Putchar (output character) getchar (input character) printf (format output) scanf (format input) puts (input string) gets (input string) \ x0d \% doutput \ x0d \% mdShapes the actual length of data according to decimal, and m is the width of the specified output field. If the number of digits is less than m, the left side is filled with spaces; if it is greater than m, it is actually output. For example: printf("%4d, %4d, a, b "); When a= 123b= 12345, the output space is 123, 12345\x0d\%ld is the output long integer data \x0d\o is the format character. The output in octal \x0d\x is a format character. The output \x0d\u in base 16 is a format character, which is used to output unsigned numbers, and it is output in base 10.