C solving memory leaks using Valgrind

Question

So I have a php, ruby and python background and got started with C. I'm using Valgrind to check if my programs are not doing silly things but I get this kind of output quite frequently:

14072== <...VALGRIND HEADER & COPYRIGHT...>
14158== HEAP SUMMARY:
14158==     in use at exit: 137,084 bytes in 196 blocks
14158==   total heap usage: 247 allocs, 51 frees, 149,496 bytes allocated
14158== 
14158== 7 bytes in 1 blocks are definitely lost in loss record 3 of 74 at
14158==    0x4C2745D: malloc in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so
14158==    by 0x50F3369: strdup in /usr/lib64/libc-2.18.so
14158==    by 0x4E51B34: readline in /usr/lib64/libedit.so.0.0.43
14158==    by 0x40083C: main in /home/<program location>/
14158== 
14158== LEAK SUMMARY:
14158==    definitely lost: 7 bytes in 1 blocks
14158==    indirectly lost: 0 bytes in 0 blocks
14158==      possibly lost: 0 bytes in 0 blocks
14158==    still reachable: 137,077 bytes in 195 blocks
14158==         suppressed: 0 bytes in 0 blocks
14158== Reachable blocks (those to which a pointer was found) are not shown.
14158== To see them, rerun with: --leak-check=full --show-leak-kinds=all
14158== 
14158== For counts of detected and suppressed errors, rerun with: -v
14158== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)

This valgrind debug output is from this quick REPL that simply shouts back user input on the screen or quits in case input equals to :exit:

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

#include <editline/readline.h>

static char prompt[5]   = "repl>";
static char* cmd_exit = ":exit";

int main(int argc, char** argv) {

    puts("Press Ctrl+c to Exit\n");

    while(1) {
        char* input = readline(prompt);
        add_history(input);
        if(strcmp(input,cmd_exit) == 0) {
            puts("Bye!");
            return 0;
        }
        printf("%s\n", input);
        free(input);
    }
    return 0;
}

I've already tried to free both prompt and cmd_exit variables before function main returns a value but leak is still there according to Valgrind.

...
    free(prompt);
    free(cmd_exit);
    return 0;
}

Some questions:

• How to get rid of this reported memory leak?
• Should I be using static variables in this situation?
• Should I free those static variables too?

Any good advice or material is welcome.

Answer 1

You only need to free things you malloced, so you don't need to free the static variables.

As your readline function seems to use strdup and that uses malloc/calloc, you need to free input. You do this in the loop which is good, but you missed it when you exit the loop:

if(strcmp(input,cmd_exit) == 0) {
        puts("Bye!");
        free(input); 
        return 0;
    }

By the way:

static char prompt[6]   = "repl>";

The text "repl>" has 6 chars, r, e, p, l, > and a nul byte to mark the end of the string.