C - Error with strcat

Question

What's the error in this code? I've got an error

error C4996: 'strcat': This function or variable may be unsafe. Consider using strcat_s instead

What it's mean? Another question - the declaration of the struct and the function prototype is legal?

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

void main()
{
    char *join(char *, char *);
    printf("%s \n", join("duck", "soup"));   
}

char *join(char *s1, char *s2)
{
    struct {
        char buf[256];
    } string;
string.buf = "abcd";\\ new line. error l-value.
    return strcat(strcpy(string.buf, s1), s2);
}

The new line - why there is an error? isn't string.buf a char pointer? what's the problem with char *s="abcd"? Thanks!:)

Stupid compiler complains about strcat instead of strcat_s. Completely ignoring the crash that is most likely once the function returns (see answer below). — gnasher729, Feb 26 '15 at 10:25
I think there is an answer to that already in this question and plenty of others in the web. — codingEnthusiast, Feb 26 '15 at 10:28
The function prototype is legal (old style), what is not legal is `void main()`. — David Ranieri, Feb 26 '15 at 10:37
@dvirnaim The function called at program startup is named main. The implementation declares no prototype for this function. **It shall be defined with a return type of int** and with no parameters or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared) — David Ranieri, Feb 26 '15 at 10:42

score 5 · Answer 1 · answered Feb 26 '15 at 10:23

5

The error message is entirely self-explanatory. It's not clear what it is you don't understand. For example, in your join function, terrible things would happen if the two strings together are longer than 255 characters.

Also, your join function is totally broken. It returns a pointer to a buffer that no longer exists once it returns since you allocated it on the stack that you are returning from.

answered Feb 26 '15 at 10:23

David Schwartz

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Not that strcat_s does you much good when there are more than 255 characters. You still don't get the correct concatenation of the two strings. If you don't check the return value, it's a silent bug in your code instead of a crash. If you do check the return value, you still don't have the strings concatenated. – gnasher729 Feb 26 '15 at 10:29

score 2 · Answer 2 · answered Feb 26 '15 at 10:24

2

That's your implementation trying to be helpful.

You can stop it from doing that by adding the following #define at the top of your code

#define _CRT_SECURE_NO_WARNINGS

or follow the suggestion and use strcat_s().

answered Feb 26 '15 at 10:24

pmg

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score 1 · Accepted Answer · edited Feb 26 '15 at 10:42

The first message is because strcat doesn't check if the target storage is big enough to hold the concatenated string. You might get a buffer overflow. strcat_s has an additional parameter which is the buffer length. It will make sure there is no buffer overflow.

Regarding you second question, the code in join() is bogus. What you do is declare a local variable that is an array of 256 char. As a local variable it will be "destroyed" when join() terminates.
You then copy s1 into this buffer. Note that the buffer could be too small and you would get a buffer overflow.

Then you call strcat with the local variable buffer as first argument. The result is that s2 will be appended to s1 in the local variable buffer. The value return by strcat is a pointer on the buffer. When join() returns, the buffer is "destroyed" and the pointer becomes invalid.

Note that another problem in you code is to declare the function join() inside of main. You must move this line out of the main function.

The way you defined your struct and string variable is correct. It is the way you perform the join() that is bogus.

thanks!!I have another question. I wrote "string.buf = "abcd";" but it had an error "left operand must be l-value". why is that? string.buf isn't a pointer to char? — Dvir Naim, Feb 26 '15 at 10:37
It means you can't assign something to `string.buf`. It is an array, not a pointer. You have to copy the string `"abcd"` into `string.buf` by using `strcpy` or preferably `strcpy_s` that make sure there is no buffer overflow. — chmike, Feb 26 '15 at 10:48

C - Error with strcat

3 Answers3