Address 0x0 is not stack'd, malloc'd or (recently) free'd

Question

I'm very new to C, and can't seem to figure out what's wrong with the following code.

int main() {
    char filen[] = "file.txt";
    FILE *file = fopen ( filen, "r" );
    if ( file != NULL )
    {
        char line [ 128 ];
        while ( fgets ( line, sizeof line, file ) != NULL ) /* read a line */
        {
            int i;
            char *result;
            for(i=0; i< NUM;i++)
            {
                char *rep;
                rep = (char *) malloc (sizeof(mychars[i][0]));
                strcpy(rep, mychars[i][0]);
                char *with;
                with = (char *) malloc (sizeof(mychars[i][1]));
                strcpy(with, cgichars[i][1]);
                result = (char *) malloc (sizeof(char) * 128);
                result = str_replace(line, rep, with);
            }


            fputs(result, stdout);
        }
    }
    fclose ( file );


    return 0;
}

Valgrind is giving me this error:

==4266== Invalid read of size 1
==4266==    at 0x4C286D2: __GI_strlen (mc_replace_strmem.c:284)
==4266==    by 0x5118A8D: fputs (iofputs.c:37)
==4266==    by 0x400A0F: main (repl.c:35)
==4266==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

repl.c corresponds to the line beginning with fputs toward the end of this code.

Also, mychars is a two dimensional array that looks like this:

char *mychars[NUM][2] = {
  "a", "97",
  "b", "98",
  ....

Can someone please tell me how to fix this? Also, any pointers on how I should improve my current code (especially with malloc) would be much appreciated.

Edit: Code for str_replace

char *str_replace(char *str, char *orig, char *rep) {
  char buffer[4096];
  char *p;

  if(!(p = strstr(str, orig)))
    return NULL;

  strncpy(buffer, str, p-str);
  buffer[p-str] = '\0';
  sprintf(buffer+(p-str), "%s%s", rep, p+strlen(orig));

  return buffer;

}

EDIT 2 New Code for str_replace, and main

For testing purposes, I've replaced my str_replace method with the one found here:

What is the function to replace string in C?

And my main is changed slightly:

int main() {
    static const char filen[] = "file.txt";
    FILE *file = fopen ( filen, "r" );
    if ( file != NULL )
    {
        char line [ 128 ];
        while ( fgets ( line, sizeof line, file ) != NULL ) /* read a line */
        {
            int i;
            char *result;
            for(i=0; i< NUM;i++)
            {
                char *rep;
                rep = (char *) malloc (sizeof(mychars[i][0]));
                strcpy(rep, mychars[i][0]);
                char *with;
                with = (char *) malloc (sizeof(mychars[i][1]));
                strcpy(with, mychars[i][1]);
                result = str_replace(line, rep, with);
            }


            fputs(result, stdout);
        }
    }
    fclose ( file );


    return 0;
}

But I'm still getting

==6730== Invalid read of size 1
==6730==    at 0x4C286D2: __GI_strlen (mc_replace_strmem.c:284)
==6730==    by 0x5118A8D: fputs (iofputs.c:37)
==6730==    by 0x400995: main (repl.c:29)
==6730==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Perhaps the most frustrating part of this is not knowing what these Invalid read errors are.

EDIT 3 I've updated the code in the center of the for loop as such:

        int i;
        char* result;
        result = &line[0];
        for(i=0; i< NUM_CGICHARS;i++)
        {
            char *rep;
            rep = (char *) malloc (sizeof(char));
            strcpy(rep, cgichars[i][1]);
            char *with;
            with = (char *) malloc (sizeof(char)*3);
            strcpy(with, cgichars[i][0]);
            result = str_replace(result, rep, with);
            fputs(result, stdout);
            free(rep);
            free(with);
        }

And now I'm starting to get output! However, after only two iterations, I get a segmentation fault, with valgrind giving me a whole bunch of this:

==9130== Invalid read of size 1
==9130==    at 0x4C286D2: __GI_strlen (mc_replace_strmem.c:284)
==9130==    by 0x5118A8D: fputs (iofputs.c:37)
==9130==    by 0x4009DF: main (teststep1.c:27)
==9130==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Answer 1

In these two lines

            result = (char *) malloc (sizeof(char) * 128);
            result = str_replace(line, rep, with);

you first allocate space for result that you then loose immediately after by overwriting it with the return of str_replace. That function probably returns 0, so your fputs fails.

BTW, don't cast the return of malloc, in C this is superfluous and may hide the fact that you forgot to include the prototype.

Edit: Your str_replace function is completely wrong in its memory handling. Never return the pointer to a local variable, the space isn't valid after you have left the function.