I am trying to implement mv command in xv6-riscv operating system, In mv.c, I want to create a file given a path, Which system call should I use to create a file as I could not find the create() call in riscv to create a file given a path.
mv.c
#include "kernel/types.h"
#include "user/user.h"
#include "kernel/stat.h"
int mv(char * path1, char * path2)
{
int file1 = open(path1, 0);
int file2 = create(path2, 1); // What should I use over here ? as create is not a call in riscv xv6
if(file1 == -1)
{
printf("Invalid Path 1\n");
return -1;
}
if(file2 == -1)
{
printf("Invalid Path 2\n");
}
}
Related
I am a complete noob when it comes to linux system calls interacting with c-code.
So far I have been able to open a single file, but that's about it. I'm unsure of how I would take a second file and combine both of those into a third.
For example, I have file1 with simple text contents, and file2 with the same, how could I combine both contents into file3 using only linux system calls? I know I have to use lseek to change the pointer, but unsure of how to utilize that.
here is what I have so far... I apologize for the scarcity:
This takes file1 and copies it to file2, I believe
#include <fcntl.h>
#include <unistd.h>
int copyfile(const char *file1, const char *file2)
{
int infile, outfile;
ssize_t nread;
char buffer[BUFSIZE]
if( (infile = open(file1, O_RDONLY)) == -1 )
return (-1);
if( (infile = open(file2, O_WRONLY|O_CREATE|O_TRUNC, PERM)) == -1 )
{
close (infile);
return (-2);
}
/*read from file1 BUFSIZE chars at a time*/
while ( nread = read (infile, buffer, BUFSIZE) )
{
// write buffer to output file
if (write (outfile, buffer, nread) < nread)
{
close(infile);
close(outfile);
return (-3);
}
}
close (infile)
close (outfile)
if (nread == -1)
return (-4);
else
return(0);
}
The files will be entered within the terminal as such:
lastnameCat.c file1 file2 file3
such that file1 and file2 are added together, and sent into file3.
You can use the copy_file_range system call for this. It is faster than using read and write calls as the copying is done inside the kernel. From the man page:
The copy_file_range() system call performs an in-kernel copy between two file descriptors without the additional cost of transferring data from the kernel to user space and then back into the kernel.
Here is an example of using it:
#define _GNU_SOURCE
#include <fcntl.h>
#include <limits.h>
#include <stdint.h>
#include <unistd.h>
int do_copy(int infd, int outfd)
{
ssize_t bytes = 0;
do
{
bytes = copy_file_range(infd, NULL, outfd, NULL, SSIZE_MAX, 0);
}
while(SSIZE_MAX == bytes);
return bytes;
}
int concatenate(const char *inpath1, const char *inpath2, const char *outpath)
{
int infd1 = -1;
int infd2 = -1;
int outfd = -1;
int res = -1;
infd1 = open(inpath1, O_RDONLY);
if(infd1 < 0)
goto close;
infd2 = open(inpath2, O_RDONLY);
if(infd2 < 0)
goto close;
outfd = open(outpath, O_WRONLY|O_CREAT|O_TRUNC);
if(outfd < 0)
goto close;
res = do_copy(infd1, outfd);
if(res < 0)
goto close;
res = do_copy(infd2, outfd);
close:
if(infd1 >= 0)
close(infd1);
if(infd2 >= 0)
close(infd2);
if(outfd >= 0)
close(outfd);
return res;
}
The loop in do_copy allows for very large files which may exceed the maximum copy possible in a single call.
Based on your comments, it sounds like this doesn't have to be a C program as long as it is user friendly. As long as you can guarantee that it will be run in linux, just create a shell script and name it whatever you want. You could even give the shell script the same name as your c program executable and users wouldn't be able to tell the difference:
#!/bin/bash
cat $1 $2 > $3
Say that you name this script lastnameCat and make it executable with chmod +x ./lastnameCat. From then on you could simply do:
$ ./lastnameCat file1 file2 file3
You could also name this script lastnameCat.c if you wanted, but that is a bit deceptive in my opinion since it is not a C file, it is a bash script.
follwing code has written to open a file and write data to terminal using sysyem calls in linux.
