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more progress on I/O demo

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Perry Kivolowitz 2022-12-26 12:59:57 -06:00
parent 65eb208440
commit 46ec454c70
1 changed files with 13 additions and 2 deletions
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15
section_1/funcs/file_ops.s
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@ -19,11 +19,13 @@ fd .req w28
main: stp x29, x30, [sp, -16]! main: stp x29, x30, [sp, -16]!
stp x27, x28, [sp, -16]! stp x27, x28, [sp, -16]!
bl open_file bl open_file
// w0 will contain either the file descriptor of the new // w0 will contain either the file descriptor of the new
// file or -1 for a failure. Note that the value in w0 // file or -1 for a failure. Note that the value in w0
// has also been copied to "fd" - a register alias. // has also been copied to "fd" - a register alias.
cmp w0, wzr cmp w0, wzr
bge 1f bge 1f
// If we get here, the open has failed. Use perror() to // If we get here, the open has failed. Use perror() to
// print a meaningful error and branch to exit. The return // print a meaningful error and branch to exit. The return
// code of the program will be set to non-zero inside fail. // code of the program will be set to non-zero inside fail.
@ -37,6 +39,7 @@ main: stp x29, x30, [sp, -16]!
// after printing an error message. // after printing an error message.
bl write_data bl write_data
cbz w0, 10f cbz w0, 10f
// If we get here, there was an error in write_data. Print // If we get here, there was an error in write_data. Print
// a reasonable error message then branch to the clean usleep // a reasonable error message then branch to the clean usleep
// code. // code.
@ -49,6 +52,7 @@ main: stp x29, x30, [sp, -16]!
// lseek(). // lseek().
10: bl seek_zero 10: bl seek_zero
cbz x0, 20f cbz x0, 20f
// If we get here, the seek failed. Cause a reasonable // If we get here, the seek failed. Cause a reasonable
// message to be printed then branch to the clean up code. // message to be printed then branch to the clean up code.
ldr x0, =sf ldr x0, =sf
@ -75,9 +79,13 @@ main: stp x29, x30, [sp, -16]!
Explanation of the magic numbers: Explanation of the magic numbers:
int open(const char *pathname, int flags, mode_t mode); int open(const char *pathname, int flags, mode_t mode);
octal 102 for flags is O_RDRW | O_CREAT octal 102 for flags is O_RDRW | O_CREAT
octal 600 for mode is rw------- i.e. read and write for octal 600 for mode is rw------- i.e. read and write for
the owner but no permissions for anyone else. the owner but no permissions for anyone else.
There is a version of open() that takes two parameters. However,
if O_CREAT is specified, the three parameter version is required.
*/ */
.equ O_FLAGS, 0102 .equ O_FLAGS, 0102
@ -94,9 +102,9 @@ open_file:
ret ret
/* This function uses perror to print a meaningful error /* This function uses perror() to print a meaningful error
message in the event of a failure. The string value message in the event of a failure. The string value
passed to perror arrives to us as a pointer in x0. passed to perror() arrives to us as a pointer in x0.
*/ */
fail: fail:
@ -107,6 +115,9 @@ fail:
ret ret
/* ssize_t write(int fd, const void *buf, size_t count); /* ssize_t write(int fd, const void *buf, size_t count);
This function will write a string to the file descriptor contained
in "fd" (a register alias).
*/ */
write_data: write_data: