Using of a system call in c - c

Question

I would like to create a program name patch witch recives a FILENAME and a string.In the file there is the string 'allis'.I need to replace each time the string 'allis' appears in the text with the string I recieved as an input.
I need to use only system call and not the standat libary.
This is an example code on how the code should look like(this just example of the structure it is not the mission, the code writes "allis" word instead of "hello world"):
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc , char* argv[], char* envp[])
{
char buf[6];
int fd = open(argv[1],O_WRONLY,0777);
if (fd == -1)
exit(1);
write(fd,"hello world!\n",13);
close(fd);
fd = open(argv[1], O_RDONLY);
if (fd == -1)
exit(1);
read(fd,buf,5);
buf[5]='\0';
close(fd);
fd = open(argv[1],O_WRONLY,0777);
if (fd == -1)
exit(1);
write(fd,"allis",5);
close(fd);
return 0;
}
I need to use the system call lseek but I dont know how I should use it because I dont know where in the text 'allis' is located and what happens if the string i get as an input is bigger than 'allis' I need to move all the text to the right or something because I will override other text.
EDIT:
I wrote a code which check char by char (instead of allis its shira and it replace it with sergay)It actually worked. and than I tried to change the code to work with argv[1] and argv[2] as needed and it didnt work I revert back to the code that originally worked for me and its just doesnt work now!! The problem is that the buf doesnt take 5 character (only 1)
and believe me or not it worked before.
The code :
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#define SYS_WRITE 1
#define STDOUT 1
extern int system_call();
int main(int argc , char* argv[], char* envp[])
{
char buf[5];
int index1=0,index2=0;//the index of the file
int fd1;
int fd = open("file.txt", O_RDONLY);
if (fd == -1)
return 1;
fd1 = open("copytext.txt",O_WRONLY,0777);
if (fd1 == -1)
return 1;
int i =0;
while (fd!=-1){//while there is still info to read
printf("enter the %d loop\n",i);
i++;
if(read(fd,buf,5) == 0 ) break;//read 5 chars from the file //buf[5]='\0';//!! not sure about this
printf("the buffer is: %s\n",buf);
if ((buf[0]=='s') && (buf[1]=='h') && (buf[2]=='i') && (buf[3]=='r') && (buf[4]=='a'))// if (strcmp(buf,"shira")==0)//if we found shira
{
write(fd1,"sergay",6);//replace shira
index2 = index2 + 6;
if(lseek(fd1,index2,SEEK_SET) < 0) break;
index1 = index1 + 5;//size of shira
if(lseek(fd,index1,SEEK_SET) < 0) break;
}
else {//we did not find shira;
write(fd1,buf,1);//write only one
index2++;
if(lseek(fd1,index2,SEEK_SET) < 0) break;
index1++;
if(lseek(fd,index1,SEEK_SET) < 0) break;
}
}
printf("get out of the loop\n");
char buffer[index2];
fd1 = open("copytext.txt", O_RDONLY);
if (fd1 == -1)
return 1;
if(lseek(fd1,0,SEEK_SET) < 0) return 1;
if(read(fd1,buffer,index2) == 0 ) return 1;
fd = open("file.txt",O_WRONLY,0777);
if (fd == -1)
return 1;
if(lseek(fd,0,SEEK_SET) < 0) return 1;
write(fd,buffer,index2);
close(fd);
close(fd1);
return 0;
}