Question

I started learning C, and I don’t understand what I’m doing wrong. Here is a simple code of a function that returns the pid+”.data”.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

char * getfilename(){
    char name[60];
    sprintf(name,"%i.data",getpid());
    return name;
}

void main(){
    char* name = getfilename();
    printf("%s",name);
}

outputs: ��#�a.

So I guess that I’m doing something wrong.

Answer 1

char * getfilename(){
    char name[60];
    sprintf(name,"%i.data",getpid());
    return name;
}

You cannot access name object after getfilename has returned. The lifetime of the automatic object name ends at the getfilename last }. Accessing it after the function returns is undefined behavior.

As a temporary fix you can specify name as static and it will work. But what you should do is to have the getfilename function accepts a pointer argument where the filename will be written.

EDIT:

Why I don’t suggest to use strdup?

• strdup is not a Standard C function. strdup lives in the POSIX world. For portability reasons whenever I can, I prefer to use Standard C functions.
• strdup performs a hidden malloc call and you have not to forget to perform a free. This is contrary to all functions of the Standard C library which never call malloc (or actually that never appear to call malloc). strdup is a bad API design.
• strdup is performing a copy of a string. Why do you need to perform an extra copy? Just write the string in a place you can retrieve it.