modify_ldt (2)

#include <sys/types.h>

int modify_ldt(int func, void *ptr, unsigned long bytecount);

Note: There is no glibc wrapper for this system call; see NOTES.

modify_ldt() reads or writes the local descriptor table (ldt) for a process. The ldt is a per-process memory management table used by the i386 processor. For more information on this table, see an Intel 386 processor handbook.

When func is 0, modify_ldt() reads the ldt into the memory pointed to by ptr. The number of bytes read is the smaller of bytecount and the actual size of the ldt.

When func is 1, modify_ldt() modifies one ldt entry. ptr points to a user_desc structure and bytecount must equal the size of this structure.

The user_desc structure is defined in <asm/ldt.h> as:

struct user_desc {
    unsigned int  entry_number;
    unsigned long base_addr;
    unsigned int  limit;
    unsigned int  seg_32bit:1;
    unsigned int  contents:2;
    unsigned int  read_exec_only:1;
    unsigned int  limit_in_pages:1;
    unsigned int  seg_not_present:1;
    unsigned int  useable:1;
};

In Linux 2.4 and earlier, this structure was named modify_ldt_ldt_s.

On success, modify_ldt() returns either the actual number of bytes read (for reading) or 0 (for writing). On failure, modify_ldt() returns -1 and sets errno to indicate the error.

EFAULT

ptr points outside the address space.

EINVAL

ptr is 0, or func is 1 and bytecount is not equal to the size of the structure modify_ldt_ldt_s, or func is 1 and the new ldt entry has invalid values.

ENOSYS

func is neither 0 nor 1.

This call is Linux-specific and should not be used in programs intended to be portable.

Glibc does not provide a wrapper for this system call; call it using syscall(2).

vm86(2)

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