I tried to limit the number of n bytes copied to dest (here str1) by using strncpy(). The dest is big enough for the n bytes, but the output produced garbage when dest is smaller than the source (here argv[1]). This looks different, when i make dest large enough to hold the source.

Here's the code:

#include <stdio.h>                                                              
#include <string.h>                                                             
                                                                                
int main(int argc, char *argv[])                                                
{                                                                               
/* // Works when str1[?] is 20:                                                                    
  char str1[20];                                                                
  char str2[20]; */                                                             
                                                                                
  // produces garbage, str1 is smaller than argv[1] but big enough for the 4 bytes copied                                                            
  char str1[10];                                                                
  char str2[10];                                                                
                                                                                
  printf("argc = %i\n", argc);                                                  
  if (argc <= 1 || argc > 2){                                                   
    printf("Input Example:\n"                                                   
           "  %s SEGMENT:OFFSET\n"                                              
           "  %s 0100:F00A\n", argv[0], argv[0]);                               
    return 0;                                                                   
  }                                                                             
  printf("strlen(argv[1]) = %li, argv[1] = %s\n"                                
          , strlen(argv[1]), argv[1]);                                          
                                                                                
  // str1                                                                       
  strncpy(str1, argv[1], 4); // copying 4 bytes to str1                                                   
  printf("Copy only 4 Bytes -> sizeof(str1) = %li, "                            
         "strlen(str1) = %li, str1 = %s\n", sizeof(str1), strlen(str1), str1);  
                                                                                
  // str2                                                                       
  strncpy(str2, argv[1], 3); // copying 3 bytes to str2                                                   
  printf("Copy only 3 Bytes -> sizeof(str2) = %li, "                            
         "strlen(str2) = %li, str2 = %s\n", sizeof(str2), strlen(str2), str2);  
                                                                                
  return 0;                                                                     
}                      

The input of 0100:F00A produces:

./test.bin 0100:F00A
argc = 2
strlen(argv[1]) = 9, argv[1] = 0100:F00A
Copy only 4 Bytes -> sizeof(str1) = 10, strlen(str1) = 8, str1 = 0100�U
Copy only 3 Bytes -> sizeof(str2) = 10, strlen(str2) = 3, str2 = 010

Expected was just str1 = 0100.

I also do wonder why str2 is correct, its initial array size is as small as str1.

When i change

char str1[10] 

to

char str1[20]

and by doing so make it larger than the input of argv[1], then the output is correct:

./test.bin 0100:F00A
argc = 2
strlen(argv[1]) = 9, argv[1] = 0100:F00A
Copy only 4 Bytes -> sizeof(str1) = 20, strlen(str1) = 4, str1 = 0100
Copy only 3 Bytes -> sizeof(str2) = 20, strlen(str2) = 3, str2 = 010

It seems like strncpy is first copying everything to dest and then cutting away the rest after it. But is that the way how strncpy works? I assumed it is only copying what is need, the 4 bytes.

CodePudding user response：

from man page https://linux.die.net/man/3/strncpy

The strncpy() function is similar, except that at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

ie in your case not null is placed in dest

CodePudding user response：

From the GLIBC manual page for strncpy():

Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

You are observing the effect.