more pointer problems

For some reason the change in pointer location made from a function does not carry back. When I run this and input something like 1 + 2, I get stack underflow.

Code:

#include <stdio.h>    /* supports I/O system */
#include <stdlib.h>   /* misc declarations */
#include <string.h>   /* supports string functions */

#define COLS 80

void calculator( char exp[ ] );

void push( char ch, char *stack, char *bos );
char pop( char *stack, char *tos );
int gets_s( char *buf, int size );  /* fgets with stdin */

int main( void ) {
  char exp[ COLS ];  /* the expressions */
  
  gets_s( exp, COLS );
  
  calculator( exp );

  return 0;
}

void calculator( char exp[ ] ) {
  static char *stack;    /* pointer to some free stack memory */
  static char *tos;  /* top of stack */
  static char *bos;  /* bottom of stack */
  stack = ( char * ) malloc( COLS * sizeof( char ) );

  if ( !stack ) {
    printf( "Allocation Failure" );
    exit( 1 );
  }
  tos = stack;
  bos = stack + ( COLS - 1 );
  
  printf( "before parse: %s\n", exp );
  push( 'a', stack, bos );

  pop( stack, tos );

}

void push( char ch, char *stack, char *bos ) {
  if ( stack > bos ) {
    printf( "Stack Full\n" );
  }
  else {
    *stack = ch;
    stack++;
    *stack = '\0';
    //printf( "pushed: %s\n", stack - 1 );
  }
}

char pop( char *stack, char *tos ) {
  stack--;
  if ( stack < tos ) {
    printf( "Stack Underflow\n" );
    return NULL;
  }
  
  return ( *stack );
}

int gets_s( char *buf, int size ) {
  int error;  /* returns null on error */
  int c;      /* a single "character" */
  char *ptr;  /* pointer to first \n in string */

  if ( fgets( buf, size, stdin ) != NULL ) {
    if ( ( ptr = strchr( buf, '\n' ) ) != NULL ) {
      *ptr = '\0';
      error = 1;
    }
    else {
      while ( ( ( c = getchar( ) ) != '\n' ) && ( c != EOF ) ) { }
      error = NULL;
    }
  }
  else {
    error = NULL;
  }

  return ( error );
}

What OS/compiler are you using? When I compile it using MSVC 6 and XP, run it and enter '1 + 2', I get no errors.

Borland 5.5 free command line compiler. I don't get errors on compile, I get run time error, the in program code error "stack underflow".

Use ptr-to-ptr

Code:

//Function call would be
push( 'a', &stack, bos );

//function prototype
void push( char ch, char **stack, char *bos );

//function definition
void push( char ch, char **stack, char *bos ) {
  if ( (*stack) > bos ) {
    printf( "Stack Full\n" );
  }
  else {
    **stack = ch;
    (*stack)++;
    **stack = '\0';
    //printf( "pushed: %s\n", stack - 1 );
  }
}