Pfthb

I read a make_tempory_file demonstration in APUE, and confused about:

char good_template = "/tmp/dirXXXXXX"; /* right way */
char *bad_template = "/tmp/dirXXXXXX"; /* wrong way*/

void make_temp(char *template);

int main()

 char good_template = "/tmp/dirXXXXXX"; /* right way */
 char *bad_template = "/tmp/dirXXXXXX"; /* wrong way*/

 printf("trying to create first temp file...n");
 make_temp(good_template);
 printf("trying to create second temp file...n");
 make_temp(bad_template);
 exit(0);


void make_temp(char *template)

 int fd;
 struct stat sbuf;

 if ((fd = mkstemp(template)) < 0)

 err_sys("can’t create temp file");
 printf("temp name = %sn", template);
 close(fd);
 if (stat(template, &sbuf) < 0)
 
 if (errno == ENOENT)

 printf("file doesn’t existn");
 else
 err_sys("stat failed");
 
 else
 
 printf("file existsn");
 unlink(template);
 

The instruction explain it:

The difference in behavior comes from the way the two template strings are declared. For the first template, the name is allocated on the stack, because we use an array variable. For the second name, however, we use a pointer. In this case, only the memory for the pointer itself resides on the stack; the compiler arranges for the string to be stored in the read-only segment of the executable. When the mkstemp function tries to modify the string, a segmentation fault occurs.

I tried to understand the statement but stuck with stack

Does it refer to the stack where the arrow point to?

When the mkstemp function tries to modify the string, a segmentation fault occurs.

The reason is because the string declared as char *bad_template = "/tmp/dirXXXXXX"; the bad_template is assigned to a character string literal.

In C, modifying a string literal is undefined behaviour

Reference - Section 6.4.5-6 of C99 Standard

A character string literal is a sequence of zero or more multibyte characters enclosed in
double-quotes, as in "xyz". A wide string literal is the same, except prefixed by the letter L.

If the program attempts to modify such an array, the behavior is undefined.