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4. System Calls

4.1 System call numbers

System calls are identified by their numbers. The number of the call foo is __NR_foo. For example, the number of _llseek used above is __NR__llseek, defined as 140 in /usr/include/asm-i386/unistd.h. Different architectures have different numbers.

Often, the kernel routine that handles the call foo is called sys_foo. One finds the association between numbers and names in the sys_call_table, for example in arch/i386/kernel/entry.S.


The world changes and system calls change. Since one must not break old binaries, the semantics associated to any given system call number must remain fully backwards compatible.

What happens in practice is one of two things: either one gets a new and improved system call with a new name and number, and the libc routine that used to invoke the old call is changed to use the new one, or the new call (with new number) gets the old name, and the old call gets "old" prefixed to its name.

For example, long ago user IDs had 16 bits, today they have 32. __NR_getuid is 24, and __NR_getuid32 is 199, and the former belongs to the 16-bit version of the call, the latter to the 32-bit version. Looking at the associated kernel routines, we find that these are sys_getuid16 and sys_getuid, respectively. (Thus, sys_getuid does not have number __NR_getuid.) Looking at glibc, we find code somewhat like

int getuid32_available = UNKNOWN;

uid_t getuid(void) {
        if (getuid32_available == TRUE)
                return INLINE_SYSCALL(getuid32, 0);
        if (getuid32_available == UNKNOWN) {
                uid_t res = INLINE_SYSCALL(getuid32, 0);

                if (res == 0 || errno != ENOSYS) {
                        getuid32_available = TRUE;
                        return res;
                getuid32_available = FALSE;
        return INLINE_SYSCALL(getuid, 0);

For an example where the name was moved and the old call got a name prefixed by "old", see __NR_oldolduname, __NR_olduname, __NR_uname, belonging to sys_olduname, sys_uname, sys_newuname, respectively. One also has __NR_oldstat, __NR_stat, __NR_stat64 belonging to sys_stat, sys_newstat, sys_stat64, respectively. And __NR_umount, __NR_umount2 belonging to sys_oldumount, sys_umount, respectively. And __NR_select, __NR__newselect belonging to old_select, sys_select, respectively.

These moving names are confusing - now you have been warned: the system call with number __NR_foo does not always belong to the kernel routine sys_foo().

4.2 The call

What happens? The assembler for a call with 0 parameters (on i386) is

#define _syscall0(type,name) \
type name(void) \
{ \
long __res; \
__asm__ volatile ("int $0x80" \
        : "=a" (__res) \
        : "0" (__NR_##name)); \
__syscall_return(type,__res); \
Thus, the basic ingredient is the assembler instruction INT 0x80. This causes a programmed exception and calls the kernel system_call routine. Some relevant code fragments:

/* include/asm-i386/hw_irq.h */
#define SYSCALL_VECTOR              0x80

/* arch/i386/kernel/traps.c */

/* arch/i386/kernel/entry.S */
#define GET_CURRENT(reg) \
        movl $-8192, reg; \
        andl %esp, reg

#define SAVE_ALL \
        cld; \
        pushl %es; \
        pushl %ds; \
        pushl %eax; \
        pushl %ebp; \
        pushl %edi; \
        pushl %esi; \
        pushl %edx; \
        pushl %ecx; \
        pushl %ebx; \
        movl $(__KERNEL_DS),%edx; \
        movl %edx,%ds; \
        movl %edx,%es;

#define RESTORE_ALL     \
        popl %ebx;      \
        popl %ecx;      \
        popl %edx;      \
        popl %esi;      \
        popl %edi;      \
        popl %ebp;      \
        popl %eax;      \
1:      popl %ds;       \
2:      popl %es;       \
        addl $4,%esp;   \
3:      iret;

        pushl %eax                      # save orig_eax
        testb $0x02,tsk_ptrace(%ebx)    # PT_TRACESYS
        jne tracesys
        cmpl $(NR_syscalls),%eax
        jae badsys
        call *SYMBOL_NAME(sys_call_table)(,%eax,4)
        movl %eax,EAX(%esp)             # save the return value

        cli                             # need_resched and signals atomic test
        cmpl $0,need_resched(%ebx)
        jne reschedule
        cmpl $0,sigpending(%ebx)
        jne signal_return

We transfer execution to system_call, save the original value of the EAX register (it is the number of the system call), save all other registers, verify that we are not being traced (otherwise the tracer must be informed and entirely different things happen), make sure that the system call number is within range, and call the appropriate kernel routine from the table sys_call_table. Upon return we check a few things and when all is well restore the registers and call IRET to return from this INT.

