string_to_uint64_t_and_function_isuint64_t, C (gcc)

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#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <inttypes.h>
#include <string.h>
#include <time.h>
#include <sys/timeb.h>

static __inline__ uint64_t string_to_u64(const char * s) {
  uint64_t i;
  char c ;
  int scanned = sscanf(s, "%" SCNu64 "%c", &i, &c);
  if (scanned == 1) return i;
  if (scanned > 1) {
    // TBD about extra data found
    return i;
  }
  // TBD failed to scan;
  return 0;
}

static __inline__ int is_uint64_t(const char *uint64_t_as_string){ 	//string must contains number. Then return 1. Else - return 0.
    uint64_t uint64_t_value;										//define variable to save integer
    char temp_string_uint64_t[256];									//define temp variable to save integer as string
	
	uint64_t_value = string_to_u64(uint64_t_as_string);				//write value of number
	sprintf(temp_string_uint64_t, "%" PRIu64, uint64_t_value);		//write number as string in temp variable
	
	if(!strcmp(temp_string_uint64_t, uint64_t_as_string)){			//compare stings (0 if comparison true) !0 = 1 - true;
		/*
		printf (	"temp_string_uint64_t \'%s\' == integer %s."
					"Number is valid and gives uint64_t_value = %" PRIu64 "\n",
					temp_string_uint64_t, uint64_t_as_string, uint64_t_value
		);
		*/
		return 1;//return true
		
	}else{
		/*
		printf (	"temp_string_uint64_t \'%s\' != integer %s."
					"Number is not valid. uint64_t_value = %" PRIu64 "\n",
					temp_string_uint64_t, uint64_t_as_string, uint64_t_value
		);
		*/
		return 0;//else false

}
}

int main(void)
{
    int x; //1 or 0
	char temp[256] = "18446744073709551615"; //(2^64-1) = 18446744073709551615
	
	x = is_uint64_t(temp);

if(x){
        printf("is_uint64_t(%s) = %d", temp, x);
    }
}

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Compilation time: 0.13 sec, absolute running time: 0.15 sec, cpu time: 0.09 sec, memory peak: 3 Mb, absolute service time: 0,29 sec