Linux simulate write() system call failure using ptrace() - linux

Question

I am trying to simulate failure of write() system call.
I have read that return value -1 (in EAX) indicates error in system call and errno gives the exact reason for failure.
I am trying to intercept system call write() return -1 in EAX register and set the "errno" to some error value.
puts() internally uses write(), which is system call number 4.
If i do perror("Error:") in the child, it should show the error corresponding to "errno" which i would like to set.
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <sys/reg.h> /* For constants ORIG_EAX etc */
#include <stdio.h>
#include <sys/user.h>
#include <errno.h>
int main()
{ pid_t child;
int status;
long orig_eax,eax,params[3];
int ret_val=-1,insyscall=0;
struct user_regs_struct regs;
child = fork();
if(child == 0) {
ptrace(PTRACE_TRACEME, 0, NULL, NULL);
execl("/home/kashi/Documents/2nd_Sem/MyPrgms/ptrace/ramana/write", "write", NULL);
//execl("/bin/ls","ls",NULL);
}
else {
while(1)
{
wait(&status);
if(WIFEXITED(status))
break;
orig_eax = ptrace(PTRACE_PEEKUSER,
child, 4 * ORIG_EAX, NULL);
if(orig_eax == 4)
{
ptrace( PTRACE_GETREGS, child, 0, &regs );
printf("Write called with %ld, %ld %ld\n",regs.ebx, regs.ecx,regs.edx);
if(insyscall == 0)
{
/* Syscall entry */
printf("In %d\n",insyscall);
insyscall = 1;
}
else
{
/* Syscall exit */
regs.orig_eax=-1;
**errno=11; //This errno should be set in the child process, how to do it?**
ptrace( PTRACE_SETREGS, child, 0, &regs );
eax = ptrace(PTRACE_PEEKUSER, child, 4 * ORIG_EAX, NULL);
printf("Write returned with %ld\n", eax);
insyscall = 0;
}
}
//printf("The child made a "
// "system call %ld\n", regs.orig_eax);
ptrace(PTRACE_SYSCALL, child, NULL, NULL);
}
}
return 0;
}
The tracee program (testwrite.c) is:
#include<stdio.h>
#include<unistd.h>
#include<sys/ptrace.h>
#include<signal.h>
#include<errno.h>
//int display(char *p);
int main()
{
printf("Hi Kashi\n");
perror("Error: ");
return 0;
}
output:
[kashi#localhost ramana]$ ./test.sh
In 0
Hi Kashi
Write returned with -1
In 0
**Error: : Success**
Write returned with -1
The perror("Error:") displays text message corresponding to "errno". I am not able to set this "errno" in child process, how can i do it?