I am attempting to get the function pointer by using dlopen and dlsym, however I have been unable to get it working correctly. It fails when trying to doing the dlsym call. Following is my code.
Any help please?
#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
int test() {
printf("%s", "test()");
return 123;
}
int main() {
char * functionname = "test";
void* handle = dlopen(NULL,RTLD_LAZY|RTLD_GLOBAL);
if (!handle) {
fprintf(stderr, "Couldn't open handle: %s\n",
dlerror());
exit(1);
}
int (*fun)() = (int (*)())dlsym(handle, functionname);
if (fun == NULL) {
fprintf(stderr, "Couldn't find function: %s\n",functionname);
exit(1);
}
int a = fun();
printf("result: %d \n", a);
}
Probably you need to specify to the linker to export the symbols as dynamic. With gcc you have to use -rdynamic.
You can check the exported dynamic symbols with objdump -T.
Related
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#define MALLOC(p,s) {\
if (!((p) = malloc(s))) { \
fprintf(stderr, "insufficient memory");\
exit(EXIT_FAILURE);\
}\
}
#define IS_EMPTY(first) (!first)
typedef struct listNode* listPointer;
typedef struct listNode {
int data;
listPointer link;
}listNode;
void printList(listPointer first);
int main(void)
{
int x;
int tmpData;
listPointer first = NULL;
listPointer tmpLink = NULL;
FILE* fp = NULL;
if (!(fp = fopen("in.txt", "r"))) {
fprintf(stderr, "cannot open the file");
exit(EXIT_FAILURE);
}
while (!feof(fp)) {
fscanf(fp, "%d", &tmpData);
MALLOC(tmpLink, sizeof(listNode));
if (IS_EMPTY(first)) {
MALLOC(first, sizeof(listNode));
*tmpLink = *first;
}
tmpLink->data = tmpData;
tmpLink = tmpLink->link;
}
printList(first);
}
void printList(listPointer first)
{
for (; first; first = first->link) {
printf("%d ", first->data);
}
printf("\n");
}
We know that we can implement the insert function.
But I'm really curious about why this doesn't work.
What "first" refers to and what "tmpLink" refers to is the same
After implementing the link list while updating tmpLink,
I'm going to use "first" to print later.
I've spent almost a day just thinking about this, and I've tried debugging it, but I don't know why.
Got this segfault, and I can't seem to get around it. Narrowed it down to the pthread_join() function. I'm loading libpthread dynamically.
int main(int argc, char **argv)
{
void *lib_handle;
create pthread_c;
join pthread_j;
pthread_t thrd_id;
int rc;
char *error;
lib_handle = dlopen("/lib/x86_64-linux-gnu/libpthread.so.0", RTLD_NOW);
if (!lib_handle)
{
fprintf(stderr, "%s\n", dlerror());
exit(1);
}
pthread_c = dlsym(lib_handle, "pthread_create");
pthread_j = dlsym(lib_handle, "pthread_join");
if ((error = dlerror()) != NULL)
{
fprintf(stderr, "%s\n", error);
exit(1);
}
rc = pthread_c(&thrd_id, NULL, sub, (void *)NULL);
pthread_j(thrd_id, NULL); // CAUSES SEGFAULT
printf ("testing");
dlclose(lib_handle);
return 0;
}
void* sub (void* a)
{
printf("Hello Thread, I'm the World!\n");
}
The printf() statement shows that pthread_create() is working as it should. But I need to call pthread_join() otherwise the program terminates before the thread is spun up.
Turns out you must declare the join and create typedefs to use pthread_t instead of int from sys/types.h
typedef int (*create)(pthread_t, void*, void*, void*);
typedef void (*join) (pthread_t, void*);
I think i was using an int for create which worked, but doesn't work for join()
Could anyone give some simple examples (function names are good) for reading text files line by line (binary is OK if text is really hard) in a FreeBSD kernel module, from a given directory?
Really appreciate your kind help.
Here's a sample kernel module that'll cat your /etc/motd on load:
// kernel module motd catter.
