Returing and printing a string from a function in C

This is because you are returning a pointer to an object allocated in automatic memory - an undefined behavior.

If you want to return a string from a function, you need to return either a dynamically-allocated block, or a statically allocated block.

Another choice is to pass the buffer into the function, and provide the length as the return value of the function, in the way the file reading functions do it:

size_t getBitstring(unsigned short int instr, char* buf, size_t buf_size) {
    ... // Fill in the buffer without going over buf_size
    printf("don't get excited yet %s\n", binaryNumber);
    return strlen(buf); 
}

Here is how you call this function:

char binaryNumber[] = "0000000000000000";
size_t len = getBitstring(instr, binaryNumber, sizeof(binaryNumber));

Now binaryNumber is an automatic variable in the context of the caller, so the memory would be around while the caller needs it.

This is a good example of a problem people hit when they're learning C that they probably wouldn't hit if they were operating in Java or another more modern language that doesn't expose the details of memory layout to the user.

While everyone's answer here is probably technically correct, I'm going to try a different tack to see if I can answer your question without just giving you a line of code that will fix it.

1. First you need to understand what's going on when returning a variable defined inside a function, like this:

   void f(void) {
       int x = 0;
   
       /* do some crazy stuff with x */
   
       return x;
   }
   

   What happens when you call return x;? I'm probably omitting some details here, but what is essentially going on is that the calling context gets the value stored inside the variable named x at that time. The storage that is allocated to store that value is no longer guaranteed to contain that value after the function is over.

2. Second, we need to understand what happens when we refer to an array by its name. Say we have a 'string':

   char A[] = "12345";
   

   In C, this is actually equivalent to declaring an array of characters that ends in a \0:

   char A[6] = { '1' , '2' , '3' , '4' , '5' , '\0' };
   

   Then this is sort of like declaring six chars A[0], A[1], ... , A[5]. The variable A is actually of type char * i.e. it is a pointer containing the memory address storing A[0] (the beginning of the array).

3. Finally, we need to understand what happens when you call printf to print a string like this:

   printf("%s", A);
   

   What you're saying here is "print all the bytes starting at memory address A until you hit a byte that contains \0".

So, let's put it all together. When you run your code, the variable binaryNumber is a pointer containing the memory address of the first character of the array: binaryNumber[0], and this address is what's returned by the function. BUT, you've declared the array inside of getBitString, so as we know that the memory allocated for the array is no longer guaranteed to store the same values after getBitString is over.

When you run printf("%s", a) you're telling C to "print all the bytes starting at memory address a until you get to a byte containing \0 -- but since that memory address is only guaranteed to contain valid values inside getBitString, what you get is whatever garbage it happens to contain at the time when you call it outside of getBitString.

So what can you do to resolve the problem? Well you have several options, here are is a (non-exhaustive) list:

1. You declare binaryString outside of getBitString so that it's still valid when you try to access it in main
2. You declare binaryString as static as some others have suggested, which is effectively the same thing as above, except that the actual variable name binaryString is only valid inside the function, but the memory allocated to store the array is still valid outside the function.
3. You make a copy of the string using the strdup() function before you return it in your function. Remember that if you do this, you have to free() the pointer returned by strdup() after you're done with it, otherwise what you've got is a memory leak.

Your binaryNumber character array only exists inside of your getBitstring function. Once that function returns, that memory is no longer allocated to your binaryNumber. To keep that memory allocated for use outside of that function you can do one of two things:

Return a dynamically allocated array

char* getBitstring(unsigned short int instr) {
    // dynamically allocate array to hold 16 characters (+1 null terminator)
    char* binaryNumber = malloc(17 * sizeof(char));
    memset(binaryNumber, '0', 16); // Set the first 16 array elements to '0'
    binaryNumber[16] = '\0'; // null terminate the string

    //....
    //doing things to binaryNumber
    //.....

    return binaryNumber;
}

int main(int argc, char *argv[]) {
    char *a = getBitstring(0x1234);
    printf("%s", a);
    free(a); // Need to free memory, because you dynamically allocated it
    return 0;
}

or pass the array into the function as an argument

void* getBitstring(unsigned short int instr, char* binaryNumber, unsigned int arraySize ) {
    //....
    //doing things to binaryNumber (use arraySize)
    //.....
}

int main(int argc, char *argv[]) {
    char binaryNumber[] = "0000000000000000";
    getBitstring(0x1234, binaryNumber, 16); // You must pass the size of the array
    printf("%s", binaryNumber);
    return 0;
}

Others have suggested making your binaryNumber array static for a quick fix. This would work, but I would avoid this solution, as it is unnecessary and has other side effects.

Create your return type in dynamic way with malloc() function. create 16+1 blocks for end of string. this is not safe but easy to understand.

char * binaryNumber = (char*) malloc(17*sizeof(char));//create dynamic char sequence
strcpy(binaryNumber,"0000000000000000");//assign the default value with String copy

The final result will be;

char* getBitstring(unsigned short int instr) {
//this is what the code below is going to convert into. It is set to default
//as a 16 bit string full of zeros to act as a safety default.
char * binaryNumber = (char*) malloc(17*sizeof(char));//create dynamic char sequence
strcpy(binaryNumber,"0000000000000000");//assign the default value with String copy function
//....
//doing things to binaryNumber
//.....
printf("don't get excited yet %s\n", binaryNumber); //note, this works
return binaryNumber;
}
 int main(int argc, char *argv[]) {
 char *a = getBitstring(0x1234);
 printf("%s", a); //this fails
 return 0;
}

of course include the <string.h> library.

your char binaryNumber[] is local to the function getBitstring(). you cannot return a local variable to other function.

The scope of binaryNumber is over when getBitstring() finishes execution. So, in your main(), char *a is not initialized.

The workaround:

1. define the array as static so that it does not go out-of-scope. [Not a good approach, but works]

or

2. use dynamic memory allocation and return the pointer. [don't forget to free later, to avoid memory leak.]

IMO, the second approach is way better.

You need static:

static char binaryNumber[] = "0000000000000000";

Without static keyword, the value of automatic variable is lost after function returns. Probably you know this.