Returning structs - c

Question

This is probably a really stupid question, but
I have an array of structs outside of int main
typedef struct{
char c;
int k;
}factor_t;
and I declared
factor_t *factors = malloc(INIT*sizeof(*factors));
where INIT is 10
After running my function, I have an array of structs each which holds a char, c, and integer, k - e.g., factors[5].c could hold "b" or "d" or "e" and factors[5].k could hold "3" or "33" or "333"
I need to somehow insert these into a string, but I can't seem to
strcat(destination,c or k);
they both give me pointer to integer errors, destination is a char*
How would I go about putting these into a string? I'm aiming to get a string that looks like
ck
ck
ck
that is, a pattern of "ck\n" per struct, where c = char and k = integer
I use strcat(destination, "\n"); for the \n and it works, but I can't do the same with c and k

Answer 1

Calculate the length of the string and output with that offset.
#include <stdio.h>
#include <string.h>
typedef struct{
char c;
int k;
}factor_t;
void struct_cat(char *str, factor_t f) {
sprintf(str + strlen(str), "%c%d", f.c, f.k);
}
int main(void) {
factor_t fac = {'b', 33};
char buf[100] = "hogehoge";
struct_cat(buf, fac);
puts(buf);
return 0;
}

Answer 2

strcat appends a copy of the source string to the destination string. It expects c to be a null terminated string not a single char
If you want to add a single char to an array that is larger than n and the null terminating char is at index n
destination[n] = c;
destination[n+1] = '\0;
you have to be certain that destination is large enough.
If you want to format print additional data to destination string, again make sure destination is large enough and do :
sprintf(destination + n, "%c%d\n", c, k);
Or if you know how that destination has m chars left :
snprintf(destination + n, m, "%c%d\n", c, k);
With this if you attempt to print more than m chars the extra ones will be discarded.

Answer 3

You can use sprintf to do so . -
size_t len=strlen(destination); // calculate before concatenation
sprintf(&destination[len], "%c%d\n", factors[5].c,factors[5].k); // string with newline
destination should be of type char *.

Answer 4

If you need separate this feature use function (like #MikeCAT). But use of snprintf() and strncat() does not allow to go beyond the array bounds:
void strncat_struct(char *buffer, size_t buffer_size, factor_t f)
{
char tmp_buf[32];
snprintf(tmp_buf, sizeof(tmp_buf), "%c, %d\n", f.c, f.k);
strncat(buffer, tmp_buf, buffer_size);
}
int32_t main(int32_t argc, char **argv)
{
//...
char buffer[256] = {0};
for(i = 0; i < INIT; i++) {
strncat_struct(buffer, sizeof(buffer), factors[i]);
}
//...
}
Without using additional function. It is theoretically faster, couse there is no need to calculate string length:
int32_t main(int32_t argc, char **argv)
{
//...
char buffer[256];
char *buf_ptr = buffer;
size_t buf_size = sizeof(buffer);
for(i = 0; i < INIT; i++) {
int32_t printed;
printed = snprintf(buf_ptr, buf_size, "%c, %d\n", factors[i].c, factors[i].k);
buf_ptr += printed;
buf_size -= printed;
}
//...
}