Simple Logger in C

This is a simple logger in C which logs the output of a C program to a file.

It has got following features:
1. Logging the time of compilation
2. Logging the file name from which the log is coming from
3. Line number at which the log function is called from.

Currently it outputs the logs to a file "log.txt" in the same directory as the logger file , but it can be customized as per usage.

To compile :
gcc -o output logger.c test.c

To Run :
./output

Open file "log.txt" in the same directory to see the logs.

Happy Logging !!!

//logger.h
void log_print(char* filename, int line, char *fmt,...);
#define LOG_PRINT(...) log_print(__FILE__, __LINE__, __VA_ARGS__ )

//logger.c
#include <stdio.h>
#include <stdarg.h>
#include <time.h>
#include <string.h>
#include <stdlib.h>
#include "logger.h"
FILE *fp ;
static int SESSION_TRACKER; //Keeps track of session

char* print_time()
{
    int size = 0;
    time_t t;
    char *buf;
    
    t=time(NULL); /* get current calendar time */
    
    char *timestr = asctime( localtime(&t) );
    timestr[strlen(timestr) - 1] = 0;  //Getting rid of \n
    
    size = strlen(timestr)+ 1 + 2; //Additional +2 for square braces
    buf = (char*)malloc(size);
    
    memset(buf, 0x0, size);
    snprintf(buf,size,"[%s]", timestr);
   
    return buf;
}
void log_print(char* filename, int line, char *fmt,...)
{
    va_list         list;
    char            *p, *r;
    int             e;

    if(SESSION_TRACKER > 0)
      fp = fopen ("log.txt","a+");
    else
      fp = fopen ("log.txt","w");
    
    fprintf(fp,"%s ",print_time());
    fprintf(fp,"[%s][line: %d] ",filename,line);
    va_start( list, fmt );

    for ( p = fmt ; *p ; ++p )
    {
        if ( *p != '%' )//If simple string
        {
            fputc( *p,fp );
        }
        else
        {
            switch ( *++p )
            {
                /* string */
            case 's':
            {
                r = va_arg( list, char * );

                fprintf(fp,"%s", r);
                continue;
            }

            /* integer */
            case 'd':
            {
                e = va_arg( list, int );

                fprintf(fp,"%d", e);
                continue;
            }

            default:
                fputc( *p, fp );
            }
        }
    }
    va_end( list );
    fputc( '\n', fp );
    SESSION_TRACKER++;
    fclose(fp);
}

//test.c
#include <stdio.h>
#include "logger.h"
int main()
{
  
  int x = 199,i=0;
  char *s = "Bitter Truth";

  while(i<5)
  {
   LOG_PRINT("Hello World ");
   LOG_PRINT("Zing is back !!! %s %d",s,x++);
   i++;
  }
}