(This was for the i386 architecture. All details differ on other architectures, but the basic idea is the same: store the syscall number and the syscall parameters somewhere the kernel can find them, in registers, on the stack, or in a known place of memory, do something that causes a transfer to kernel code, etc.)

4.3 System call parameters

On i386, the parameters of a system call are transported via registers. The system call number goes into %eax, the first parameter in %ebx, the second in %ecx, the third in %edx, the fourth in %esi, the fifth in %edi, the sixth in %ebp.

Ancient history

Earlier versions of Linux could handle only four or five system call parameters, and therefore the system calls select() (5 parameters) and mmap() (6 parameters) used to have a single parameter that was a pointer to a parameter block in memory. Since Linux 1.3.0 five parameters are supported (and the earlier select with memory block was renamed old_select), and since Linux 2.3.31 six parameters are supported (and the earlier mmap with memory block was succeeded by the new mmap2).

4.4 Error return

Above we said: typically, the kernel returns a negative value to indicate an error. But this would mean that any system call only can return positive values. Since the negative error returns are of the form -ESOMETHING, and the error numbers have small positive values, there is only a small negative error range. Thus

#define __syscall_return(type, res) \
do { \
        if ((unsigned long)(res) >= (unsigned long)(-125)) { \
                errno = -(res); \
                res = -1; \
        } \
        return (type) (res); \
} while (0)
Here the range [-125,-1] is reserved for errors (the constant 125 is version and architecture dependent) and other values are OK.

What if a system call wants to return a small negative number and it is not an error? The scheduling priority of a process is set by setpriority() and read by getpriority(), and this value ranges from -20 (top priority) to 19 (lowest priority background job). The library routines with these names use these numbers, but the system call getpriority() returns 20 - P instead of P, moving the output interval to positive numbers only.

Or, similarly, the subfunctions PEEK* of ptrace return the contents of a memory word in the traced process, and any value is possible. However, the system call returns this value in the data argument, and glibc does something like

        res = sys_ptrace(request, pid, addr, &data);
        if (res >= 0) {
                errno = 0;
                res = data;
        return res;
so that a user program has to do
        errno = 0;
        res = ptrace(PTRACE_PEEKDATA, pid, addr, NULL);
        if (res == -1 && errno != 0)
                /* error */

4.5 Interrupted system calls

Above we saw in ret_from_sys_call the test on sigpending: if a signal arrived while we were executing kernel code, then just before returning from the system call we first call the user program's signal handler, and when this finishes return from the system call.

When a system call is slow and a signal arrives while it was blocked, waiting for something, the call is aborted and returns -EINTR, so that the library function will return -1 and set errno to EINTR. Just before the system call returns, the user program's signal handler is called.

(So, what is "slow"? Mostly those calls that can block forever waiting for external events; read and write to terminal devices, but not read and write to disk devices, wait, pause.)

This means that a system call can return an error while nothing was wrong. Usually one will want to redo the system call. That can be automated by installing the signal handler using a call to sigaction with the SA_RESTART flag set. The effect is that upon an interrupt the system call is aborted, the user program's signal handler is called, and afterwards the system call is restarted from the beginning.

Why is this not the default? It was, for a while, but often it is necessary to react to a signal while the reacting is not done by the signal handler itself. It is difficult to do nontrivial things in a signal handler since the rest of the program is in an unknown state, and most signal handlers just set a flag that is tested elsewhere.

A demo:

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>

int got_interrupt;

void intrup(int dummy) {
        got_interrupt = 1;

void die(char *s) {
        printf("%s\n", s);

int main() {
        struct sigaction sa;
        int n;
        char c;

        sa.sa_handler = intrup;
        sa.sa_flags = 0;
        if (sigaction(SIGINT, &sa, NULL))
        sa.sa_flags = SA_RESTART;
        if (sigaction(SIGQUIT, &sa, NULL))

        got_interrupt = 0;
        n = read(0, &c, 1);
        if (n == -1 && errno == EINTR)
                printf("read call was interrupted\n");
        else if (got_interrupt)
                printf("read call was restarted\n");

        return 0;

Here Ctrl-C will interrupt the read call, while after Ctrl-\ the read call is restarted.

Partial success

There are other cases where a syscall has to be done in several steps. Instead of just calling the system call write() it may be necessary to do

ssize_t my_write(int fd, const void *buf, size_t count) {
        ssize_t res;

        while (count) {
                res = write(fd, buf, count);
                if (res < 0)
                        return res;
                buf += res;
                count -= res;
        return 0;
even when writing to an ordinary disk file. Indeed, since 2.6.16 there is a limit MAX_RW_COUNT in read_write.c that causes a maximal write size of INT_MAX & PAGE_CACHE_MASK which may be 2^31-1-4095 = 2147479552. This might violate POSIX. Usually a write only returns a short count when interrupted by a signal, or when the disk is full, or the max file size is reached.

