c passing array of structs and delete one

Question

OK - please be patient for long winded explanation - I assume this is something just completely stupid on my part, because it's been (several) years since working in c. I have a couple of weird issues that I'm dealing with.

FIRST:

Have structure

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

struct ts{
  char *fname;
  char *lname;
  char *fingers;
  char *toes;
};

void delelement(char *, struct ts *);
int i;

int main(int argc, char **argv){
  struct ts *ex=(struct ts*)malloc(sizeof(struct ts));

  ex[0].fname="joe";
  ex[0].lname="bob";
  ex[0].fingers="11";
  ex[0].toes="9";

  ex[1].fname="billy";
  ex[1].lname="bronco";
  ex[1].fingers="10";
  ex[1].toes="10";

  ex[2].fname="martha";
  ex[2].lname="sue";
  ex[2].fingers="12";
  ex[2].toes="20";

  delelement("billy", ex);

  return 0;
}

now we get to the part I'm having problems with. now for debugging I loop through and print out the values in the array of structs - this works (nevermind I'm not returning a value in this function - problem I'm running into is before we even get to that)

void delelement(char *delwhat, struct ts *passedex){

  //struct ts *tempex=(struct ts*)malloc(sizeof(struct ts));
  for(i=0; i<sizeof(passedex)-1; i++){
    printf("passedex[%d].fname is %s\n", i, passedex[i].fname);    
    printf("passedex[%d].lname is %s\n", i, passedex[i].lname);
    printf("passedex[%d].fingers is %s\n", i, passedex[i].fingers);
    printf("passedex[%d].toes is %s\n", i, passedex[i].toes);
  }
  return;
}

now THAT works fine - prints out information correctly.

now let's simply remove the comment and define the temporary array of structs

void delelement(char *delwhat, struct ts *passedex){

  struct ts *tempex=(struct ts*)malloc(sizeof(struct ts));
  for(i=0; i<sizeof(passedex)-1; i++){
    printf("passedex[%d].fname is %s\n", i, passedex[i].fname);    
    printf("passedex[%d].lname is %s\n", i, passedex[i].lname);
    printf("passedex[%d].fingers is %s\n", i, passedex[i].fingers);
    printf("passedex[%d].toes is %s\n", i, passedex[i].toes);

  }
  return;
}

BOOM - segfault
passedex[0].fname is joe
passedex[0].lname is bob
passedex[0].fingers is 11
passedex[0].toes is 9
passedex[1].fname is billy
Segmentation fault

OK so I tried a different approach - which kinda works

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

struct ts{
  char *fname;
  char *lname;
  char *fingers;
  char *toes;
};

void delelement(char *, struct ts *, struct ts *);
int i;

int main(int argc, char **argv){
  struct ts *ex=(struct ts*)malloc(sizeof(struct ts));
  struct ts *tempex=(struct ts*)malloc(sizeof(struct ts));

  ex[0].fname="joe";
  ex[0].lname="bob";
  ex[0].fingers="11";
  ex[0].toes="9";

  ex[1].fname="billy";
  ex[1].lname="bronco";
  ex[1].fingers="10";
  ex[1].toes="10";

  ex[2].fname="martha";
  ex[2].lname="sue";
  ex[2].fingers="12";
  ex[2].toes="20";

  delelement("billy", ex, tempex);

  return 0;
}

void delelement(char *delwhat, struct ts *passedex, struct ts *tempex){

  //struct ts *tempex=(struct ts*)malloc(sizeof(struct ts));

  for(i=0; i<sizeof(passedex)-1; i++){
    printf("passedex[%d].fname is %s\n", i, passedex[i].fname);    
    printf("passedex[%d].lname is %s\n", i, passedex[i].lname);
    printf("passedex[%d].fingers is %s\n", i, passedex[i].fingers);
    printf("passedex[%d].toes is %s\n", i, passedex[i].toes);
  }
  return;
}

