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The best way to get a sense for the AT&T assembly syntax is to look at an actual AT&T-style s file produced by gcc Doing this actually has two benefits: First of all, it will help you become familiar with the AT&T mnemonics and formatting conventions In addition, you may find it useful, when struggling to figure out how to call a C library function from assembly, to create a short C program that calls the function of interest and then examines the s file that gcc produces when it compiles your C program The dateisc program which follows was part of my early research, and I used it to get a sense for how ctime() was called at the assembly level Obviously, for this trick to work you must have at least a journeyman understanding of the AT&T mnemonics (I discuss ctime() and other time-related C library calls in detail in the next chapter) You don't automatically get a s file every time you compile a C program The s file is created, but once gas assembles the s file to a binary object code file (typically a o file), it deletes the s file If you want to examine a s file created by gcc, you must compile with the -S option (Note that this is an uppercase S Case matters big time in the Unix world!) The command would look like this: gcc dateisc -S Note that the output of this command is the assembly source file only If you specify the -S option, gcc understands that you want to generate assembly source rather than an executable program file, so all it will generate is the s file To compile a C program to an executable program file, you must compile it again without the -S option Here's dateisc It does nothing more than print out the date and time as returned by the standard C library function ctime(): #include <timeh> #include <stdioh> int main() { time_t timeval; (void)time(&timeval); printf("The date is: %s", ctime(&timeval)); exit(0); } It's not much of a program, but it does illustrate the use of three C library function calls, time(), ctime(), and printf() When gcc compiles the preceding program (dateisc), it produces the file dateiss, which follows I have manually added the equivalent Intel mnemonics as comments to the right of the AT&T mnemonics, so
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you can see what equals what in the two systems (Alas, neither gcc nor any other utility I have ever seen will do this for you!) file "dateisc" version "0101" gcc2_compiled: section rodata LC0: string "The date is: %s" text align 4 globl main type main,@function main: pushl %ebp # push ebp movl %esp,%ebp # mov ebp,esp subl $4,%esp # sub esp,4 leal -4(%ebp),%eax # lea eax,ebp-4 pushl %eax # push eax call time # call time addl $4,%esp # add esp,4 leal -4(%ebp),%eax # lea eax,ebp-4 pushl %eax # push eax call ctime # call ctime addl $4,%esp # add esp,4 movl %eax,%eax # mov eax,eax pushl %eax # push eax pushl $LC0 # push dword LC0 call printf # call printf addl $8,%esp # add esp,8 pushl $0 # push dword 0 call exit # call exit addl $4,%esp # add esp,4 p2align 4,,7 L1: leave # leave ret # ret Lfe1: size main,Lfe1-main ident "GCC: (GNU) egcs-29166 19990314/Linux (egcs-112 release)" One thing to keep in mind when reading this is that dateiss is assembly language code produced mechanically by a compiler, and not by a human programmer! Some things about the code (such as why the label L1 is present but never referenced) are less than ideal and can only be explained as artifacts of gcc's compilation machinery In a more complex program there may be some customary use of a label L1 that doesn't exist in a program this simple Some quick things to note here while reading the preceding listing: When an instruction does not take operands (call, leave, ret), it does not have an operand-size suffix Calls and returns look pretty much alike in both Intel and AT&T syntax When referenced, the name of a message string is prefixed by a dollar sign ($) the same way that numeric literals are In NASM, a named string variable is considered a variable and not a literal This is just another AT&T peccadillo to be aware of Note that the comment delimiter in the AT&T scheme is the pound sign (#) rather than the semicolon used in nearly all Intel-style assemblers, including NASM
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