creating pipe between father and child process - c

Question

I'm trying to create a pipe between father and child process.
in this pipe, the child process will write data and the father will read and print it.
I don't know why but if I enter a big string the data got wrong, for strings with +- 7 words it still do fine.
I guess it is about the size of the buffer but can't fix it.
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
/* in this code i will make a child process with fork command
then i will create pipe using pipe commands.
i will transfer data from the child process to the father process
omriziner code
*/
void main(int argc,char *argv[])
{
if(argc < 2){
printf("prototype error \n<Enter any data you wana write> \n");
return;
}
int fd[2]; // creating array with 2 places for 2 fd'stdio
// fd[0] is set to read file in the pipe
//fd[1] is set to write file in the pipe
int piperes;
pid_t childpid;
char buff[5];
char * data = "learning to the exam";
printf("father pid %d:\n",getpid());
printf ("size of data is %d \n",(int)sizeof(argv[1]));
printf ("size of buff is %d \n",(int)sizeof(buff));
piperes = pipe(fd);
if(piperes < 0){
perror("PIPE ERR");
exit(1);
}
printf("Pipe succeed \n");
if((childpid = fork()) == -1){ // fork will create a child process
perror("FORK ERR");
exit(1);
}
// when fork suceed - the pid of the child will return in the parent and 0 will return in the child
// when fork fail - the pid will be -1
printf("Fork succeed, fork return is %d and process pid is %d :\n",childpid,getpid());
if(childpid == 0){ // if pid zero , wer in the child prcs
close(fd[0]);
write(fd[1],argv[1],sizeof(argv[1])); // send data to the write fd of the pipe
printf("data was written to fd[1] by pid : %d \n",getpid());
exit(0);
}
else{ // in this case, we're in the father process
close(fd[1]);
read(fd[0],buff,sizeof(argv[1])+1);
printf("Recived data is ''%s''", buff);
printf("By pid : %d \n",getpid());
exit(1);
}
}

Answer 1

sizeof(argv[1])
This does not do what you think it does.
sizeof is evaluated at compile-time1, and in this case will return 8 (assuming you're on a 64-bit machine), because argv[1] is a pointer.
Because you want the length of the string (which can only be known at run-time), you should instead use:
strlen(argv[1])
1 - There are cases where sizeof is evaluated at run-time. This is not one of them.