segmentation fault , Memory not being allocated

Question

The code is as follows , Seems like nothing wrong with it at all. My gcc doesnt find alloc.h

print(node *q) 39 { 40 node *p=q; 41 do 42 { 43 printf("%d",p->n); 44 if(p->ptr != NULL) 45 p=p->ptr; 46 else

(gdb) p p $1 = (node *) 0x0

And the code where memory is allocated is

 if(p== NULL) { p=(node *)malloc(sizeof(node)); if(p== NULL) printf("The malloc failed\n"); p->n=num; p->ptr=NULL; }

When I run this in debugger there is no message of malloc failed.

Can anyone help. Regards

Sraddha

 add(node **q) { node *p=*q; int num; printf("Enter the number you want to add"); scanf("%d", &num); if(p== NULL) { p=(node *)malloc(sizeof(node)); if(p== NULL) printf("The malloc failed\n"); p->n=num; p->ptr=NULL; } }

Is p passed in to the allocator function as an argument (of type node *) by any chance? — Mat
– Mat, Commented Feb 10, 2012 at 11:24
Could you edit your post to show the rest of that loop please, and also say where the segv occurs? And, as Mat suggests, also show the whole allocation function. — Useless
– Useless, Commented Feb 10, 2012 at 11:24
The standard header where the prototype for malloc() is, is <stdlib.h>. You don't need "alloc.h" (whatever that is) ... — pmg
– pmg, Commented Feb 10, 2012 at 11:35

hmjd · Accepted Answer · 2012-02-10 11:38:02Z

5

You need to assign to *q in the add() function, not the local p:

add(node **q) { int num; printf("Enter the number you want to add"); scanf("%d", &num); if(*q == NULL) { *q = malloc(sizeof(node)); /* no need to cast return value. */ /* Corrected if logic to not access failed malloc. */ if(*q == NULL) { printf("The malloc failed\n"); } else { *q->n=num; *q->ptr=NULL; } } }

answered Feb 10, 2012 at 11:38

hmjd

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4 Comments

sraddhaj Over a year ago

Yes I realize now . Thanks hmjd . I ll try this.

Lundin Over a year ago

The comment about malloc in this answer is important, read this.

sraddhaj Over a year ago

hmjd , You were absolutely right , Yes I had failed to change the pointer to the queue head . Thanks very much.

sraddhaj Over a year ago

Thanks I didnt know . :D Ok !

Lundin · Accepted Answer · 2012-02-10 13:35:20Z

In addition to the issues addressed by hmjd's answer, you might want to consider a different program design. You have made function does three entirely different things: interaction with the user, allocation of memory and the actual algorithm. This program design was the actual culprit that caused the bug.

Instead, you might want to consider a more object-oriented approach:

int prompt_user (void) // this function interacts with the user ("GUI") { int num; printf("Enter the number you want to add"); scanf("%d", &num); getchar(); // discard line feed character from stdin return num; } void init_node (node* new_node, int num) { new_node->n = num; new_node->ptr = NULL; } // this code is the caller: { node* q = NULL; ... int num = prompt_user(); if(q == NULL) { q = malloc(sizeof(node)); if(q == NULL) { // error handling } } init_node(q, num); ... free(q); }

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