I have made a c code intended to create a file at root directory.
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
const char *path="/";
int main(){
FILE *fp;
umask(0);
chdir(path);
fp=fopen("test.txt","w+");
fclose(fp);
return 0;
}
the compilation gives no errors but when I execute the file, the following error appears:
kwagjj#kwagjj-Inspiron-3420:~$ gcc -Wall a2.c -o a2
kwagjj#kwagjj-Inspiron-3420:~$ ./a2
Segmentation fault (core dumped)
how am I using the umask function wrong?
My money is on fopen() failing (probably due to insufficient permissions) and returning NULL, and fclose(NULL) causing the segfault.
Related
For an assignment I have we are to find vulnerabilities in a certain C program and exploit them using various buffer overflow attacks. However when I run the .out file in the terminal with it's input argument it just stalls and doesn't do anything.
Even when I run GDB, that just lags too. I'm not looking for a solution to the assignment, I'm just looking for reasons why it's not running?
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
void partialwin()
{
printf("Achieved 1/2!\n");
}
void fullwin(){
printf("Achieved 2/2\n");
}
void vuln(){
char buffer[36];
gets(buffer);
printf("Buffer contents are %s\n",buffer);
}
int main(int argc,char**argv){
vuln();
}
Providing your sourc file is called assignment1.c and you're using gcc this should work, $ being your command prompt (which could be different on your platform)
$ gcc assignment1.c
$ a.out
Hello
Buffer contents are Hello
$
ORIGINAL:
I've been trying to implement a basic shared memory program using the Windows Subsystem for Linux (aka Bash on Ubuntu on Windows). Here is the code I wrote:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define KEY 1374
int shmid;
char *dataPtr;
int main (void) {
shmid = shmget((key_t)KEY,1024, IPC_CREAT | 0666);
dataPtr = shmat(shmid, (void*)0,0);
if(dataPtr == (char*)(-1))
perror("shmat error");
strcpy(dataPtr,"test");
sleep(10);
shmdt(dataPtr);
shmctl(shmid, IPC_RMID, NULL);
}
I compiled it with GCC without any error or warning, but when I tried to run it, I got this error message:
shmat error: Function not implemented
Segmentation fault (core dumped)
I tried to get to the bottom of this error, but when I googled it, I
literally got nothing.
So the question is: did I horribly messed-up something very simple, or is it possible that the shared-memory system isn't implemented on "Ubuntu on Windows"?
EDIT:
I tried to run my code on my faculty's linux server, and it worked fine. So apparently the problem is with the Bash on Ubuntu on Windows system. The shared-memory system probably isn't implemented at all.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/shm.h>
int main()
{
int shmid;
char *viraddr;
char buffer[BUFSIZ];
shmid=shmget(1234,BUFSIZ,0666|IPC_CREAT);
viraddr=(char*)shmat(shmid, 0,0);
while(1)
{
puts("Enter some text:");
fgets(buffer,BUFSIZ,stdin);
strcat(viraddr,buffer);
if(strncmp(buffer,"end",3)==0)
break;
}
shmdt(viraddr);
exit(0);
}
I am learning shared memory.I want to realize the communication between two processes. When I run this program, it always indicates this error -- segmentation fault core dumped. Any help on this? Thanks.
Here both viraddr and buffer have the same size. For strcat() it is recommended dst size to be atleast a byte larger than the src size. viraddr should have enough space to hold buffer data just to avoid buffer overrun.
Perhaps, sometimes the memory allocation returns FFFFF and then throws Segmentation Fault Core Dumped. Reason being you don't have enough rights to access the Memory for your processes.
Simply use sudo to compile and run the code. For instance say you have these files
shm1.c
shm2.c
shm.h
Then your commands need to be fired as following
sudo gcc shm1.c -o s1
sudo gcc shm2.c -o s2
sudo ./s1 &
sudo ./s2
If you are willing to do it on 2 different terminals, just fire sudo ./s1 in terminal1 & sudo ./s2 in different terminal 2
I'm trying to discover libVLC sdk in windows 7. Every time I compile the code, I'm getting this error
main.c:(.text+0xf): undefined reference to `libvlc_clock'
I've included the path. This is my code
#include <stdio.h>
#include <inttypes.h>
#include <vlc\vlc.h>
#include <vlc\libvlc.h>
int main()
{
int64_t time = libvlc_clock();
return 0;
}
and this is the line in prompt command in windows 7
gcc main.c -o test -I"C:\Program Files (x86)\VideoLAN\VLC\sdk\include"
For testing LD_PRELOAD, I wrote my own getpid, which prints something before calling the original getpid using dlsym. The code is given below.
#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <dlfcn.h>
typedef pid_t (*getpidType)(void);
pid_t getpid(void)
{
printf("Hello, getpid!\n");
getpidType f = (getpidType)dlsym(RTLD_NEXT, "getpid");
return f();
}
However when I use such getpid in my program and run it using LD_PRELOAD, by typing LD_PRELOAD=./prelib.so ./prog, I get the following error.
./prog: symbol lookup error: ./prelib.so: undefined symbol: dlsym
But If I do LD_PRELOAD=./prelib.so bash -c 'echo $$', there is no such error. Any idea what is going on here.
Linking it with libdl.so.2 by using -ldl in the makefile solved the problem.