Easy concatenation and writing to a file in C

concat.c

#include <lib.h>      // provides _syscall and message
#include <unistd.h>   // provides function prototype
#include <stdio.h>    // printf

int main() {

  int processID = 10;
  long timestamp = 123456789;
  char fromState[20] = "RDY";
  char toState[20] = "RUN";
  char buffer[100];
  
	FILE *fp = fopen("proc-states", "w+");
	if (fp == NULL) { printf("Error opening file!\n"); 	return 0; }

	char buffer[100];
	sprintf(buffer, "%d\t%ld\t%s\t%s\n", processID, timestamp, fromState, toState);
	fprintf(fp, "%s\n", buffer);
	fclose(fp);
  
  return 0;
}