For循环卡在C（cygwin）的函数调用中，非常奇怪的行为我无法理解

Question

I have this function someoneDidSomething() , it's meant to generate a random phrase like "Mandy loved Jesus!" for example.

The problem is, when I call it only one time and exit the program it works just fine. Even if i call the program few times repeatedly. But when I call it in a loop the output looks like the image below.

PS: After some time I get a segmentation fault.

I'm pretty much confused :D - this makes no sense.

void main ()
{
    for (int i = 0; i <= 500; i++)
    {
        char message[255];
        strcpy (message, someoneDidSomething());
        printf ("Because %s\n", message);
        sleep (1);
    }
}

char* someoneDidSomething()
{
    static char message[255];
    strcat (message, getPerson());
    strcat (message, " ");
    strcat (message, getVerb());
    strcat (message, " ");
    strcat (message, getSomething());
    strcat (message, ".");
    return message;
}

Called one time output:

Because Mandy loved Jesus.

Called in a for loop:

Because Mandy loved Jesus.Hammond felt the World.Ashlynn knew Jesus.

Answer 1

here:

static char message[255];
strcat (message, getPerson());

the first time it works because global memory ( static is zeroed in most systems, although adding a = {0}; doesn't hurt), so strcat acts like strcpy .

But subsequent calls keep the value in message . message grows and grows, so not the result you want, and after a while, buffer overflows triggers a segmentation fault.

You need strcpy for the first step.

static char message[255];
strcpy (message, getPerson());

Since you copy the result in the caller, why not passing the buffer directly:

    char message[255];
    someoneDidSomething(message);

then your routine starts with:

void someoneDidSomething(char *message)
{
    strcpy(message, getPerson());

you may also consider a single sprintf instead of all those function calls...

and doesn't return anything (no need). Also allows your method to be called safely in a multithreaded environment (well safely if the length of the resulting string doesn't exceed the length of the buffer, eternal problem)

Answer 2

In addition to the answer by @Jean-François Fabre, you can fix your program by replacing static memory with dynamic memory. Just make sure to free() everything allocated:

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

char* someoneDidSomething (void);

int main ()
{
    for (int i = 0; i <= 10; i++)
    {
        char* message = someoneDidSomething();
        printf ("Because %s\n", message);
        free(message);
    }
}

char* someoneDidSomething (void)
{
    char* message = malloc(256);
    message[0] = '\0'; // strcat expects a null terminated string!

    strcat (message, "Bob");
    strcat (message, " ");
    strcat (message, "jumped");
    strcat (message, " ");
    strcat (message, "high");
    strcat (message, ".");

    return message;
}

Answer 3

you should not declare a static variable. please just try as follow. it would work.

char* someoneDidSomething()
{
char message[255]; //just remove static keyword
strcat (message, getPerson());
strcat (message, " ");
strcat (message, getVerb());
strcat (message, " ");
strcat (message, getSomething());
strcat (message, ".");
return message;
}