scanf into a string array [duplicate]

Question

Could someone please explain why this is not working correctly? I get stack{0 through 4] to be the same as what ever is in stack[4]

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

int stack_pointer = 0;
char *stack[30]; //MAX NUMBER OF VALUES IN STACK IS 30

int main(int argc, char** argv) {

    char command_line[256];

    while(stack_pointer < 5) {  //Just store 5 inputs
        printf("repl> ");
        scanf("%s",command_line);
        stack[stack_pointer] = command_line;  
        stack_pointer++;
    }   

    int i = 0;
    for(i = 0; i<5; i++) {
        printf("stack[%d] =  %s\n", i, stack[i]);
    }
}

EX)

repl> 1
repl> 2
repl> 3
repl> 4
repl> 5
stack[0] = 5
stack[1] = 5
stack[2] = 5
stack[3] = 5
stack[4] = 5

stack[stack_pointer] = strdup(command_line);

chux
– chux

2014-02-18 16:26:57 +00:00
Commented Feb 18, 2014 at 16:26 — chux
– chux, Commented Feb 18, 2014 at 16:26
Pointer is pointing to the same place(to command_line).

BLUEPIXY
– BLUEPIXY

2014-02-18 16:28:14 +00:00
Commented Feb 18, 2014 at 16:28 — BLUEPIXY
– BLUEPIXY, Commented Feb 18, 2014 at 16:28

isedev · Accepted Answer · 2014-02-18 16:37:51Z

3

You are storing the char pointer command_line in each stack position and overwriting the contents of the command_line buffer for each input. So each stack position points to the same buffer and its contents will be the last input written to it.

You need to make a copy of the buffer each time and store a pointer to the copy in each stack position. For example:

stack[stack_pointer] = strdup(command_line);

Note that at some point, in the real program, you will need to free the memory allocated by strdup. When you've finished using the contents of the stack:

while (stack_pointer >= 0) free(stack[--stack_pointer]);

edited Feb 18, 2014 at 16:37

answered Feb 18, 2014 at 16:31

isedev

19.7k3 gold badges65 silver badges60 bronze badges

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

3 Comments

halfbit Over a year ago

I would add a comment of freeing the memory strdup creates

isedev Over a year ago

@halfbit: good point - updated.

user3324343 Over a year ago

isedev: Thank you so much! It makes sense now!

halfbit · Accepted Answer · 2014-02-18 16:32:55Z

1

your stack[pointer] points to the beginning of command_line

so all elements of the stack array are pointing to the same position, that is holding the last result

you are overwriting this memory position with each loop.

answered Feb 18, 2014 at 16:32

halfbit

4,0213 gold badges36 silver badges50 bronze badges

Comments

halfbit · Accepted Answer · 2014-02-18 16:48:49Z

edit: oops, duplicated my answer, sry

your stack[pointer] points to the beginning of command_line

so all elements of the stack array are pointing to the same position, that is holding the last result

you are overwriting this memory position with each loop.

edit: other solution not the best, but showing pointers:

int stack_pointer = 0;
char *stack[30]; //MAX NUMBER OF VALUES IN STACK IS 30

int main(int argc, char** argv) {

   char command_line[32000];  // not save! just for ex. here never do that in real life
   char *p=command_line;      // p points to start of ...

   while(stack_pointer < 5) {  //Just store 5 inputs
      printf("repl> ");
      scanf("%s",p);          // p points to start of (next) command line position
      stack[stack_pointer] = p;  
      stack_pointer++;

      p+=strlen(p)+1;           // move the 'p' pointer after the last string (and its '0x00')
   } 
}

Collectives™ on Stack Overflow

scanf into a string array [duplicate]

3 Answers 3

3 Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Linked

Related