not declared a size for string1, meaning that you will always get an error, fix it with something like this:

char string1[100]

If 100 is the maximum length of your input.

Or read your input character by character.

And, to get rid of the warnings, add #include "string.h" to where your #include statements are.


You forgot to allocate space for string1, scanf will not allocate memory for you, you have to do that yourself. Furthermore, string2 points at non-writeable memory and it doesn't have enough room to append string1 to it anyway so your strcat would overflow even if you had char string2[] = "tar czvf YourNewFile.tar.gz ";.

Here's an annotated version of something that's closer to what you really want:

#include <stdio.h>  /* printf, sprintf, fgets */
#include <string.h> /* strcat, strlen */
#include <stdlib.h> /* malloc */

#define TAR "tar czvf YourNewFile.tar.gz"

int main(void) {
    char path[100] = { 0 };  /* Initialize to all 0 bytes.           */
    char *cmd;               /* We'll allocate space for this later. */
    int   len;

    printf("Type in your file path: ");
    fgets(path, sizeof(path), stdin);   /* Read at most 100 characters into path */

    /*
     * Remove the trailing newline (if present).
     */
    len = strlen(path);
    if(path[len - 1] == '\n')
        path[len - 1] = '\0';

    /*
     * Allocate room for our command. 
     */
    cmd = malloc(
          strlen(TAR)   /* Room for the base tar command.         */
        + 1             /* One more for the space.                */
        + strlen(path)  /* Room for the path we read.             */
        + 1             /* One more for the final nul terminator. */
    );

    /*
     * You could also use a bunch of strcpy and strcat stuff for
     * this but sprintf is less noisy and safe as long as you've
     * properly allocated your memory.
     */
    sprintf(cmd, "%s %s", TAR, path);

    /*
     * This is vulnerable to unpleasant things in `path` (such as spaces,
     * &, >, <, ...) but this will do as a learning exercise. In real life
     * you'd probably want to use fork and exec for this to avoid the  
     * interface issues with the shell.
     */
    system(cmd);  

    /*
     * Free the memory we allocated.
     */
    free(cmd);

    /*
     * You need a return value because of "int main(...)". Zero is
     * the standard "all's well" return value.
     */
    return 0;
}

Someone please let me know if I've made any off-by-one errors.

You can find reference material for the functions in the above over here.


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