C Tips

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strncpy()

The typical error would look like this:

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

int main()
{
  const char *src = "123456789";
  char dst[5];

  strncpy(dst, src, sizeof(dst));

  for (int cnt=0; cnt < sizeof(dst); cnt++)
  {
    printf("%2d. 0x%x '%c'\n", cnt, dst[cnt], dst[cnt]);
  }
}

We see that the terminating '\0' is missing:

$ ./main 
 0. 0x31 '1'
 1. 0x32 '2'
 2. 0x33 '3'
 3. 0x34 '4'
 4. 0x35 '5'
$

If we cannot be sure that the source string is less or equal in length to the destination, we have to write the following construct:

  strncpy(dst, src, sizeof(dst)-1);
  dst[sizeof(dst)-1] = '\0';