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sysctl ? read/write system parameters |
#include <unistd.h> #include <linux/unistd.h> #include <linux/sysctl.h> _syscall1(int, _sysctl, struct __sysctl_args *, args); int _sysctl(struct __sysctl_args *args); |
The _sysctl call reads and/or writes kernel parameters. For example, the hostname, or the maximum number of open files. The argument has the form struct __sysctl_args { int *name; /* integer vector describing variable */ int nlen; /* length of this vector */ void *oldval; /* 0 or address where to store old value */ size_t *oldlenp; /* available room for old value, overwritten by actual size of old value */ void *newval; /* 0 or address of new value */ size_t newlen; /* size of new value */ }; This call does a search in a tree structure, possibly resembling a directory tree under /proc/sys, and if the requested item is found calls some appropriate routine to read or modify the value. |
#include <linux/unistd.h> #include <linux/types.h> #include <linux/sysctl.h> _syscall1(int, _sysctl, struct __sysctl_args *, args); int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp, void *newval, size_t newlen) { struct __sysctl_args args={name,nlen,oldval,oldlenp,newval,newlen}; return _sysctl(&args); } #define SIZE(x) sizeof(x)/sizeof(x[0]) #define OSNAMESZ 100 char osname[OSNAMESZ]; int osnamelth; int name[] = { CTL_KERN, KERN_OSTYPE }; main(){ osnamelth = SIZE(osname); if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0)) perror("sysctl"); else printf("This machine is running %*s\n", osnamelth, osname); return 0; } |
Upon successful completion, _sysctl returns 0. Otherwise, a value of ?1 is returned and errno is set to indicate the error. |
ENOTDIR |
name was not found. |
EPERM |
No search permission for one of the encountered ‘directories’, or no read permission where oldval was nonzero, or no write permission where newval was nonzero. |
||
EFAULT |
The invocation asked for the previous value by setting oldval non-NULL, but allowed zero room in oldlenp. |
This call is Linux-specific, and should not be used in programs intended to be portable. A sysctl call has been present in Linux since version 1.3.57. It originated in 4.4BSD. Only Linux has the /proc/sys mirror, and the object naming schemes differ between Linux and BSD 4.4, but the declaration of the sysctl(2) function is the same in both. |
The object names vary between kernel versions. THIS MAKES
THIS SYSTEM CALL WORTHLESS FOR APPLICATIONS. Use the
/proc/sys interface instead. |
proc(5) |