how to write integer var to file using write() function in c - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pthread.h>
void *func(void *ptr);
int file;
int main()
{
pthread_t thread1, thread2;
int iret1, iret2;
int p;
p=1;
file=open("file1.txt", O_CREAT | O_TRUNC | O_RDWR , 0666);
iret1 = pthread_create(&thread1, NULL, func, (void *)&p);
pthread_join(thread1, NULL);
close(file);
exit(0);
}
void *func(void *ptr)
{
int *num;
int i;
num = (int *)ptr;
printf("%d ", *num);
for (i=0; i<10; i++)
{
printf("%d", *num);
write(file, *num, sizeof(*num));
}
}
How to write integer var to file using write() function in c?
This is my code. The problem is in the func(). If I use chars or const int it's working fine.

Answer 1

First, read the man page of write(). It writes bytes, not element types.
So, what you are trying to achieve, cannot be accomplished directly with write(). you need to use snprintf() to convert the int to char string, which you can use with write(). Please check the following code.
1. Define a char array, print the value of the int pointer to that array using snprintf().
2. Use the char array as the argument of write(). It'll work.
NOTE: It's always a best practice to add some error check to the system calls and library calls. It provides many useful information in the case they fail.
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pthread.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
void *func(void *ptr);
int file;
int main()
{
pthread_t thread1, thread2;
int iret1, iret2;
int p;
p=1;
file=open("file1.txt", O_CREAT | O_TRUNC | O_RDWR , 0666);
iret1 = pthread_create(&thread1, NULL, func, (void *)&p);
pthread_join(thread1, NULL);
close(file);
exit(0);
}
void *func(void *ptr)
{
int *num;
int i, ret = -1;
num = (int *)ptr;
char buf[4] = {0}; //to use with write()
printf("%d\n", *num);
for (i=0; i<10; i++)
{
printf("%d", *num);
memset(buf, 0, sizeof (buf));
snprintf(buf, sizeof *num, "%d", *num); //print to buffer
ret = write(file, buf, strlen(buf)); //write the buf to file
if (ret < 0) //check for erroneous condition
printf("ret is %d, errno %d\n", ret, errno);
}
}

Answer 2

write(fd, &var, sizeof(i)); is a short fix for this question.