// Doug Luce doug#forephypodia.con.com
#include <sys/param.h>
#include <sys/vnode.h>
#include <sys/fcntl.h>
#include <sys/module.h>
#include <sys/kernel.h>
#include <sys/namei.h>
#include <sys/proc.h>
#include <sys/sbuf.h>
static int catfile(const char *filename) {
struct sbuf *sb;
static char buf[128];
struct nameidata nd;
off_t ofs;
ssize_t resid;
int error, flags, len;
NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, filename, curthread);
flags = FREAD;
error = vn_open(&nd, &flags, 0, NULL);
if (error)
return (error);
NDFREE(&nd, NDF_ONLY_PNBUF);
ofs = 0;
len = sizeof(buf) - 1;
sb = sbuf_new_auto();
while (1) {
error = vn_rdwr(UIO_READ, nd.ni_vp, buf, len, ofs,
UIO_SYSSPACE, IO_NODELOCKED, curthread->td_ucred,
NOCRED, &resid, curthread);
if (error)
break;
if (resid == len)
break;
buf[len - resid] = 0;
sbuf_printf(sb, "%s", buf);
ofs += len - resid;
}
VOP_UNLOCK(nd.ni_vp, 0);
vn_close(nd.ni_vp, FREAD, curthread->td_ucred, curthread);
uprintf("%s", sbuf_data(sb));
return 0;
}
static int EventHandler(struct module *inModule, int inEvent, void *inArg) {
switch (inEvent) {
case MOD_LOAD:
uprintf("MOTD module loading.\n");
if (catfile("/etc/motd") != 0)
uprintf("Error reading MOTD.\n");
return 0;
case MOD_UNLOAD:
uprintf("MOTD module unloading.\n");
return 0;
default:
return EOPNOTSUPP;
}
}
static moduledata_t moduleData = {
"motd_kmod",
EventHandler,
NULL
};
DECLARE_MODULE(motd_kmod, moduleData, SI_SUB_DRIVERS, SI_ORDER_MIDDLE);
This was cobbled together mostly from bits of https://svnweb.freebsd.org/base/release/10.1.0/sys/kern/vfs_mountroot.c?revision=274417&view=markup
There's no nice scanning/parsing facilities native kernel-side, so
that's usually done the hard way.
I'm getting started with mruby and found out the hard way that an error was occurring in my code, but there was no error reporting. Maybe I am doing something wrong. How can I get errors to appear when they occur?
Excerpt from C code:
mrb = mrb_open();
FILE *f = fopen("example.rb", "r");
mrb_load_file(mrb, f);
fclose(f);
// more C code...
Ruby code which fails without reporting:
# example.rb
def my_method
call_undefined_method
end
my_method()
Ruby code which rescues from the error to display that there was an error:
# example.rb
def my_method
call_undefined_method
rescue => e
puts "Error: #{e.message}"
end
my_method()
You can get exception via mrb->exc
#include <mruby.h>
#include <mruby/variable.h>
#include <mruby/string.h>
#include <stdio.h>
int
main(int argc, char* argv[]) {
mrb_state* mrb = mrb_open();
FILE *f = fopen("example.rb", "r");
mrb_value obj = mrb_load_file(mrb, f);
fclose(f);
if (mrb->exc) {
// Error
obj = mrb_funcall(mrb, mrb_obj_value(mrb->exc), "inspect", 0);
fwrite(RSTRING_PTR(obj), RSTRING_LEN(obj), 1, stdout);
putc('\n', stdout);
} else {
obj = mrb_funcall(mrb, obj, "inspect", 0);
fwrite(RSTRING_PTR(obj), RSTRING_LEN(obj), 1, stdout);
putc('\n', stdout);
}
mrb_close(mrb);
return 0;
}
I have a client server program where client writes a command on PIPE for server. While reading the command from Server it reads only first char of command and throws error. Can anyone help me with this?
