How to write int to file using write system call and read them exactly as written?

This is as simple as it can get (note that stdio.h is only included for printf; the read/write works without it):

#include <unistd.h>#include <fcntl.h>#include <stdio.h>int main(){    // Open file with write permission (create if doesn't exist).    int fd = open("float.txt", O_CREAT | O_WRONLY);    float val = 1.5f;    if (fd != -1) {        write(fd, &val, sizeof(val));        close(fd);    }    // Test read.    fd = open("float.txt", O_RDONLY);    float new_val;    if (fd != -1) {        read(fd, &new_val, sizeof(new_val));        printf("new_val = %f\n", new_val);        close(fd);    }    return 0;}

int foo;write (fd, &foo, sizeof foo);

As ascertained in comments, the problem is more to convert a number to a decimal numeral than to use the write call.

To write an int or float that is readable by humans, you must convert it to a numeral, then write the characters of that numeral. For float, if the internal representation uses a different base (e.g., binary) than the numeral (e.g., decimal), then this requires a lot of work to do correctly. I would recommend either using existing open-source code or a scientific paper on the topic.

The code to convert an int to a decimal numeral is fairly straightforward:

#include <stdlib.h> //  For size_t./*  IntToString converts the int x to a decimal numeral, which is written to s.    No terminal null character is written.  The number of characters written is    returned.  s MUST point to space with enough room for the numeral,    including a leading '-' if x is negative.*/size_t IntToString(char *s, int x){    //  Set pointer to current position.    char *p = s;    //  Set t to absolute value of x.    unsigned t = x;    if (x < 0) t = -t;    //  Write digits.    do    {        *p++ = '0' + t % 10;        t /= 10;    } while (t);    //  If x is negative, write sign.    if (x < 0)        *p++ = '-';    //  Remember the return value, the number of characters written.    size_t r = p-s;    //  Since we wrote the characters in reverse order, reverse them.    while (s < --p)    {        char t = *s;        *s++ = *p;        *p = t;    }    return r;}#include <stdio.h>  //  For printf.//  Demonstrate IntToString.static void Demonstrate(int x){    char buf[100];    size_t n = IntToString(buf, x);    printf("IntToString(%d) = %.*s.\n", x, (int) n, buf);}#include <limits.h> //  For INT_MIN and INT_MAX.int main(void){    Demonstrate(0);    Demonstrate(1);    Demonstrate(9);    Demonstrate(10);    Demonstrate(INT_MAX-1);    Demonstrate(INT_MAX);    Demonstrate(-1);    Demonstrate(-9);    Demonstrate(-10);    Demonstrate(INT_MIN+1);    Demonstrate(INT_MIN);    return 0;}