[Bug c/58237] New: gcc fails to detect obvious resource leaks

[dcb314 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58237

            Bug ID: 58237
           Summary: gcc fails to detect obvious resource leaks
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dcb314 at hotmail dot com

Some code has obvious resource leaks

# include <stdio.h>

void f1(const char *str)
{
    FILE * fp = fopen(str, "r");
    char buf[10];

    while (fgets(buf, 10, fp) != NULL)
    {
        /* Do something with buf */
    }
    /* Missing call to fclose. Need warning here for resource leak */
}

It would be nice if gcc could notice the obvious missing call
to fclose and produce a warning about a resource leak.

To help some test driven development, here are some additional test cases.

This case is slightly more subtle, but still a warning would
be nice for when flag == 0.

void f2(const char *str, int flag)
{
    FILE * fp = fopen(str, "r");
    char buf[10];

    while (fgets(buf, 10, fp) != NULL)
    {
        /* Do something with buf */
    }
    /* fclose only sometimes called.
     * Still a leak for the case when flag == 0.
     */
    if (flag)
        fclose(fp);
}

For this one, all bets are off since we don't know what function
f31() does with fp.

extern void f31( FILE * fp);

void f3(const char *str)
{
    FILE * fp = fopen(str, "r");
    char buf[10];

    while (fgets(buf, 10, fp) != NULL)
    {
        /* Do something with buf */
    }
    /* Not sure if fclose executed by f31 or not. Say nothing */
    f31(fp);
}

Here is the obvious case where we shouldn't say anything

void f4(const char *str)
{
    FILE * fp = fopen(str, "r");
    char buf[10];

    while (fgets(buf, 10, fp) != NULL)
    {
        /* Do something with buf */
    }
    /* Nothing to say here. */
    fclose(fp);
}

And here is another one where producing a warning message
seems counter - productive.

void main(int argc, const char * argv[])
{
    FILE * fp = fopen(argv[0], "r");
    char buf[10];

    while (fgets(buf, 10, fp) != NULL)
    {
        /* Do something with buf */
    }
    /* Nothing to say here, because we are in main. */
}

Of course, there are many other matched pairs of functions
(open, close), (opendir, closedir) etc that could also have
resource leak warnings. 

A useful first step would be to pick one pair and implement for that.
Then extend to other pairs.

Here is cppcheck doing what I want

$ ~/cppcheck/cppcheck/cppcheck aug24a.cc
Checking aug24a.cc...
[aug24a.cc:16]: (error) Resource leak: fp
[aug24a.cc:32]: (error) Resource leak: fp
$