Manpage of SCANF


Section: Linux Programmer's Manual (3)
Updated: 2017-09-15


scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conversion  


#include <stdio.h>int scanf(const char *format, ...);int fscanf(FILE *stream, const char *format, ...);int sscanf(const char *str, const char *format, ...);#include <stdarg.h>int vscanf(const char *format, va_list ap);int vsscanf(const char *str, const char *format, va_list ap);int vfscanf(FILE *stream, const char *format, va_list ap);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

vscanf(), vsscanf(), vfscanf():



The scanf() family of functions scans input according to formatas described below. This format may contain conversion specifications; the results from such conversions, if any, are stored in the locations pointed to by the pointerarguments that follow format. Each pointerargument must be of a type that is appropriate for the value returned by the corresponding conversion specification.

If the number of conversion specifications in formatexceeds the number of pointerarguments, the results are undefined. If the number of pointerarguments exceeds the number of conversion specifications, then the excess pointerarguments are evaluated, but are otherwise ignored.

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(3) and reads input from the stream pointer streamusing a variable argument list of pointers (see stdarg(3). 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(3) and vsprintf(3) functions respectively.

The formatstring consists of a sequence of directiveswhich describe how to process the sequence of input characters. If processing of a directive fails, no further input is read, and scanf() returns. A "failure" can be either of the following: input failure, meaning that input characters were unavailable, or matching failure, meaning that the input was inappropriate (see below).

A directive is one of the following:

A sequence of white-space characters (space, tab, newline, etc.; see isspace(3)). This directive matches any amount of white space, including none, in the input.
An ordinary character (i.e., one other than white space or aq%aq). This character must exactly match the next character of input.
A conversion specification, which commences with a aq%aq (percent) character. A sequence of characters from the input is converted according to this specification, and the result is placed in the corresponding pointerargument. If the next item of input does not match the conversion specification, the conversion fails---this is a matching failure.

Each conversion specificationin formatbegins with either the character aq%aq or the character sequence "%n$" (see below for the distinction) followed by:

An optional aq*aq assignment-suppression character: scanf() reads input as directed by the conversion specification, but discards the input. No corresponding pointerargument is required, and this specification is not included in the count of successful assignments returned by scanf().
For decimal conversions, an optional quote character (aq). This specifies that the input number may include thousands' separators as defined by the LC_NUMERICcategory of the current locale. (See setlocale(3).) The quote character may precede or follow the aq*aq assignment-suppression character.
An optional aqmaq character. This is used with string conversions (%s, %c, %[), and relieves the caller of the need to allocate a corresponding buffer to hold the input: instead, scanf() allocates a buffer of sufficient size, and assigns the address of this buffer to the corresponding pointerargument, which should be a pointer to a char *variable (this variable does not need to be initialized before the call). The caller should subsequently free(3) this buffer when it is no longer required.
An optional decimal integer which specifies the maximum field width. Reading of characters stops either when this maximum is reached or when a nonmatching character is found, whichever happens first. Most conversions discard initial white space characters (the exceptions are noted below), and these discarded characters don't count toward the maximum field width. String input conversions store a terminating null byte (aq\0aq) to mark the end of the input; the maximum field width does not include this terminator.
An optional type modifier character. For example, the ltype modifier is used with integer conversions such as %dto specify that the corresponding pointerargument refers to a long intrather than a pointer to an int.
A conversion specifierthat specifies the type of input conversion to be performed.

The conversion specifications in formatare of two forms, either beginning with aq%aq or beginning with "%n$". The two forms should not be mixed in the same formatstring, except that a string containing "%n$" specifications can include %%and %*. If formatcontains aq%aq specifications, then these correspond in order with successive pointerarguments. In the "%n$" form (which is specified in POSIX.1-2001, but not C99), nis a decimal integer that specifies that the converted input should be placed in the location referred to by the n-th pointerargument following format.  


The following type modifier characterscan appear in a conversion specification:
Indicates that the conversion will be one of d, i, o, u, x, X, or nand the next pointer is a pointer to a short intor unsigned short int(rather than int).
As for h, but the next pointer is a pointer to a signed charor unsigned char.
As for h, but the next pointer is a pointer to an intmax_tor a uintmax_t. This modifier was introduced in C99.
Indicates either that the conversion will be one of d, i, o, u, x, X, or nand the next pointer is a pointer to a long intor unsigned long int(rather than int), or that the conversion will be one of e, f, or gand the next pointer is a pointer to double(rather than float). Specifying two lcharacters is equivalent to L. If used with %cor %s, the corresponding parameter is considered as a pointer to a wide character or wide-character string respectively.
Indicates that the conversion will be either e, f, or gand the next pointer is a pointer to long doubleor the conversion will be d, i, o, u, or xand the next pointer is a pointer to long long.
equivalent to L. This specifier does not exist in ANSI C.
As for h, but the next pointer is a pointer to a ptrdiff_t. This modifier was introduced in C99.
As for h, but the next pointer is a pointer to a size_t. This modifier was introduced in C99.

