Add string to string in C with safe functions

Question

I want to copy the a file name to a string and append ".cpt" to it. But I am unable to do this with safe functions (strcat_s). Error: "String is not null terminated!". And I did set '\0', how to fix this using safe functions?

size = strlen(locatie);
size++;
nieuw = (char*)malloc(size+4);
strcpy_s(nieuw, size, locatie);
nieuw[size] = '\0';
strcat_s(nieuw, 4, ".cpt"); // <-- crash
puts(nieuw);

Solution 1

The size parameter of the _s functions is the size of the destination buffer, not the source. The error is because there is no null terminator in nieuw in the first for characters. Try this:

size = strlen(locatie);
size++;
int nieuwSize = size + 4;
nieuw = (char*)malloc(nieuwSize );
strcpy_s(nieuw, nieuwSize, locatie);
nieuw[size] = '\0';
strcat_s(nieuw, nieuwSize, ".cpt"); // <-- crash
puts(nieuw);

Solution 2

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

int main(void)
{
    char old[] = "hello";
    size_t size = strlen(old) + 5;;
    char *new_name = (char*)malloc(size);
    new_name[size-1] = '\0';
    strcpy_s(new_name, size, old);
    strcat_s(new_name, size, ".cpp");
    printf("%s\n", new_name);
    return 0;
}

Solution 3

Maybe some standard solution?

const char * const SUFFIX = ".cpt";
size = strlen(locatie) + strlen(SUFFIX) + 1;  // +1 for NULL
nieuw = (char*)malloc(size);
snprintf(nieuw, size, "%s%s", locatie, SUFFIX);

Comments

[Admin]

asprintf is not available for windows.

[another.anon.coward]

+1, Though I have no experience with MS function just curious after reading documentation of strcpy_s, isn't nieuw[size] = '\0'; redundant?

[shf301]

Yes it is redundant. strcpy_s will always null terminate the string.

[kakyo]

much nicer. I can't believe how much harder it is to get strcat_s working right.