How can I insert an integer into a block of memory?

Question

Is there any way I can put an integer into a string of known size and if yes how can I read it? For example an (int a=10) into a (char string[200]).

Answer 1

Is there any way I can put an integer into a string of known size and if yes how can I read it? For example an (int a=10) into a (char string[200]).

Yes, there is use the string version of printf

snprinf(string,200,"%d",a);

It is also possible to generate the string with a loop with div and mod by 10 if you want to do above by hand. See another answer for an example of this https://stackoverflow.com/a/49615617/391691

To get your int back from the string sscanf, atoi or atol can be used

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

void main(){
   int a;
   char string[200];


   a = 10;
   snprintf(string,200,"%d",a);
   printf("As string:%s As int:%d\n",string,atoi(string));
}

The interpretation of this question is still not clear to me, this represents an alternative interpretation of the question compared to previous answers https://stackoverflow.com/a/58584291/391691

Answer 2

Is there any way I can put an integer into a string of known size and if yes how can I read it? For example an (int a=10) into a (char string[200]).

In this answer I work with the minimal question stated, which does not give details about any of the following:

• Representation / encoding of the integer in string
• Architecture this should run on
• The position where to insert the integer

If this question is about converting an integer to its ASCII representation the answer by @Simson is what you are looking for.

C allows you to "put"^[1] an integer into any position of a c style string by casting the char array to an int*.

// string: the array (char*)
// pos: position in the array (size_t)
// to_insert: the value to insert (int)
(*(int*)(&string[pos])) = to_insert;

But be warned: This is a sledgehammer approach. You have to be very careful when doing this or your code might show undefined behavior on different platforms. For example you have to manually check and answer:

• Memory alignment: will I try to insert the integer at a position which is not addressable by the architecture?
• Endianness: is the way that I write this integer to the array actually what I want?
• sizeof(int): does the integer still fit into the array? Maybe consider using any fixed length integer types such as uint16_t, uint32_t, ...
• and possibly a few more

The following example shows the usage of the method discussed:

#include <stdio.h>

void insert_at_pos(char* arr, size_t pos, int to_insert){
  (*(int*)(&arr[pos])) = to_insert;
}

int main(void) {
  char string[200] = {0};
  insert_at_pos(string, 3, 10);

  //print char by char as hex
  for (size_t i=0; i < sizeof(string)/sizeof(*string); ++i){
    printf("%x ",(unsigned char)string[i]);
  }
}

Running the example:

$ ./main
0 0 0 a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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[1]: And by "put" I understand write as-is.