scanf

Input format conversion

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Contents

Interface
Description
Conversions
Return Values
See Also
Bugs
Page Comments

Interface

#include <stdio.h>int  scanf (const char * restrict format, ...)
int  fscanf (FILE * restrict stream, const char * restrict format, ...)
int  sscanf (const char * restrict str, const char * restrict format, ...)

#include <stdarg.h>int  vscanf (const char * restrict format, va_list ap)
int  vsscanf (const char * restrict str, const char * restrict format, va_list ap)
int  vfscanf (FILE * restrict stream, const char * restrict format, va_list ap)

Description

The scanf family of functions scans input according to a format as described below. This format may contain conversion specifiers the results from such conversions, if any, are stored through the pointer arguments. The scanf function reads input from the standard input stream stdin, fscanf reads input from the stream pointer stream, and sscanf reads its input from the character string pointed to by str. The vfscanf function is analogous to vfprintf and reads input from the stream pointer stream using a variable argument list of pointers (see stdarg). The vscanf function scans a variable argument list from the standard input and the vsscanf function scans it from a string; these are analogous to the vprintf and vsprintf functions respectively. Each successive pointer argument must correspond properly with each successive conversion specifier (but see the * and %n conversions below). All conversions are introduced by the % (percent sign) character or %n sequence. #include <the> pointer will be the n th argument after the format string. The format string may also contain other characters. White space (such as blanks, tabs, or newlines) in the format string match any amount of white space, including none, in the input. Everything else matches only itself. Scanning stops when an input character does not match such a format character. Scanning also stops when an input conversion cannot be made (see below).

Extended locale versions of these functions are documented in scanf_l. See xlocale for more information.

The code below attempts to open the text file "fred.txt" in the current directory and read each character in it until the EOF symbol is encountered. Reading from the file is achieved using the fscanf function.

Example 1

#include <stdio.h>
 
int main()
{
  FILE *in;
  if (in = fopen("fred.txt", "rt"))
  {
    for (char c; !feof(in); fscanf(in, "%c", &c));
    fclose(in);
  }
  return 0;
}

Conversions

Following the % character or %n sequence introducing a conversion there may be a number of flag characters, as follows: <table cellspacing="0" class="refpage" style="margin-left:25px"> <tr> <td valign="top" nowrap> * </td> <td valign="top"> Suppresses assignment. The conversion that follows occurs as usual, but no pointer is used; the result of the conversion is simply discarded. </td> </tr> <tr> <td valign="top" nowrap> hh </td> <td valign="top"> Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a char (rather than int). </td> </tr> <tr> <td valign="top" nowrap> h </td> <td valign="top"> Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a short int (rather than int). </td> </tr> <tr> <td valign="top" nowrap> l (ell) </td> <td valign="top"> Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a long int (rather than int), that the conversion will be one of a, e, f, or g and the next pointer is a pointer to double (rather than float), or that the conversion will be one of c, s or [ and the next pointer is a pointer to an array of wchar_t (rather than char). </td> </tr> <tr> <td valign="top" nowrap> ll (ell ell) </td> <td valign="top"> Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a long long int (rather than int). </td> </tr> <tr> <td valign="top" nowrap> L </td> <td valign="top"> Indicates that the conversion will be one of a, e, f, or g and the next pointer is a pointer to long double. </td> </tr> <tr> <td valign="top" nowrap> j </td> <td valign="top"> Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a intmax_t (rather than int). </td> </tr> <tr> <td valign="top" nowrap> t </td> <td valign="top"> Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a ptrdiff_t (rather than int). </td> </tr> <tr> <td valign="top" nowrap> z </td> <td valign="top"> Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a size_t (rather than int). </td> </tr> <tr> <td valign="top" nowrap> q </td> <td valign="top"> (deprecated.) Indicates that the conversion will be one of dioux or n and the next pointer is a pointer to a long long int (rather than int). </td> </tr> </table> \n \n In addition to these flags, there may be an optional maximum field width, expressed as a decimal integer, between the % and the conversion. If no width is given, a default of "infinity" is used (with one exception, below); otherwise at most this many bytes are scanned in processing the conversion. In the case of the lc, ls and l[ conversions, the field width specifies the maximum number of multibyte characters that will be scanned. Before conversion begins, most conversions skip white space; this white space is not counted against the field width.

