Editing Inline assembler (section)

=== A system call in GCC ===

Calling an operating system directly is generally not possible under a system using 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 function]]s for system calls are written using inline assembler.

The following C code example shows an x86 system call wrapper in [[AT&T syntax|AT&T assembler syntax]], using the [[GNU Assembler]]. Such calls are normally written with the aid of macros; the full code is included for clarity. In this particular case, the wrapper performs a system call of a number given by the caller with three operands, returning the result.<ref>{{man|2|syscall|Linux}}</ref>

To recap, GCC supports both ''basic'' and ''extended'' assembly. The former simply passes text verbatim to the assembler, while the latter performs some substitutions for register locations.<ref name=GCCEXT/>

<syntaxhighlight lang="c">
extern int errno;

int syscall3(int num, int arg1, int arg2, int arg3)
{
  int res;
  __asm__ (
    "int $0x80"        /* make the request to the OS */
    : "=a" (res),      /* return result in eax ("a") */
      "+b" (arg1),     /* pass arg1 in ebx ("b") [as a "+" output because the syscall may change it] */
      "+c" (arg2),     /* pass arg2 in ecx ("c") [ditto] */
      "+d" (arg3)      /* pass arg3 in edx ("d") [ditto] */
    : "a"  (num)       /* pass system call number in eax ("a") */
    : "memory", "cc",  /* announce to the compiler that the memory and condition codes have been modified */
      "esi", "edi", "ebp"); /* these registers are clobbered [changed by the syscall] too */

/* 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;
}
</syntaxhighlight>