C语言最大难点揭秘：编程的祸根！

void f1(char *explanation){char p1;
    p1 = malloc(100);            (void) sprintf(p1,"The f1 error occurred because of '%s'.",                           explanation);            local_log(p1);}

int getkey(char *filename){    FILE *fp;int key;
    fp = fopen(filename, "r");fscanf(fp, "%d", &key);return key;        }

void f2(int datum){int *p2;
/* Uh-oh!  No one has initialized p2. */            *p2 = datum;       ...        }

/* Allocate once, free twice. */void f3(){char *p;
    p = malloc(10);     ...free(p);     ...free(p);        }
/* Allocate zero times, free once. */void f4(){char *p;
/* Note that p remains uninitialized here. */free(p);}

void f8(){struct x *xp;
   xp = (struct x *) malloc(sizeof (struct x));   xp.q = 13;   ...free(xp);   .../* Problem!  There's no guarantee that  the memory block to which xp points  hasn't been overwritten. */return xp.q;       }

/********* ...** Note that any function invoking protected_file_read()* assumes responsibility eventually to fclose() its* return value, UNLESS that value is NULL.*********/FILE *protected_file_read(char *filename){    FILE *fp;
    fp = fopen(filename, "r");    if (fp) {...    } else {...    }    return fp;}
        /******** ...** Note that the return value of get_message points to a* fixed memory location.  Do NOT free() it; remember to* make a copy if it must be retained ...*********/char *get_message(){    static char this_buffer[400];
            ...    (void) sprintf(this_buffer, ...);    return this_buffer;        }

        /********* ...* While this function uses heap memory, and so * temporarily might expand the over-all memory* footprint, it properly cleans up after itself.* ********/        int f6(char *item1){    my_class c1;    int result;            ...    c1 = new my_class(item1);    ...            result = c1.x;    delete c1;    return result;}/********* ...* Note that f8() is documented to return a value* which needs to be returned to heap; as f7 thinly* wraps f8, any code which invokes f7() must be* careful to free() the return value.*********/int *f7(){    int *p;
    p = f8(...);    ...    return p;}

static char *important_pointer = NULL;void f9(){if (!important_pointer) important_pointer = malloc(IMPORTANT_SIZE);            ...if (condition)/* Ooops!  We just lost the reference        important_pointer already held. */important_pointer = malloc(DIFFERENT_SIZE);            ...        }

int main(){char p[5];strcpy(p, "Hello, world.");puts(p);}