Advanced Programming in the UNIX® Environment

ptg10805159

162 StandardI/O Library Chapter 5

Refer to Section 7.3 of Kernighan and Ritchie[ 1988 ] for additional details on

handling variable-length argument lists with ISO StandardC.Beawarethat the

variable-length argument list routines provided with ISO C—the<stdarg.h>header

and its associated routines — differ from the<varargs.h>routines that wereprovided

with older UNIX systems.

Formatted Input

Formatted input is handled by the threescanffunctions.

#include <stdio.h>

int scanf(const char *restrict format,...);

int fscanf(FILE *restrict fp,const char *restrictformat,...);

int sscanf(const char *restrict buf,const char *restrictformat,...);

All three return: number of input items assigned,

EOFif input error or end of file beforeany conversion

Thescanffamily is used to parse an input string and convert character sequences into

variables of specified types. The arguments following the format contain the addresses

of the variables to initialize with the results of the conversions.

The format specification controls how the arguments areconverted for assignment.

The percent sign (%) indicates the beginning of a conversion specification. Except for

the conversion specifications and white space, other characters in the format have to

match the input. If a character doesn’t match, processing stops, leaving the remainder

of the input unread.

Thereare three optional components to a conversion specification, shown in square

brackets below:

%[*][fldwidth][m][lenmodifier]convtype

The optional leading asterisk is used to suppress conversion. Input is converted as

specified by the rest of the conversion specification, but the result is not stored in an

argument.

Thefldwidthcomponent specifies the maximum field width in characters. The

lenmodifiercomponent specifies the size of the argument to be initialized with the

result of the conversion. The same length modifiers supported by theprintffamily of

functions aresupported by thescanffamily of functions (see Figure5.8 for a list of the

length modifiers).

Theconvtypefield is similar to the conversion type field used by theprintf

family,but thereare some differences. One difference is that results that arestored in

unsigned types can optionally be signed on input. For example,−1will scan as

4294967295 into an unsigned integer.Figure5.10 summarizes the conversion types

supported by thescanffamily of functions.

The optionalmcharacter between the field width and the length modifier is called

theassignment-allocation character.Itcan be used with the%c,%s,and%[conversion