How to write data into an external file on Linux?
For instance, if I start the code with .include "output.s" and want to write "1111 0000" into it, then how to do this.
>Solution :
The best way to do file I/O is to use the libc. Envision how you would do it in C, then do the same thing in assembly. For example, in C you could do
int main() {
ssize_t n;
int fd;
char buf[9] = "1111 0000";
fd = creat("output.s", 0666);
if (fd == -1) {
perror("output.s");
return (EXIT_FAILURE);
}
n = write(fd, buf, sizeof buf);
if (n == -1) {
perror("output.s");
return (EXIT_FAILURE);
}
close(fd);
return (EXIT_SUCCESS);
}
Next, translate the code step by step into assembly:
.data
name: .string "output.s" # the file name
buf: .ascii "1111 0000"
buflen= .-buf # the length of buf
.text
.globl main
.type main, @function
main: push %rbx # free rbx and align stack
lea name(%rip), %rdi # file name
mov $0666, %esi # initial permissions
call creat # create the file
cmp $-1, %eax # was the call successful?
je .Lfail # if not, process error
mov %eax, %ebx # stash a copy of the file descriptor
mov %eax, %edi # file descriptor
lea buf(%rip), %rsi # buffer
mov $buflen, %edx # buffer length
call write # write data
cmp $-1, %rax # was the call successful?
je .Lfail # if not, process error
mov %ebx, %edi # file descriptor
call close # close the file
xor %eax, %eax # return EXIT_SUCCESS
pop %rbx # restore %rbx
ret
.Lfail: lea name(%rip), %rdi # the file name
call perror # print an error message
mov $1, %eax # return EXIT_SUCCESS
pop %rbx # restore %rbx
ret
.size main, .-main