Can't combine an integer with a string

I am trying to combine an integer with a string. There is a function called strcat() in string library. However, I can't do it with strcat() because the function does not take an integer as parameter.

So I used sprintf() function to convert an integer to string. After doing it, I combined two strings together using strcat() function. I am getting neither error nor output.

This is the code I am working on:

#include "stdio.h"
#include "string.h"
#include "stdint.h"

char* header = "ABC";
uint8_t data_length = 10;
char* data_length_str;
uint8_t command = 0xA0;

int main()
{
    sprintf(data_length_str,"%d",data_length);
    strcat(header,data_length_str);
    printf("Header = %s",header);

    return 0;
}

I want to see an output like "ABC10".I also tried this in a project that contain microcontroller. I got hard fault handler error.

What seems to be a problem here?

The destination address passed to sprintf and strcat needs to identify a pre-existing region of storage which is large enough to hold the destination string. A declaration like char data_length_string[20]; will create a 20-byte region of storage, and indicate that when data_length_string appears in most kinds of expression, the compiler should substitute a pointer which identifies that region of storage.

A declaration like char header[40] = "ABC"; will behave similarly, but the 40-byte region of storage will be initialized with the characters 'A', 'B', and 'C', which will then be followed by 37 zeroes.

Note that functions like sprintf and strcat will blindly assume that the destination buffer is large enough to hold the resulting string. If it isn't, the functions will likely overwrite information which is used for other purposes, such as keeping track of how functions are nested, with consequences that are likely to be disastrous. Such functions should only be used in cases where it's practical to compute an absolute an upper bound on the length of strings, and make arrays large enough to accommodate that worst case.

You can do something as follows

#include "stdio.h"
#include "string.h"
#include "stdint.h"

#define DATA_LENGTH (10U)
#define HEADER_SIZE (3U)
#define DEST_BUFFER_SIZE (HEADER_SIZE + DATA_LENGTH)

char header[HEADER_SIZE] = "ABC";
char data_length_str[DEST_BUFFER_SIZE];

int main()
{
    sprintf(data_length_str, "%d", DATA_LENGTH);
    strcat(header, data_length_str);
    printf("Header = %s", header);

    return 0;
}

You will need to assign some memory to data_length_str either dynamically or as a static array. You can also define it as an array if you know the maximum length of your data.