extra characters are printed when file contents are read into a array in c

Question

I just want to read a file and store it in a array. I tried the following code

#include<stdio.h>
#include<string.h>
#include<fcntl.h> 
int main()
{
int fd;
char buffer[500];
fd=open("input.txt",O_RDONLY);
read(fd,buffer,500);
printf("\nTHE FILE CONTENTS ARE:\n%s",buffer);
} 

input.txt contains:

COPY   START 0000  
FIRST  STL   RETADR  
CLOOP  JSUB  RDREC  
       LDA   LENGTH  
       COMP  #0 
       JEQ   ENDFIL  
       JSUB  WRREC  
       J     CLOOP 
ENDFIL LDA   =C'EOF'  
       STA   BUFFER  
       LDA   #3  
       STA   LENGTH  
       JSUB  WRREC  
       J     @RETADR  
       USE   CDATA  
       RETADR RESW  1
       LENGTH RESW  
       USE   CBLKS
       BUFFER RESB 4096
       BUFEND EQU   *
       MAXLEN EQU    BUFEND-BUFFER

when i printing i am getting extra characters at the end.

end line that i get is

MAXLEN EQU BUFEND-BUFFER EQU BUFEND-BUFFER

why is this?how to solve?

Answer 1

You have a buffer overflow in your code. You fill the char buffer[500]; argument with 500 characters from the file, but then you printf it with %s , which expects a zero-terminated string. But buffer is not zero-terminated, hence printf will try to read buffer[500] , buffer[501] , etc until it finds a zero terminator ( '\\0' ).

I suggest you define buffer to be 501 characters long, fill it with 500 characters and set buffer[500] to '\\0' so that printf will not attempt to read past buffer[500] .

Edit: Actually to print only the contents read from the file you should check the return value of the read call and put the '\\0' character next to last character written into buffer , eg:

int fd;
char buffer[501];
fd = open("input.txt", O_RDONLY);
const ssize_t r = read(fd, buffer, 500);
if (r < 0) {
    // Handle error
} else {
    buffer[r] = '\0';
    printf("\nTHE FILE CONTENTS ARE:\n%s",buffer);
}

See the manual page for read for more information ( man 3p read ).