C (linux) passing array of structs to delete element

Question

I have the following issues in my C Program,

FIRST, I have the following 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;
}

For debugging I loop through and print the values in the array of structs - this works (nevermind I'm not returning a value in this function - the 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 kind of 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

I am getting errors here. 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
  //realloc if needed - gotta get here first - pass segfault and/or jumbled data

  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.

Can you use the code sample markup {} in the editor when pasting your C code? It's really hard to read this way. — froody
– froody, Commented Mar 29, 2011 at 15:27

Community · Accepted Answer · 2017-05-23 12:19:54Z

2

You only allocated one struct, not 3 as you think.

You should do something like :

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

What you do is OK with the const char (string literal) assignments. But you should change your definition of struct ts to:

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

This way your compiler will warn you if you try to change the contents of those strings.

Otherwise you can use malloc() and strcpy() for your char*'s in the struct.And

ex[0].fname = malloc(sizeof(char) * 128);
//...
strcpy(ex[0].fname , "myString");
//...

Also :

This code snippet looks a bit weird

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

  for(i=0; i<sizeof(passedex)-1; i++){
      //...
  }
  return;
}

I think you mean

void delelement(char *delwhat, struct ts *passedex , int array_size)
{
    int i;
    for( i=0 ; i < array_size ; i++ )
    {
         //...

You need to pass the size of the array as an argument. You may find some posts on this link interesting : newbie questions about malloc and sizeof

edited May 23, 2017 at 12:19

CommunityBot

11 silver badge

answered Mar 29, 2011 at 15:28

user418748

Sign up to request clarification or add additional context in comments.

8 Comments

florin Over a year ago

You need to allocate memory before strcpy'ing into a char* too.

user418748 Over a year ago

@florin, which part did I forgot ?

baldrunner2049 Over a year ago

wrong. why should he allocate the struct char ptrs? his sample is clear, he assigns a const char* to the struct field fname and so on, he doesnt memcpy; you use malloc() before making that assignment you'll get a true segfault bloodshed :P :P :P no downvote for pointing the 3-struct malloc mess ;)

user418748 Over a year ago

@adirau , what do you mean you use malloc() before making that assignment you'll get a true segfault bloodshed ? If you were to strcpy() you should first use malloc() and then strcpy().

baldrunner2049 Over a year ago

citing: "You never allocated memory in your struct's chars, hence the Undefined Behaviour - SegFault." ; he really doesn't want to malloc his char * struct members; he's just operating on them with const char * assignments, no copying; he does make a mess later by interreferencing char *s, etc but those don't cause the segfault; only the bad ts struct malloc does;

|

baldrunner2049 · Accepted Answer · 2011-03-29 15:31:22Z

1

you are allocating space only for one ts struct

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

fact that you can access and work fine with the memory after your allocated space it's just pure luck

answered Mar 29, 2011 at 15:31

baldrunner2049

9,0944 gold badges33 silver badges38 bronze badges

Comments

Lindydancer · Accepted Answer · 2014-09-05 20:55:32Z

1

Next problem:

for(i=0; i<sizeof(passedex)-1; i++){

The sizeof operator does not return the number of element in an array, it returns the number of bytes one passedex occupies. As this is a pointer, it will have a value like 4 or 8, depending on the kind of machine you are using.

In fact, in C, there is no way of knowing how big an array, if all you have is a pointer to it. This means that you have to pass in new parameter informing your routine how big the array is.

edited Sep 5, 2014 at 20:55

answered Mar 29, 2011 at 15:38

Lindydancer

26.3k4 gold badges54 silver badges72 bronze badges

Collectives™ on Stack Overflow

C (linux) passing array of structs to delete element

3 Answers 3

8 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

8 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related