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This program will simply write some hardcoded records to disk. It will:
Open the file
Write three records
Close the file
Type the following code into a file called write-records.s:
.include "linux.s" .include "record-def.s" .section .data #Constant data of the records we want to write #Each text data item is padded to the proper #length with null (i.e. 0) bytes. #.rept is used to pad each item. .rept tells #the assembler to repeat the section between #.rept and .endr the number of times specified. #This is used in this program to add extra null #characters at the end of each field to fill #it up record1: .ascii "Fredrick\0" .rept 31 #Padding to 40 bytes .byte 0 .endr .ascii "Bartlett\0" .rept 31 #Padding to 40 bytes .byte 0 .endr .ascii "4242 S Prairie\nTulsa, OK 55555\0" .rept 209 #Padding to 240 bytes .byte 0 .endr .long 45 record2: .ascii "Marilyn\0" .rept 32 #Padding to 40 bytes .byte 0 .endr .ascii "Taylor\0" .rept 33 #Padding to 40 bytes .byte 0 .endr .ascii "2224 S Johannan St\nChicago, IL 12345\0" .rept 203 #Padding to 240 bytes .byte 0 .endr .long 29 record3: .ascii "Derrick\0" .rept 32 #Padding to 40 bytes .byte 0 .endr .ascii "McIntire\0" .rept 31 #Padding to 40 bytes .byte 0 .endr .ascii "500 W Oakland\nSan Diego, CA 54321\0" .rept 206 #Padding to 240 bytes .byte 0 .endr .long 36 #This is the name of the file we will write to file_name: .ascii "test.dat\0" .equ ST_FILE_DESCRIPTOR, -4 .globl _start _start: #Copy the stack pointer to %ebp movl %esp, %ebp #Allocate space to hold the file descriptor subl $4, %esp #Open the file movl $SYS_OPEN, %eax movl $file_name, %ebx movl $0101, %ecx #This says to create if it #doesn't exist, and open for #writing movl $0666, %edx int $LINUX_SYSCALL #Store the file descriptor away movl %eax, ST_FILE_DESCRIPTOR(%ebp) #Write the first record pushl ST_FILE_DESCRIPTOR(%ebp) pushl $recordl call write_record addl $8, %esp #Write the second record pushl ST_FILE_DESCRIPTOR(%ebp) pushl $record2 call write_record addl $8, %esp #Write the third record pushl ST_FILE_DESCRIPTOR(%ebp) pushl $record3 call write_record addl $8, %esp #Close the file descriptor movl $SYS_CLOSE, %eax movl ST_FILE_DESCRIPTOR(%ebp), %ebx int $LINUX_SYSCALL #Exit the program movl $SYS_EXIT, %eax movl $0, %ebx int $LINUX_SYSCALL
This is a fairly simple program. It merely consists of defining the data we want to write in the . data section, and then calling the right system calls and function calls to accomplish it. For a refresher of all of the system calls used, see Appendix C.
You may have noticed the lines:
.include "linux.s" .include "record-def.s"
These statements cause the given files to basically be pasted right there in the code. You don't need to do this with functions, because the linker can take care of combining functions exported with .globl. However, constants defined in another file do need to be imported in this way.
Also, you may have noticed the use of a new assembler directive, .rept. This directive repeats the contents of the file between the .rept and the .endr directives the number of times specified after .rept. This is usually used the way we used it - to pad values in the . data section. In our case, we are adding null characters to the end of each field until they are their defined lengths.
To build the application, run the commands:
as write-records.s -o write-record.o as write-record.s -o write-record.o ld write-record.o write-records.o -o write-records
Here we are assembling two files separately, and then combining them together using the linker. To run the program, just type the following:
./write-records
This will cause a file called test.dat to be created containing the records. However, since they contain non-printable characters (the null character, specifically), they may not be viewable by a text editor. Therefore we need the next program to read them for us.
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