Reusing memory space allocated by malloc()

Question

I am a newbie to C and have a couple questions regarding the usage of malloc(): Is it OK to reuse a block of memory allocated? More specifically, what I am trying to do is as below (trying to parse a file using fgets):

#include <stdio.h>
#include <stdlib.h>
main() {
    /*... some code here to open a file and give pointer to fp */

    char *s=(char*) malloc(MAX_CHAR_IN_LINE);

    do {
        //QUESTION: is this right to reuse s in each iteration?
        fgets(s,MAX_CHAR_IN_LINE,fp);
    } while (*s!=NULL);

    free(s);
}

Thank you!

---

Thanks for the answer! Below is a summary of relevant follow-up Q/As

Q: I fear following situation might happen: In 1st iteration, 5 char were read in, say "abcde"; In 2nd iteration, only 3 char were read in, (a shorter line), say, "fgh". do I end up with "fghde" which seems odd? – user3424826

A: In C, strings are null-terminated. So yes, some of the old string will still be there, but there will be a NUL character that marks the end of the new string. In the future, if you have a specific concern (like that one), present it upfront, so that it can be addressed without all the back-and-forth. It just wastes everyone's time. – Jonathon Reinhart

Q: Maybe I shall rephrase my question as this: is it necessary to clear the allocated space each time before reusing it? (purge every byte before reusing it)

A: The answer to that question is: It depends. If it is a C-string you're working with, then no, its unnecessary (because of the null-terminator, like I mentioned). If it's a dynamically-allocated (malloc'd) struct, then yes, you should memset(p, 0, sizeof(*p)), or manually set every member to zero. – Jonathon Reinhart

Answer 1

Yes, that's okay. between 'malloc' and 'free', the memory is yours to do what you wish with it.

I should add: if malloc fails it will return 0 (an invalid address). If that happens your example will segfault on the first iteration. To account for this, you may modify your program as follows:

#include <stdio.h>
#include <stdlib.h>
int main() {
    /*... some code here to open a file and give pointer to fp */

    char *s=(char*) malloc(MAX_CHAR_IN_LINE);
    if(s == NULL) {
        printf("Error: malloc failed.");
        return 1;
    }

    do {
        //QUESTION: is this right to reuse s in each iteration?
        fgets(s,MAX_CHAR_IN_LINE,fp);
    } while (*s!=NULL);

    free(s);
    return 0;
}

Answer 2

The usage of malloc and free is OK, but while(*s != NULL) may not be correct in running. The following may be better.

while (!feof(fp)) {
    fgets(s,MAX_CHAR_IN_LINE,fp);
}