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I would think that strncat() should fail because it is being passed a negative number for the count, but that third parameter is of type size_t, which, according to the page, is: вЂ¦isВ 20-25-1 which is -6 which means: strncat( fullName, ' ', -6 ) // error! LetвЂ™s say FULLNAME_SIZE is 20, but the string copied there was 25 (a buffer overrun): FULLNAME_SIZE- strlen(fullName)-1 But what if, somehow, the existing string was too large? If strlen(fullName) was larger than FULLNAME_SIZE, the math would break. We are assuming that the length of the data in the buffer is never greater than what the buffer can hold. Strncat( fullName, lastName, FULLNAME_SIZE- strlen(fullName)-1 ) Strncat( fullName, ' ', FULLNAME_SIZE- strlen(fullname)-1 )

Strncpy( fullName, firstName, FULLNAME_SIZE-1 )

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That problem is with strlen(), which we are using for strncat() to determine how much data is already in the destination buffer: // Create full name by starting with first name. In the previous installment I mentioned there is a potential problem. In our quest to find the safest (or at least a safer) way to copy or append strings, we have been discussing the C library functions strncpy() and strncat(). This entry was posted in C Programming on Januby Allen Huffman. Look for an upcoming article that will expand on this with replacement functions you can use to make things easier and more efficient. If (strncmp( buffer, firstName, FIRSTNAME_SIZE ) = 0)ДЇollow these three simple steps and you will greatly reduce your chance of a buffer overrun that could cause unexpected problems. Instead of strcmp(buffer, firstname), do this: #define BUFSIZE 30 // size of desination bufferД¬har buffer = // Initialize with zeros. If the max amount is copied, strncpy() will not null terminate the destination, so you need to do that yourself. Instead of using strcpy(), which does absolutely no checking to see if itвЂ™s copying more data than the destination buffer can hold, use strncpy() to limit how much can be copied.

Os9 68k emulator mac macos dsk image how to#

вЂ“ Updated note about the problem with strlen(), referencing the discussion in part 5, and the solution in the summary.ДЄnd now, a simple one-page summary of how to make copying/appending C strings safer and hopefully avoid potential buffer overrun crashes (or other problems).

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