C-equivalent of the 'setw' function

No, since the stream used in C doesn't maintain state the way the stream object does.

You need to specify with e.g. printf() using a suitable formatting code.

printf ("%5d", 42);

Will print 42 using 5 spaces. Read the man pages of printf to understand how character padding, overflow and other nuances work.

EDIT: Some examples -

int x = 4000;
printf ("1234567890\n");
printf ("%05d\n", x);
printf ("%d\n", x);
printf ("%5d\n", x);
printf ("%2d\n", x);

Gives the output

1234567890
04000
4000
 4000
4000

Notice that the %2d was too small to handle the number passed to it, yet still printed the entire value.

Another option is to define the format string as a variable:

char print_format[] = "%5d"; printf(print_format, 42);

The above is similar to C++ setw, in that you can set the contents of the variable before printing. Many occasions require dynamic formatting of the output. This is one method to achieve it.