The following conversions are available: <table cellspacing="0" class="refpage" style="margin-left:25px"> <tr> <td valign="top" nowrap> % </td> <td valign="top"> Matches a literal '%'. That is, "%%" in the format string matches a single input '%' character. No conversion is done, and assignment does not occur. </td> </tr> <tr> <td valign="top" nowrap> d </td> <td valign="top"> Matches an optionally signed decimal integer; the next pointer must be a pointer to int. </td> </tr> <tr> <td valign="top" nowrap> i </td> <td valign="top"> Matches an optionally signed integer; the next pointer must be a pointer to int. The integer is read in base 16 if it begins with '0x' or '0X', in base 8 if it begins with '0', and in base 10 otherwise. Only characters that correspond to the base are used. </td> </tr> <tr> <td valign="top" nowrap> o </td> <td valign="top"> Matches an octal integer; the next pointer must be a pointer to unsigned int. </td> </tr> <tr> <td valign="top" nowrap> u </td> <td valign="top"> Matches an optionally signed decimal integer; the next pointer must be a pointer to unsigned int. </td> </tr> <tr> <td valign="top" nowrap> x, X </td> <td valign="top"> Matches an optionally signed hexadecimal integer; the next pointer must be a pointer to unsigned int. </td> </tr> <tr> <td valign="top" nowrap> a, A, e, E, f, F, g, G </td> <td valign="top"> Matches a floating-point number in the style of strtod. The next pointer must be a pointer to float (unless l or L is specified.) </td> </tr> <tr> <td valign="top" nowrap> s </td> <td valign="top"> Matches a sequence of non-white-space characters; the next pointer must be a pointer to char, and the array must be large enough to accept all the sequence and the terminating NUL character. The input string stops at white space or at the maximum field width, whichever occurs first. \n \n If an l qualifier is present, the next pointer must be a pointer to wchar_t, into which the input will be placed after conversion by mbrtowc. </td> </tr> <tr> <td valign="top" nowrap> S </td> <td valign="top"> The same as ls. </td> </tr> <tr> <td valign="top" nowrap> c </td> <td valign="top"> Matches a sequence of width count characters (default 1); the next pointer must be a pointer to char, and there must be enough room for all the characters (no terminating NUL is added). The usual skip of leading white space is suppressed. To skip white space first, use an explicit space in the format. \n \n If an l qualifier is present, the next pointer must be a pointer to wchar_t, into which the input will be placed after conversion by mbrtowc. </td> </tr> <tr> <td valign="top" nowrap> C </td> <td valign="top"> The same as lc. </td> </tr> <tr> <td valign="top" nowrap> [ </td> <td valign="top"> Matches a nonempty sequence of characters from the specified set of accepted characters; the next pointer must be a pointer to char, and there must be enough room for all the characters in the string, plus a terminating NUL character. The usual skip of leading white space is suppressed. The string is to be made up of characters in (or not in) a particular set; the set is defined by the characters between the open bracket [ character and a close bracket ] character. The set excludes those characters if the first character after the open bracket is a circumflex ^. To include a close bracket in the set, make it the first character after the open bracket or the circumflex; any other position will end the set. The hyphen character - is also special; when placed between two other characters, it adds all intervening characters to the set. To include a hyphen, make it the last character before the final close bracket. For instance, '[^]0-9-]' means the set "everything except close bracket, zero through nine, and hyphen". The string ends with the appearance of a character not in the (or, with a circumflex, in) set or when the field width runs out.

If an l qualifier is present, the next pointer must be a pointer to wchar_t, into which the input will be placed after conversion by mbrtowc. </td> </tr> <tr> <td valign="top" nowrap> p </td> <td valign="top"> Matches a pointer value (as printed by '%p' in printf); the next pointer must be a pointer to void. </td> </tr> <tr> <td valign="top" nowrap> n </td> <td valign="top"> Nothing is expected; instead, the number of characters consumed thus far from the input is stored through the next pointer, which must be a pointer to int. This is not a conversion, although it can be suppressed with the * flag. </td> </tr> </table>

The decimal point character is defined in the program's locale (category LC_NUMERIC).

For backwards compatibility, a "conversion" of '%\0' causes an immediate return of EOF.

Return Values

These functions return the number of input items assigned, which can be fewer than provided for, or even zero, in the event of a matching failure. Zero indicates that, while there was input available, no conversions were assigned; typically this is due to an invalid input character, such as an alphabetic character for a '%d' conversion. The value EOF is returned if an input failure occurs before any conversion such as an end-of-file occurs. If an error or end-of-file occurs after conversion has begun, the number of conversions which were successfully completed is returned.

Bugs

Earlier implementations of scanf treated %D, %E, %F, %O and %X as their lowercase equivalents with an l modifier. In addition, scanf treated an unknown conversion character as %d or %D, depending on its case. This functionality has been removed.

Numerical strings are truncated to 512 characters; for example, %f and %d are implicitly %512f and %512d.

The scanf family of functions do not correctly handle multibyte characters in the format argument.

Page Comments

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int	`scanf` (const char * restrict format, ...)
int	`fscanf` (FILE * restrict stream, const char * restrict format, ...)
int	`sscanf` (const char * restrict str, const char * restrict format, ...)

int	`vscanf` (const char * restrict format, va_list ap)
int	`vsscanf` (const char * restrict str, const char * restrict format, va_list ap)
int	`vfscanf` (FILE * restrict stream, const char * restrict format, va_list ap)