To read the value of the file descriptor (fd) it should assign a value. As we know in if else statement, from if part else part or else if part one part will implement at a time. So according to following code fd will have a value only at else if line. But when I pass a file name and run this program it opens the file. File opening is happen in while loop from read(() system call. But while loop is in else part and since file descriptor can't have any value theoretically. So how does the read function get recognize the file exactly? This is confusing me.
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#define SIZE 10
int main(int argc, char *argv[])
{
int fd,n;
char buff[SIZE];
if(argc != 2)
{
printf("USAGE : %s\n",argv[0] );
exit(1);
}
else if ((fd = open(argv[1],0)) == -1)
{
perror("STATUS");
exit(1);
}
else
{
while((n = read(fd,buff,SIZE)) > 0)
{
write(1,buff,SIZE);
}
close(fd);
}
}
Following happens here:
Let's suppose the program is started with xyz.txt on the command line and let's suppose the xyz.txt file does exist:
if(argc != 2)
{
// we don't get here because argc == 2
printf("USAGE : %s\n",argv[0] );
exit(1);
}
else if ((fd = open(argv[1],0)) == -1) // the statement in the if clause will therefore
// be executed, fd will be something different
// from -1 because open succeeded
{
perror("STATUS"); // therefore we dont ge here either
exit(1);
}
else
{ // instead we get here and
while((n = read(fd,buff,SIZE)) > 0) // everything works as expected
{
write(1,buff,SIZE);
}
close(fd);
}
Let's say I have a file in Linux with this path:
/path/to/file/test.mp3
I want to know the path to its device. For example I want to get something like:
/dev/sdb1
How do I do this with the C programming language?
I know the terminal command to do it, but I need C functions that will do the job.
EDIT:
I have read this question before asking mine. It doesn't concretly mention code in C, it's more related to bash than to the C language.
Thanks.
You need to use stat on the file path, and get the device ID st_dev and match that to a device in /proc/partitions
Read this for how to interpret st_dev: https://web.archive.org/web/20171013194110/http://www.makelinux.net:80/ldd3/chp-3-sect-2
I just needed that inside a program I am writing...
So instead of running "df" and parsing the output, I wrote it from scratch.
Feel free to contribute!
To answer the question:
You first find the device inode using stat() then iterate and parse /proc/self/mountinfo to find the inode and get the device name.
/*
Get physical device from file or directory name.
By Zibri <zibri AT zibri DOT org>
https://github.com/Zibri/get_device
*/
#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdlib.h>
#include <libgen.h>
int get_device(char *name)
{
struct stat fs;
if (stat(name, &fs) < 0) {
fprintf(stderr, "%s: No such file or directory\n", name);
return -1;
}
FILE *f;
char sline[256];
char minmaj[128];
sprintf(minmaj, "%d:%d ", (int) fs.st_dev >> 8, (int) fs.st_dev & 0xff);
f = fopen("/proc/self/mountinfo", "r");
if (f == NULL) {
fprintf(stderr, "Failed to open /proc/self/mountinfo\n");
exit(-1);
}
while (fgets(sline, 256, f)) {
char *token;
char *where;
token = strtok(sline, "-");
where = strstr(token, minmaj);
if (where) {
token = strtok(NULL, " -:");
token = strtok(NULL, " -:");
printf("%s\n", token);
break;
}
}
fclose(f);
return -1;
}
int main(int argc, char **argv)
{
if (argc != 2) {
fprintf(stderr, "Usage:\n%s FILE OR DIRECTORY...\n", basename(argv[0]));
return -1;
}
get_device(argv[1]);
return 0;
}
output is just the device name.
Example:
$ gcc -O3 getdevice.c -o gd -Wall
$ ./gd .
/dev/sda4
$ ./gd /mnt/C
/dev/sda3
$ ./gd /mnt/D
/dev/sdb1
$
Use this command to print the partition path:
df -P <pathname> | awk 'END{print $1}'
I have a linked list with an fd and a string I used to open this file in each entry. I want to open and add files to this list only if this file is not already opened, because I open and parse this files and do not want to do it twice. My idea was to compare the filename with every single name in this list, but my program do it multiple times and one file in Linux can have multiple names (soft/hard links). I think it should not be so complicated, because its easy for the OS to check, whether I already used a inode or not, r?
I already tried to open the same file with and without flock, but I always get a new fd.
