Thread: overlay of strings

can someone tell me why this gives an memory error
thank you

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


#define MAX_LEN 40

void main()
{
int pos;
char *line, *aster, *blank;

line=(char*)malloc(MAX_LEN);
aster=(char*)malloc(1);
blank=(char*)malloc(1);

line=" ";
aster="*";
blank="";

printf("enter star position (1-40):");
scanf("%d",&pos);

if (pos>0 && pos<41)
{
printf("\n");
printf(" 1 2 3 4\n");
printf("1234567890123456789012345678901234567890\n ");
puts(strcat(strncat(blank,line,pos-1),aster));
}
else
printf("out of range");
}

edit

line=" ";

this has 40 spaces

> #include <malloc.h>
The ANSI header file for malloc is stdlib.h

> void main()
This must be
int main ( )

> line=(char*)malloc(MAX_LEN);
You don't need the cast (it can hide errors)

> aster=(char*)malloc(1);
Given the use of string functions, and that all C strings have a \0, you need at least 2 bytes here.

> line=" ";
If you really want to set the memory, then you should use strcpy - this just makes the pointer point at this string constant, and throws your malloc'ed memory away
strcpy( line, " " );

> puts(strcat(strncat(blank,line,pos-1),aster));
Perhaps you should try breaking this up into several smaller steps, so you can more clearly see what is going on.
In particular
1. the destination for the copy is blank, which at best is only 1 char long.
2. strncat doesn't guarantee that a \0 will be placed in the expected position, so the following strcat could be in deep trouble.

changed to this


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


#define MAX_LEN 40

int main()
{
int pos;
char *line, *aster, *blank;

line=(char*)malloc(MAX_LEN);
aster=(char*)malloc(2);
blank=(char*)malloc(2);

strcpy(line," ");
aster="*";
blank="";

printf("enter star position (1-40):");
scanf("%d",&pos);

if (pos>0 && pos<41)
{
printf("\n");
printf(" 1 2 3 4\n");
printf("1234567890123456789012345678901234567890\n ");
puts(strcat(strncat(blank,line,pos-1),aster));
}
else
printf("out of range");
}


i tried to break that complex bit up before but it did not seem to help.

i get a windows error saying

the instruction at "0x004013e4" referenced memory at "0x004230c0". The memory could not be "written".

this is still what i was getting before.

its not important as its just an example for a book as i am learning, but i you feel like solving, thanks.

Code:

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

#define MAX_LEN 40

int main() {
    int pos;
    char *line, *aster, *blank;

    line=malloc( MAX_LEN+1 );
    aster=malloc( 2 );
    blank=malloc( MAX_LEN+1 );  // you're copying to this - better have space

    strcpy( line, "                                        " ); // 40 spaces
    strcpy( aster, "*" );
    strcpy( blank, "" );

    printf( "enter star position (1-40):" );
    scanf( "%d",&pos );

    if(pos>0 && pos<41) {
        printf( "\n" );
        printf( "         1         2         3         4\n" );
        printf( "1234567890123456789012345678901234567890\n" );
        puts( strcat( strncat( blank,line,pos-1 ),aster ) );
    } else
        printf( "out of range" );

    return 0;
}

printf("%*s\n", -pos, "*");

alex