WORKS fine... (tempex now defined in main)
passedex[0].fname is joe
passedex[0].lname is bob
passedex[0].fingers is 11
passedex[0].toes is 9
passedex[1].fname is billy
passedex[1].lname is bronco
passedex[1].fingers is 10
passedex[1].toes is 10
passedex[2].fname is martha
passedex[2].lname is sue
passedex[2].fingers is 12
passedex[2].toes is 20

now lets start assigning values to *tempex - no segfault with tempex defined in main

void delelement(char *delwhat, struct ts *passedex, struct ts *tempex){

  //struct ts *tempex=(struct ts*)malloc(sizeof(struct ts));

  for(i=0; i<sizeof(passedex)-1; i++){
    printf("passedex[%d].fname is %s\n", i, passedex[i].fname);    
    printf("passedex[%d].lname is %s\n", i, passedex[i].lname);
    printf("passedex[%d].fingers is %s\n", i, passedex[i].fingers);
    printf("passedex[%d].toes is %s\n", i, passedex[i].toes);
    tempex[i].fname=passedex[i].fname;
    tempex[i].lname=passedex[i].lname;
    tempex[i].fingers=passedex[i].fingers;
    tempex[i].toes=passedex[i].toes;
  }
  return;
}

but NOW - weirdness

passedex[0].fname is joe
passedex[0].lname is bob
passedex[0].fingers is 11
passedex[0].toes is 9
passedex[1].fname is billy
passedex[1].lname is bronco
passedex[1].fingers is joe
passedex[1].toes is bob
passedex[2].fname is 11
passedex[2].lname is 9
passedex[2].fingers is billy
passedex[2].toes is bronco

obviously I'm just missing something stupid, or understanding this wrong, but have now dug a rut that I can't get out of. Any help would be appreciated.

The goal is to have a dynamic array of structures containing char *'s. Once past this issue, there will be an instance in main (or wherever) that I wish to delete one of those structures.

What I was going for was something like...

struct ts* delelement(char *delwhat, struct ts *passedex, struct ts *tempex){

  //struct ts *tempex=(struct ts*)malloc(sizeof(struct ts));

  for(i=0; i<sizeof(passedex)-1; i++){
    printf("passedex[%d].fname is %s\n", i, passedex[i].fname);    
    printf("passedex[%d].lname is %s\n", i, passedex[i].lname);
    printf("passedex[%d].fingers is %s\n", i, passedex[i].fingers);
    printf("passedex[%d].toes is %s\n", i, passedex[i].toes);
    //load tempex with everything except the one I want to delete
    if(!(passedex[i].fname==delwhat)){
      tempex[i].fname=passedex[i].fname;
      tempex[i].lname=passedex[i].lname;
      tempex[i].fingers=passedex[i].fingers;
      tempex[i].toes=passedex[i].toes;
    }
  }
  free(passedex); //haven't got here yet - dunno if needed

  for(i=0; i<sizeof(passedex)-1; i++){
    passedex[i].fname=tempex[i].fname;
    passedex[i].lname=tempex[i].lname;
    passedex[i].fingers=tempex[i].fingers;
    passedex[i].toes=tempex[i].toes;
  }

  return passedex;
}

so it would create (or have) a temporary array of structs to work with... load that array minus the one to be deleted... reload the passed array of structs and pass it back.

Answer 1

You're not allocating enough memory for an array of structs, only for a single entry. Try this instead:

struct ts *ex=(struct ts*)malloc(sizeof(struct ts) * 3);

Where 3 is the number of array elements.

Edit: Also, if you're on a linux platform, look into valgrind.

Answer 2

You're only allocating enough space in your ex array for a single struct:

struct ts *ex=(struct ts*)malloc(sizeof(struct ts));

ex[0].fname="joe";
ex[0].lname="bob";
ex[0].fingers="11";
ex[0].toes="9";

But now you write beyond the end of the array:

ex[1].fname="billy";
ex[1].lname="bronco";
ex[1].fingers="10";
ex[1].toes="10";

I guess then your heap check routine causes the segfault when you try to allocate new memory and it finds you've corrupted it.