When you successfully open a file use fstat on the file. Check to see if the st_ino and st_dev of the struct stat filed in by fstat have already been recorded in your linked list. If so then close the file descriptor and move on to the next file. Otherwise add the file descriptor, the file name and st_ino and st_dev values to the list.
You can instead use stat to check before opening the file, but using fstat after will be slightly faster if the usual case is that file hasn't already been opened.
In situations like this, it's often useful to consider your data structures. Change to a data structure which does not allow duplicates, such as a hash table.
Maintain a set of which data you've seen before. I've used a hash table for this set. As per #RossRidge's answer, use the inode and device as the key. This allows duplicates to be discovered in O(1) time.
Here is an example implementation.
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <glib.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>
static int get_fd(GHashTable *fds, const char *filename, int mode) {
int fd;
struct stat stat;
int keysize = 33;
char key[keysize]; /* Two 64 bit numbers as hex and a separator */
/* Resolve any symlinks */
char *real_filename = realpath(filename, NULL);
if( real_filename == NULL ) {
printf("%s could not be resolved.\n", filename);
return -1;
}
/* Open and stat */
fd = open( real_filename, mode );
if( fd < 0 ) {
printf("Could not open %s: %s.\n", real_filename, strerror(errno));
return -1;
}
if( fstat(fd, &stat) != 0 ) {
printf("Could not stat %s: %s.\n", real_filename, strerror(errno));
return -1;
}
/* Make a key for tracking which data we've processed.
This uses both the inode and the device it's on.
It could be done more efficiently as a bit field.
*/
snprintf(key, keysize, "%lx|%lx", (long int)stat.st_ino, (long int)stat.st_dev);
/* See if we've already processed that */
if( g_hash_table_contains(fds, key) ) {
return 0;
}
else {
/* Note that we've processed it */
g_hash_table_add(fds, key);
return fd;
}
}
int main(int argc, char** argv) {
int mode = O_RDONLY;
int fd;
GHashTable *fds = g_hash_table_new(&g_str_hash, &g_str_equal);
for(int i = 1; i < argc; i++) {
char *filename = argv[i];
fd = get_fd(fds, filename, mode);
if( fd == 0 ) {
printf("%s has already been processed.\n", filename);
}
else if( fd < 0 ) {
printf("%s could not be processed.\n", filename);
}
else {
printf("%s: %d\n", filename, fd);
}
}
}
And here's a sample result.
$ touch one two three
$ ln one one_link
$ ln -s two two_sym
$ ./test one* two* three*
one: 3
one_link has already been processed.
two: 5
two_sym has already been processed.
three: 7
As long as you don't close the successfully and intentionally opened files, you can use nonblocking flock to prevent another lock on the same file:
#include <unistd.h>
#include <sys/file.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <assert.h>
int openAndLock(const char* fn){
int fd = -1;
if(((fd = open(fn, O_RDONLY)) >= 0) && (flock(fd, LOCK_EX|LOCK_NB) == 0)){
fprintf(stderr, "Successfully opened and locked %s\n", fn);
return fd;
}else{
fprintf(stderr, "Failed to open or lock %s\n", fn);
close(fd);
return -1;
}
}
int main(int argc, char** argv){
for(int i=1; i<argc; i++){
openAndLock(argv[i]);
}
return 0;
}
Example:
$ touch foo
$ ln foo bar
$ ./a.out foo foo
Successfully opened and locked foo
Failed to open or lock foo
$ ./a.out foo bar
Successfully opened and locked foo
Failed to open or lock bar
I am using gssapi in C for the first time. I am trying to reconstruct example on Oracle doc http://docs.oracle.com/cd/E19683-01/816-1331/sampleprogs-1/index.html.
In my .c file I call gss_str_to_oid(&min_stat, &tok, oid); and get an undefined reference error. I included #include "gssapi.h" at the top of my .c file. In gssapi.h there is a function call
OM_uint32 KRB5_CALLCONV
gss_str_to_oid(
OM_uint32 *, /* minor_status */
gss_buffer_t, /* oid_str */
gss_OID *);
So what am I doing wrong? I thought that if you included #include "gssapi.h" it would give me access to function in gssapi. Both files are in my src folder. So what am I doing wrong. I am using eclipse and from what in my makefile under targets it says all: GSS-API.
