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Top Forums Programming Newline in ANSI-C standard functions Post 303020110 by yifangt on Friday 13th of July 2018 01:13:10 PM
Old 07-13-2018
unclear with memset() with strcpy()/strncpy()

Thank you so much for your patience with my basics still, as I feel getting better to understand more.
strncpy doesn't want the size of the origin string: It wants the size of the destination buffer. ...... If you'd used strcpy, you would have been safe.
To see how things are working in the memory in order to understand those garbage left, I got error in the Block Two of following code:***buffer overflow detected ***: ./memset01 terminated
Code:
#include <stdio.h>
#include <string.h>

//More about that some said don't use strcpy() but use strncpy()!!!
int main () {
   char str1[128];
   char str2[256];
   int len1, len2;
   len1=strlen("this is string1");
   len2=strlen("That is test string2");
   printf("Length of str1[128]: %d\n", len1);
   printf("Length of str2[256]: %d\n\n", len2);

   strcpy(str1, "this is string1");
   strcpy(str2, "That is test string2");

   printf("Length of strlen(str1): %lu\n", strlen(str1));
   printf("Length of strlen(str2): %lu\n\n", strlen(str2));

/*Start of block One***********************/
   memset(str1, 'X', strlen(str1)+113); // fills the first 128 bytes of the memory area pointed to by str1 with the constant byte 'X'.
   memset(str2, 'X', strlen(str2)+108); // fills the first 128 bytes of the memory area pointed to by str2 with the constant byte 'X'.

   strncpy(str1, "this is string1", len1);
   strncpy(str2, "That is test string2", len2);
   puts("___Line 1___:");
   puts(str1);
   puts("___Line 2___:");
   puts(str2);
   printf("------------------------------------------------------------------\n");
/*End of block One***********************/

/*Start of block Two***********************/
   memset(str1, 'X', strlen(str1)+113); //fills the first 128 bytes of the memory area pointed to by str1 with the constant byte 'X'.
   memset(str2, 'X', strlen(str2)+108); //fills the first 128 bytes of the memory area pointed to by str2 with the constant byte 'X'.
   strcpy(str1, "this is string1");
   strcpy(str2, "That is test string2");

   puts("___Line 1___:");
   puts(str1);
   puts("___Line 2___:");
   puts(str2);
   printf("------------------------------------------------------------------\n");
/*End of block Two***********************/
   return(0);
}

If I switch the two blocks, there is no buffer overflow for the memset().
What is going on in the memory? Thanks!

Last edited by yifangt; 07-16-2018 at 12:00 PM.. Reason: typos
 

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STRCMP(3)						   BSD Library Functions Manual 						 STRCMP(3)

NAME
strcmp, strncmp -- compare strings LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <string.h> int strcmp(const char *s1, const char *s2); int strncmp(const char *s1, const char *s2, size_t len); DESCRIPTION
The strcmp() and strncmp() functions lexicographically compare the nul-terminated strings s1 and s2. RETURN VALUES
The strcmp() and strncmp() return an integer greater than, equal to, or less than 0, according to whether the string s1 is greater than, equal to, or less than the string s2. The comparison is done using unsigned characters, so that '200' is greater than ''. The strncmp() compares not more than len characters. SEE ALSO
bcmp(3), memcmp(3), strcasecmp(3), strcoll(3), strxfrm(3) STANDARDS
The strcmp() and strncmp() functions conform to ANSI X3.159-1989 (``ANSI C89''). NOTES
If len is zero strncmp() returns always 0. BSD
June 4, 1993 BSD
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