KVM_OPEN(3) | Library Functions Manual | KVM_OPEN(3)

# NAME

`kvm_open`, `kvm_openfiles`, `kvm_close` — initialize kernel virtual memory access

# SYNOPSIS

Kernel Data Access Library (libkvm, -lkvm)
`#include <fcntl.h>`
`#include <kvm.h>`

kvm_t *
`kvm_open`(const char *execfile, const char *corefile, const char *swapfile, int flags, const char *errstr);

kvm_t *
`kvm_openfiles`(const char *execfile, const char *corefile, const char *swapfile, int flags, char *errbuf);

int
`kvm_close`(kvm_t *kd);

# DESCRIPTION

The functions `kvm_open`() and `kvm_openfiles`() return a descriptor used to access kernel virtual memory via the kvm(3) library routines. Both active kernels and crash dumps are accessible through this interface.

execfile is the executable image of the kernel being examined. This file must contain a symbol table. If this argument is `NULL`, the currently running system is assumed, which is indicated by `_PATH_KSYMS`, if it exists, otherwise `_PATH_UNIX` is used. Both are defined in `<paths.h>`.

corefile is the kernel memory device file. It can be either /dev/mem or a crash dump core generated by savecore(8). If corefile is `NULL`, the default indicated by `_PATH_MEM` from `<paths.h>` is used.

swapfile should indicate the swap device. If `NULL`, no swap device will be used.

The flags argument indicates read/write access as in open(2) and applies only to the core file. Only `O_RDONLY`, `O_WRONLY`, and `O_RDWR` are permitted. A special value `KVM_NO_FILES` can be specified which will cause no files to be opened and the handle can only be used on live kernels on a limited subset of all kvm operations.

There are two open routines which differ only with respect to the error mechanism. One provides backward compatibility with the SunOS kvm library, while the other provides an improved error reporting framework.

The `kvm_open`() function is the Sun kvm compatible open call. Here, the errstr argument indicates how errors should be handled. If it is `NULL`, no errors are reported and the application cannot know the specific nature of the failed kvm call. If it is not `NULL`, errors are printed to stderr with errstr prepended to the message, as in perror(3). Normally, the name of the program is used here. The string is assumed to persist at least until the corresponding `kvm_close`() call.

The `kvm_openfiles`() function provides BSD style error reporting. Here, error messages are not printed out by the library. Instead, the application obtains the error message corresponding to the most recent kvm library call using `kvm_geterr`() (see kvm_geterr(3)). The results are undefined if the most recent kvm call did not produce an error. Since `kvm_geterr`() requires a kvm descriptor, but the open routines return `NULL` on failure, `kvm_geterr`() cannot be used to get the error message if open fails. Thus, `kvm_openfiles`() will place any error message in the errbuf argument. This buffer should be `_POSIX2_LINE_MAX` characters large (from `<limits.h>`).

# RETURN VALUES

The `kvm_open`() and `kvm_openfiles`() functions both return a descriptor to be used in all subsequent kvm library calls. The library is fully re-entrant. On failure, `NULL` is returned, in which case `kvm_openfiles`() writes the error message into errbuf.

The `kvm_close`() function returns 0 on success and -1 on failure.

# SEE ALSO

open(2), kvm(3), kvm_geterr(3), kvm_getprocs(3), kvm_nlist(3), kvm_read(3)

# BUGS

There should not be two open calls. The ill-defined error semantics of the Sun library and the desire to have a backward-compatible library for BSD left little choice.

June 6, 2025 | Debian