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>
#include <sys/wait.h>
#include <mqueue.h>
#include <sys/stat.h>
#include "Functions.h"
#define MSGBUFFER_SIZE 50000
pid_t serverPid;
pid_t clientPid;
typedef struct msgbuf {
int messageLength;
int messageType;
char messageText[MSGBUFFER_SIZE];
} Message_buf;
int writePIPE(int fd, Message_buf *inputMessage){
printf("\n In write pipe message length :%d",inputMessage->messageLength);
printf("\n In write pipe message Data :%s",inputMessage->messageText);
ssize_t n=write(fd,inputMessage,inputMessage->messageLength);
printf("\n Size :%d", n);
return n;
}
ssize_t readPIPE(int fd, Message_buf *outputMessage)
{
ssize_t len;
ssize_t n;
if((n=read(fd,outputMessage,sizeof(outputMessage)))==0)
{
printf("\n Error");
return 0;
}
if((len=outputMessage->messageLength)>0)
{
printf("\n Length ---->:%d",len);
if((n=read(fd,outputMessage->messageText,strlen(outputMessage->messageText)))!=len)
printf("\n ERRRRROR expected %d got %d",len,n);
}
//printf("\n In Read PIPE: %s",outputMessage->messageText);
return len;
}
void Server(int readfd,int writefd)
{
Message_buf server_MessageBuf;
ssize_t length;
if((length=readPIPE(readfd,&server_MessageBuf))==0)
{
printf("\n End of file while reading pathname");
}
//server_MessageBuf.messageText[length]='\0';
printf("\n LENGTH :%d",server_MessageBuf.messageLength);
printf("\n Printing in server: %s\n",server_MessageBuf.messageText);
}
void Client(int readfd,int writefd)
{
char inputFileName[MAX_SIZE];
char inputOperation[MAX_SIZE];
char *cCommandInput = NULL;
char *fileOperation = NULL;
char *operation = (char *) malloc(MAX_SIZE);
int commandValidateStatus = 0;
int commandInterpretationStatus=0;
Message_buf client_MessageBuf;
for(;;)
{
while(1)
{
cCommandInput = acceptInput();
fileOperation = (char *) malloc(sizeof(cCommandInput));
strcpy(fileOperation,cCommandInput);
/**Function call to determine operation read/delete/exit/invalid choice and filename*****/
commandInterpretationStatus = interpretCommand(cCommandInput,
inputOperation, inputFileName);
operation = inputOperation;
/**Function call to validate the input command******/
commandValidateStatus = validateCommand(
commandInterpretationStatus, inputOperation, inputFileName);
if(commandValidateStatus==-1)
{
printf("\n Invalid Operation");
}
/*Exit command entered***/
if (commandValidateStatus == 1)
{
/*Code to clear resources */
kill(serverPid,SIGKILL);
kill(clientPid,SIGKILL);
exit(0);
}
/***Read or Delete****/
if (commandValidateStatus == 2 || commandValidateStatus == 3)
{
printf("\n Read or Delete\n");
strcpy(client_MessageBuf.messageText,fileOperation);
client_MessageBuf.messageLength=strlen(fileOperation);
client_MessageBuf.messageType=1;
if((writePIPE(writefd,&client_MessageBuf))<0)
{
printf("\n Error writing on client side ");
}
//read(readfd,*client_MessageBuf,sizeof(client_MessageBuf));
//printf("\n Reding server responsed");
//printf("%s",client_MessageBuf.messageText);
}
}
}
}
int main()
{
int pipe1[2],pipe2[2];
pipe(pipe1);
pipe(pipe2);
pid_t pid;
pid=fork();
serverPid=pid;
if(pid==0)
{
/*Call Server*/
close(pipe1[1]);
close(pipe2[0]);
Server(pipe1[0], pipe2[1]);
}
else
{
close(pipe1[0]);
close(pipe2[1]);
Client(pipe2[0],pipe1[1]);
}
return 0;
}
It looks like the code writes and reads struct msgbuf incorrectly:
typedef struct msgbuf {
int messageLength;
int messageType;
char messageText[MSGBUFFER_SIZE];
} Message_buf;
// ...
strcpy(client_MessageBuf.messageText,fileOperation);
client_MessageBuf.messageLength=strlen(fileOperation);
client_MessageBuf.messageType=1;
if((writePIPE(writefd,&client_MessageBuf))<0)
// ....
int writePIPE(int fd, Message_buf *inputMessage){
printf("\n In write pipe message length :%d",inputMessage->messageLength);
printf("\n In write pipe message Data :%s",inputMessage->messageText);
ssize_t n=write(fd,inputMessage,inputMessage->messageLength);
printf("\n Size :%d", n);
return n;
}
The above pieces that write struct msgbuf only write the first messageLength bytes of the structure which doesn't include the length of messageLength and messageType members, i.e. it truncates the object.
When reading:
ssize_t readPIPE(int fd, Message_buf *outputMessage)
{
// ...
if((n=read(fd,outputMessage,sizeof(outputMessage)))==0)
It reads only sizeof(outputMessage) bytes, which is the size of the pointer, not the object. Even if you fix it by changing it to sizeof(*outputMessage) that won't do enough, since that would expect to read the complete object whereas the writing piece truncates the object.
A good start to fix it would be to split the message into two parts: header and payload. The header is a structure of a fixed size, e.g.:
typedef struct {
int messageType;
int payloadLength;
} MessageHeader;
The payload is a variable-length part following the header. This way it would first write the entire header object into the pipe followed by payloadLength bytes of payload. When reading it would first read again the entire header and then read exactly payloadLength bytes.
Also note, that read() and write() calls may read or write less then asked, so that case needs to be explicitly handled by keeping reading or writing until the exact number of bytes has been processed.