i tried to compile this program which does a conversion of a string of characters form ascii to binary .
i dont understand how to fix this last error .



#include<stdio.h>
#include<stdlib.h>
#include<string.h>
char* asciitobinary(char* a);
int main(int argc ,char* argv[])
{
char* b;
b= asciitobinary(argv[1]);
printf("\n%s\n",b);
return 0;

}

char* asiitobinary(char* a)
{
char* b;
int len,j=0,k=0,i;
len=strlen(a);
for(i=0;i<len;i++)
{
for(k=j+7,j=j+7;k>=j;k--)
{

if(a[i]>1)
{
b[k]=a[i]%2;
a[i]=(a[i]-b[k])/2;
}
else
b[k]=0;
}

}
b[k]='\0';
return b;
}



error is this :
/tmp/cc4P9tCC.o: In function `main':
assignment.c.text+0x1e): undefined reference to `asciitobinary'
collect2: error: ld returned 1 exit status

actually it compiled there was spelling mistakke
but not working
it is saying
segmentation fault core dumped

Which is the size of char* b in asciitobinary() ?

You are probably hinting at the correct issue, but I think your hint is too subtle to be understood by someone who would make the original error.

Pointers in C do not point anywhere usable unless your code makes them point somewhere usable.

If you just declare
char* b
it doesn't point to usable memory.

You also might want to think through the sequence of actions specified by that line.

also you may try to use debug info (compile with -g). in that case you will get much better error messages (instead of: assignment.c.text+0x1e)

You missed a change in the question. That original unclear error message was at link time, not run time and debug info would not have helped. That error was due to a typo the OP found and corrected without waiting from help from this thread.

The seg fault seems to be due to a basic misunderstanding of pointers in C. So debugging, to narrow down the moment at which the seg fault occurs, probably would not help.