Inline Assembler - Example of A System Call

Example of A System Call

Calling an operating system directly is generally impossible in the presence of protected memory. The OS runs at a more privileged level (kernel mode) than the user (user mode); a (software) interrupt is used to make requests to the operating system. This is rarely a feature in a higher-level language, and so wrapper functions for system calls are written using inline assembler.

The following C code are samples including a system call wrapper in AT&T assembler syntax with the GNU Assembler. They are normally written with the aid of macros; the full code is included for clarity.

The format of basic inline assembly is very much straightforward. Its basic form is

asm("assembly code");

Example:

asm("movl %ecx, %eax"); /* moves the contents of ecx to eax */

OR

__asm__("movb %bh, (%eax)"); /* moves the byte from bh to the memory pointed by eax */

Both asm and __asm__ are valid. __asm__ can be used if the keyword asm conflicts with something in your program.

extern int errno; int funcname(int arg1, int *arg2, int arg3) { int res; __asm__ volatile( "int $0x80" /* make the request to the OS */ : "=a" (res), /* return result in eax ("a") */ "+b" (arg1), /* pass arg1 in ebx ("b") */ "+c" (arg2), /* pass arg2 in ecx ("c") */ "+d" (arg3) /* pass arg3 in edx ("d") */ : "a" (128) /* pass system call number in eax ("a") */ : "memory", "cc"); /* announce to the compiler that the memory and condition codes have been modified */ /* The operating system will return a negative value on error; * wrappers return -1 on error and set the errno global variable */ if (-125 <= res && res < 0) { errno = -res; res = -1; } return res; }