为什么会出现错误无效指针？

Question

/* DECLARED FUNCTIONS */
char *DetectDevice(void);

int main(int argc, char *argv[])
{
    char *PathToDevice;
    PathToDevice = DetectDevice();

...

    if(close(fd) == -1)
    {
        printf("Error Closing Port");
    }else
    {
        printf("whihi!");
        free(PathToDevice);
    }

    return 0;
}

char *DetectDevice(void)
{   
    char *Usbs = malloc(1024);
    Usbs = "/dev/ttyUSB1";
    return Usbs;
}

Error Message: * glibc detected * ./test: free(): invalid pointer: 0xbec1b504

by the way... this program is compiled on a raspberry pi!

Answer 1

char *DetectDevice(void)
{   
    char *Usbs = malloc(1024);
    Usbs = "/dev/ttyUSB1";
    return Usbs;
}

In char *DetectDevice(void) you are assigned "/dev/ttyUSB1" string address to Usbs that you returns. and try to free. What address stored in Usbs by malloc is overridden by Usbs = "/dev/ttyUSB1"; statement and Usbs has address to this constant string.
"/dev/ttyUSB1" is not dynamically allocated which you by mistake tried to free!

Do like this.

char *DetectDevice(void)
    {   
        char *Usbs = malloc(1024);
        strcpy(Usbs,"/dev/ttyUSB1");
        return Usbs;
    } 

Answer 2

Usbs = "/dev/ttyUSB1"; changes Usbs to point to a string literal. This may exist in read-only memory and cannot be freed. Use

char *DetectDevice(void)
{   
    char *Usbs = malloc(1024);
    strcpy(Usbs, "/dev/ttyUSB1");
    return Usbs;
}

to copy the string, or

char *DetectDevice(void)
{   
    return strdup("/dev/ttyUSB1");
}

to allocate the string with just the right amount of memory instead.

Alternatively, you could also recognise that DetectDevice returns a read-only string

const char *DetectDevice(void)
{   
    return "/dev/ttyUSB1";
}

and remove the free from calling code instead.

Answer 3

free comes with malloc or calloc or realloc It's always in that pair.

free can be applied only on the pointers which are allocated using malloc or calloc or realloc .

When you have allocated block of memory 1024 in function DetectDevice . In the next line you have assigned the pointer to String literal "/dev/ttyUSB1" .

So the same is returned and pointed by your pointer PathToDevice and now you are trying to free it, so you are getting this kind of error message.

You should use strcpy(Usbs,"/dev/ttyUSB1") and then return Usbs .

And You have not only problem with free but You are having memory leak also. After removing free(PathToDevice) from your code you still have memory leak without any error.

( Advice : It's not good practice to first allocate memory on heap and then code such that the pointer started pointing to other address. It will cause memory leak).

Answer 4

Although it may not be your problem here you should be checking that malloc actually returns a valid address and not NULL, especially on a system with limited resources such as the raspberry pi.

ie

char *DetectDevice(void)
    {   
        char *Usbs = malloc(1024);
        if(Usbs != NULL)
        {
           strcpy(Usbs,"/dev/ttyUSB1");
        }
        else
        {
          // malloc didn't allocate memory do something about it.
        }
        return Usbs;
    } 

Answer 5

You seem to expect this attribution Usbs = "/dev/ttyUSB1" means "write the content of the right-hand string into the memory pointed to by Usbs".

This is not what it means. It means "overwrite Usbs to point to the location of the statically allocated constant string". Such a location is not managed by malloc and cannot be freed by free .

To copy the contents of your constant string into Usbs you need to use the facilities in string.h .