4.6 Sysenter and the vsyscall page

It has been observed that a 2 GHz Pentium 4 was much slower than an 850 MHz Pentium III on certain tasks, and that this slowness is caused by the very large overhead of the traditional int 0x80 interrupt on a Pentium 4.

Some models of the i386 family do have faster ways to enter the kernel. On Pentium II there is the sysenter instruction. Also AMD has a syscall instruction. It would be good if these could be used.

Something else is that in some applications gettimeofday() is a done very often, for example for timestamping all transactions. It would be nice if it could be implemented with very low overhead.

One way of obtaining a fast gettimeofday() is by writing the current time in a fixed place, on a page mapped into the memory of all applications, and updating this location on each clock interrupt. These applications could then read this fixed location with a single instruction - no system call required.

There might be other data that the kernel could make available in a read-only way to the process, like perhaps the current process ID. A vsyscall is a "system" call that avoids crossing the userspace-kernel boundary.

Linux is in the process of implementing such ideas. Since Linux 2.5.53 there is a fixed page, called the vsyscall page, filled by the kernel. At kernel initialization time the routine sysenter_setup() is called. It sets up a non-writable page and writes code for the sysenter instruction if the CPU supports that, and for the classical int 0x80 otherwise. Thus, the C library can use the fastest type of system call by jumping to a fixed address in the vsyscall page.

This page was changed to have the structure of an ELF binary (called in Linux 2.5.69. In Linux 2.5.74 the name was changed to

Concerning gettimeofday(), a vsyscall version for the x86-64 is already part of the vanilla kernel. Patches for i386 exist. (An example of the kind of timing differences: John Stultz reports on an experiment where he measures gettimeofday() and finds 1.67 us for the int 0x80 way, 1.24 us for the sysenter way, and 0.88 us for the vsyscall.)

Some details

The kernel maps a page (0xffffe000-0xffffefff) in the memory of every process. (This is the next-to-last addressable page. The last is not mapped - maybe to avoid bugs related to wraparound.) We can read it:

/* get vsyscall page */
#include <unistd.h>
#include <string.h>

int main() {
        char *p = (char *) 0xffffe000;
        char buf[4096];
#if 0
        write(1, p, 4096);
        /* this gives EFAULT */
        memcpy(buf, p, 4096);
        write(1, buf, 4096);
        return 0;
and if we do, find an ELF binary.
% ./get_vsyscall_page > syspage
% file syspage
syspage: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), stripped
% objdump -h syspage

syspage:     file format elf32-i386

Idx Name          Size      VMA       LMA       File off  Algn
  0 .hash         00000050  ffffe094  ffffe094  00000094  2**2
  1 .dynsym       000000f0  ffffe0e4  ffffe0e4  000000e4  2**2
  2 .dynstr       00000056  ffffe1d4  ffffe1d4  000001d4  2**0
  3 .gnu.version  0000001e  ffffe22a  ffffe22a  0000022a  2**1
  4 .gnu.version_d 00000038  ffffe248  ffffe248  00000248  2**2
  5 .text         00000047  ffffe400  ffffe400  00000400  2**5
  6 .eh_frame_hdr 00000024  ffffe448  ffffe448  00000448  2**2
  7 .eh_frame     0000010c  ffffe46c  ffffe46c  0000046c  2**2
  8 .dynamic      00000078  ffffe578  ffffe578  00000578  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  9 .useless      0000000c  ffffe5f0  ffffe5f0  000005f0  2**2
                  CONTENTS, ALLOC, LOAD, DATA
% objdump -d syspage

syspage:     file format elf32-i386

Disassembly of section .text:

ffffe400 <.text>:
ffffe400:       51                      push   %ecx
ffffe401:       52                      push   %edx
ffffe402:       55                      push   %ebp
ffffe403:       89 e5                   mov    %esp,%ebp
ffffe405:       0f 34                   sysenter 
ffffe407:       90                      nop    
ffffe408:       90                      nop    
        ... more nops ...
ffffe40d:       90                      nop    
ffffe40e:       eb f3                   jmp    0xffffe403
ffffe410:       5d                      pop    %ebp
ffffe411:       5a                      pop    %edx
ffffe412:       59                      pop    %ecx
ffffe413:       c3                      ret    
        ... zero bytes ...
ffffe420:       58                      pop    %eax
ffffe421:       b8 77 00 00 00          mov    $0x77,%eax
ffffe426:       cd 80                   int    $0x80
ffffe428:       90                      nop    
ffffe429:       90                      nop    
        ... more nops ...
ffffe43f:       90                      nop    
ffffe440:       b8 ad 00 00 00          mov    $0xad,%eax
ffffe445:       cd 80                   int    $0x80

