如何使用c中的write()函数将整数var写入文件

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.