I am including most of my code below.
main
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <error.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "gssapi.h"
#include "gssapi_ext.h"
#include "gss-misc.h"
/* global mech oid needed by display status, and acquire cred */
FILE *display_file;
gss_OID g_mechOid = GSS_C_NULL_OID;
void usage()
{
fprintf(stderr, "Usage: gss-client [-port port] [-d]"
" [-mech mechOid] host service msg\n");
exit(1);
}
static void parse_oid(char *mechanism, gss_OID *oid)
{
char *mechstr = 0, *cp;
gss_buffer_desc tok;
OM_uint32 maj_stat, min_stat;
if (isdigit(mechanism[0])) {
mechstr = malloc(strlen(mechanism)+5);
if (!mechstr) {
printf("Couldn't allocate mechanism scratch!\n");
return;
}
sprintf(mechstr, "{ %s }", mechanism);
for (cp = mechstr; *cp; cp++)
if (*cp == '.')
*cp = ' ';
tok.value = mechstr;
} else
tok.value = mechanism;
tok.length = strlen(tok.value);
maj_stat = gss_str_to_oid(&min_stat, &tok, oid);
if (maj_stat != GSS_S_COMPLETE) {
// display_status("str_to_oid", maj_stat, min_stat);
return;
}
if (mechstr)
free(mechstr);
}
int main(argc, argv)
int argc;
char **argv;
{
/* char *service_name, *hostname, *msg; */
char *msg;
char service_name[128];
char hostname[128];
char *mechanism = 0;
u_short port = 4444;
int use_file = 0;
OM_uint32 deleg_flag = 0, min_stat;
display_file = stdout;
/* Parse arguments. */
argc--; argv++;
while (argc) {
if (strcmp(*argv, "-port") == 0) {
argc--; argv++;
if (!argc) usage();
port = atoi(*argv);
} else if (strcmp(*argv, "-mech") == 0) {
argc--; argv++;
if (!argc) usage();
mechanism = *argv;
} else if (strcmp(*argv, "-d") == 0) {
deleg_flag = GSS_C_DELEG_FLAG;
} else if (strcmp(*argv, "-f") == 0) {
use_file = 1;
} else
break;
argc--; argv++;
}
if (argc != 3)
usage();
if (argc > 1) {
strcpy(hostname, argv[0]);
} else if (gethostname(hostname, sizeof(hostname)) == -1) {
perror("gethostname");
exit(1);
}
if (argc > 2) {
strcpy(service_name, argv[1]);
strcat(service_name, "#");
strcat(service_name, hostname);
}
msg = argv[2];
if (mechanism)
parse_oid(mechanism, &g_mechOid);
/* if (call_server(hostname, port, g_mechOid, service_name,
deleg_flag, msg, use_file) < 0)
exit(1);*/
/*
if (g_mechOid != GSS_C_NULL_OID)
(void) gss_release_oid(&min_stat, &gmechOid);
*/
return 0;
}
gssapi.h
/* New for V2 */
OM_uint32 KRB5_CALLCONV
gss_str_to_oid(
OM_uint32 *, /* minor_status */
gss_buffer_t, /* oid_str */
gss_OID *);
You just can't include the header you have to link the library either dynamically or statically. Is there some dll, lib, so, etc you need to add to your project? Without makefile or your project setup been shown in your question; I think you will not receive a very clear answer. Just including header file isn't enough, the undefined is not a compilation error but a linker error, which means its missing a reference because you are not linking the library to your program.
The documentation for GSSAPI in C and C++ in not the greatest. Turns out you need to download gssapi. Here is the link http://www.gnu.org/software/gss/manual/gss.html.
It is under download and install
So, I faced same problem.
I found out that you need to add some .so files to your project.
Just in case check that your system has libkrb5-dev packet (most likely it is already installed if you have gssapi.h).
Required files are stored in folder "/usr/lib/x86_64-linux-gnu/" (debian in my case):
I added libkdb5.so and libgssapi_krb5.so to QT .pro file and all works fine:
LIBS += -lkdb5
LIBS += -lgssapi_krb5
If you need to find that files .so use folloing commands:
apt-file update
dpkg -L libkrb5-dev