The interesting addresses here are found via

% grep ffffe 
ffffe400 A __kernel_vsyscall
ffffe420 A __kernel_sigreturn
ffffe440 A __kernel_rt_sigreturn

So __kernel_vsyscall pushes a few registers and does a sysenter instruction. And SYSENTER_RETURN pops the registers again and returns. And __kernel_sigreturn and __kernel_rt_sigreturn do system calls 119 and 173, that is, sigreturn and rt_sigreturn, respectively.

What about the jump just before SYSENTER_RETURN? It is a trick to handle restarting of system calls with 6 parameters. As Linus said: I'm a disgusting pig, and proud of it to boot.

The code involved is most easily seen from a slightly earlier patch.

A tiny demo program.

#include <stdio.h>

int pid;

int main() {
                "movl $20, %eax    \n"
                "call 0xffffe400   \n"
                "movl %eax, pid    \n"
        printf("pid is %d\n", pid);
        return 0;
This does the getpid() system call (__NR_getpid is 20) using call 0xffffe400 instead of int 0x80.

Address space randomization

The layout of the vsyscall page changes, and the entry point varies. It can be found by inspection of the ELF headers of the page.

Since Linux 2.6.18 the page itself is mapped at a random address. The right entry point can now be found by searching the ELF auxiliary vector.

/* get vsyscall address and test - compile with -m32 on x86_64 */
#include <stdio.h>
#include <stdlib.h>
#include <elf.h>

static unsigned int getsys(char **envp) {
        Elf32_auxv_t *auxv;

        /* walk past all env pointers */
        while (*envp++ != NULL)
        /* and find ELF auxiliary vectors (if this was an ELF binary) */
        auxv = (Elf32_auxv_t *) envp;

        for ( ; auxv->a_type != AT_NULL; auxv++)
                if (auxv->a_type == AT_SYSINFO)
                        return auxv->a_un.a_val;

        fprintf(stderr, "no AT_SYSINFO auxv entry found\n");

unsigned int sys, pid;

int main(int argc, char **argv, char **envp) {
        sys = getsys(envp);
"               movl $20, %eax  \n"     /* getpid system call */
"               call *sys       \n"     /* vsyscall */
"               movl %eax, pid  \n"     /* get result */
        printf("pid is %d\n", pid);
        return 0;
In the auxv vector one may find AT_SYSINFO data, which points at the vsyscall entry address, and AT_SYSINFO_EHDR data, which points at the start of the vsyscall page.

Maybe in the very beginning call *%gs:0x18 worked as replacement for the old int $0x80. I have never seen a library version that actually used 0x18. The 0x18 here is the offset of the sysinfo field in the struct tcb_head at the start of the glibc TLS (thread-local storage) segment. It is 0x10 on i386 and x86_64 (in 32-bit mode) in all sources I have examined.

Let us test, with getsys() as above.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <linux/unistd.h>
#include <asm/ldt.h>
#include <elf.h>

unsigned int sys, gs, *base;

static void getgs() {
        __asm__("movl %gs, gs\n");
        if ((gs & 7) != 3) {
                fprintf(stderr, "unexpected gs = 0x%x\n", gs);

static void getta(){
        struct user_desc u;
        int i;

        u.entry_number = (gs >> 3);
        if (syscall(__NR_get_thread_area, &u)) {
        base = (unsigned int *) u.base_addr;

        for (i=0; i<100; i++)
                if (base[i] == sys)
                        goto gotit;
        fprintf(stderr, "didn't find the sysinfo entry\n");

        printf("Enter the kernel via  call *%%gs:0x%x .\n", 4*i);

int main(int argc, char **argv, char **envp) {
        sys = getsys(envp); printf("sys = 0x%x\n", sys);
        getgs();            printf("gs = 0x%x\n", gs);
        return 0;
And now, on x86_64:
% cc -m32 -Wall demo.c -o demo
% ./demo
sys = 0x55573420
gs = 0x63
Enter the kernel via  call *%gs:0x10 .
and on i386:
% ./demo
sys = 0xffffe414
gs = 0x33
Enter the kernel via  call *%gs:0x10 .

And indeed this works:

% cat exit42.c
int main() {
"               movl $1, %eax  \n"
"               movl $42, %ebx \n"
"               call *%gs:0x10 \n"
% cc -m32 exit42.c -o x
% ./x; echo $?

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