Issue with returning char pointer from function

Question

My code, which gets called when you type "recent":

char* runRecent() {
    FILE *ffp;
    ffp = fopen("bash.txt","r");
    char line[MAXLINE];
    int fileItCount = 0;
    for (int i = 0 ; i < numLinesinFile ; i++) {
        fgets(line, sizeof(line), ffp);
        if (fileCounter - 2 < 0){
            printf("No recent commands exist.\n");
            exit(EXIT_FAILURE);
        }
        if (i == (numLinesinFile - 2) && i >= 0) {
            printf("Previous command: %s\n",line);
            char * lineRet = strdup(line);
            printf("line ret: %s\n",lineRet); //output: correct, something along the lines of "ls" or "ls -a"
            printf("line ret: %d\n",&lineRet); //output: 1528174960
            return lineRet;
        }
    }
}

The lineRet then gets passed to a function:

start(runRecent());

Where the function declaration looks like:

 void start(char inputBuf[]){
     printf("input is %s\n",inputBuf); //prints the command "recent" instead of the previous command that was executed
 }

Why is this returning the current command instead of the previous one?

Somewhere where you have access to its return value, and where it makes sense to free it. — flogram_dev
– flogram_dev, Commented Sep 20, 2015 at 2:17
@zenith I return "lineRet" from runRecent to the start function, so I'm still confused as to where the free(lineRet) would go. — sc1892353
– sc1892353, Commented Sep 20, 2015 at 2:18
All in all from the code you show along with what you tell us it should do and which input you provide and which output you expect, we have much to few info to answer this question. — alk
– alk, Commented Sep 20, 2015 at 8:48

Iharob Al Asimi · Accepted Answer · 2015-09-20 02:45:52Z

Your code has many issues,

The function does not always return a value, which can lead to undefined behavior in case the function never reaches the return statement. It is very clear why this could be the cause so I will not explain it in detail.

You can just add a default return like return NULL; at the end of the function, but then you have to check for NULL before passing the result to printf().
It also invokes undefined behavior here
```
printf("line ret: %d\n", &lineRet);
```
one of the possible explanations of your observed behavior is undefined behavior, the right way to print a pointer is
```
printf("line ret: %p\n", (void *) &lineRet);
```
anything else is considered undefined behavior by the standard.
Not checking that strdup() did not return a NULL poitner, which can happen if there is no available memory. This is very unlikely the problem, but you must check if you want your program to work under any conditions.
Not checking that fopen() did not return NULL, which can be the case when for example the file is not readable to the current user or it does not exist at all, in any case when it's not possible to open it.
start(runRecent()); automatically causes a memory leak, you don't need to free every single malloc() because memory will be released automatically when the program ends. But a few mistakes like this and you will be in serious trouble, specially because you don't check if strdup() returned a non-NULL pointer.

It turns out that it's not so hard to write a program that will eat all the system available memory, a tiny memory leak an a few minutes running are sufficient for it to happen.

NOTE: Enable compiler warnings and you would both, not ask this kind of question ever and learn more about the c language just by simple induction.

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