Using format specifiers on Linux system call write()

Question

I am trying to write to stdout using only Linux system calls described in section 2 of the manual (http://man7.org/linux/man-pages/dir_section_2.html).

ssize_t write(int fd, const void *buf, size_t count);

is the function I am trying to use, however the I am trying to incorporate format specifiers into the buffer. Ideally it would work like so:

char *adjective = argv[1];
write(1, "This is a %s example\n", adjective, 19 + strlen(adjective));

But I obviously cannot do this. How would I go about getting something similar done?

Also, I know strlen() isn't a part of section 2 of the Linux system calls. I'm including it for simplicity since counting the length of the buffer isn't part of my question.

Answer 1

Use sprintf() to format into a buffer, which you then write:

char buffer[BUFSIZE];
int size = snprintf(buffer, BUFSIZE, "This is a %s example\n", adjective);
write(STDOUT_FILENO, buffer, strlen(buffer));

If you can't use sprintf(), just write each string separately.

write(STDOUT_FILENO, "This is a ", sizeof "This is a ");
write(STDOUT_FILENO, adjective, strlen(adjective));
write(STDOUT_FILENO, " example", sizeof " example");