The following conversion specifiersare available:

Matches a literal aq%aq. That is, %%in the format string matches a single input aq%aq character. No conversion is done (but initial white space characters are discarded), and assignment does not occur.
Matches an optionally signed decimal integer; the next pointer must be a pointer to int.
Equivalent to ld; this exists only for backward compatibility. (Note: thus only in libc4. In libc5 and glibc the %Dis silently ignored, causing old programs to fail mysteriously.)
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 0xor 0X, in base 8 if it begins with 0, and in base 10 otherwise. Only characters that correspond to the base are used.
Matches an unsigned octal integer; the next pointer must be a pointer to unsigned int.
Matches an unsigned decimal integer; the next pointer must be a pointer to unsigned int.
Matches an unsigned hexadecimal integer; the next pointer must be a pointer to unsigned int.
Equivalent to x.
Matches an optionally signed floating-point number; the next pointer must be a pointer to float.
Equivalent to f.
Equivalent to f.
Equivalent to f.
(C99) Equivalent to f.
Matches a sequence of non-white-space characters; the next pointer must be a pointer to the initial element of a character array that is long enough to hold the input sequence and the terminating null byte (aq\0aq), which is added automatically. The input string stops at white space or at the maximum field width, whichever occurs first.
Matches a sequence of characters whose length is specified by the maximum field width(default 1); the next pointer must be a pointer to char, and there must be enough room for all the characters (no terminating null byte is added). The usual skip of leading white space is suppressed. To skip white space first, use an explicit space in the format.
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 null byte. 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 excludesthose 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.
Matches a pointer value (as printed by %pin printf(3); the next pointer must be a pointer to a pointer to void.
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 nota conversion and does notincrease the count returned by the function. The assignment can be suppressed with the *assignment-suppression character, but the effect on the return value is undefined. Therefore %*nconversions should not be used.


On success, these functions return the number of input items successfully matched and assigned; this can be fewer than provided for, or even zero, in the event of an early matching failure.

The value EOFis returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOFis also returned if a read error occurs, in which case the error indicator for the stream (see ferror(3)) is set, and errnois set to indicate the error.  


The file descriptor underlying streamis marked nonblocking, and the read operation would block.
The file descriptor underlying streamis invalid, or not open for reading.
Input byte sequence does not form a valid character.
The read operation was interrupted by a signal; see signal(7).
Not enough arguments; or formatis NULL.
Out of memory.
The result of an integer conversion would exceed the size that can be stored in the corresponding integer type.


For an explanation of the terms used in this section, see attributes(7).
scanf(), fscanf(),
sscanf(), vscanf(),
vsscanf(), vfscanf()
Thread safetyMT-Safe locale



The functions fscanf(), scanf(), and sscanf() conform to C89 and C99 and POSIX.1-2001. These standards do not specify the ERANGEerror.

The qspecifier is the 4.4BSD notation for long long, while llor the usage of Lin integer conversions is the GNU notation.

The Linux version of these functions is based on the GNUlibiolibrary. Take a look at the infodocumentation of GNUlibc (glibc-1.08)for a more concise description.  



The 'a' assignment-allocation modifier

Originally, the GNU C library supported dynamic allocation for string inputs (as a nonstandard extension) via the acharacter. (This feature is present at least as far back as glibc 2.0.) Thus, one could write the following to have scanf() allocate a buffer for an input string, with a pointer to that buffer being returned in *buf:

    char *buf;
    scanf("%as", &buf);

The use of the letter afor this purpose was problematic, since ais also specified by the ISO C standard as a synonym for f(floating-point input). POSIX.1-2008 instead specifies the mmodifier for assignment allocation (as documented in DESCRIPTION, above).

Note that the amodifier is not available if the program is compiled with gcc -std=c99or gcc -D_ISOC99_SOURCE(unless _GNU_SOURCEis also specified), in which case the ais interpreted as a specifier for floating-point numbers (see above).

Support for the mmodifier was added to glibc starting with version 2.7, and new programs should use that modifier instead of a.

As well as being standardized by POSIX, the mmodifier has the following further advantages over the use of a:

It may also be applied to %cconversion specifiers (e.g., %3mc).
It avoids ambiguity with respect to the %afloating-point conversion specifier (and is unaffected by gcc -std=c99etc.).


All functions are fully C89 conformant, but provide the additional specifiers qand aas well as an additional behavior of the Land lspecifiers. The latter may be considered to be a bug, as it changes the behavior of specifiers defined in C89.

Some combinations of the type modifiers and conversion specifiers defined by ANSI C do not make sense (e.g., %Ld). While they may have a well-defined behavior on Linux, this need not to be so on other architectures. Therefore it usually is better to use modifiers that are not defined by ANSI C at all, that is, use qinstead of Lin combination with d, i, o, u, x, and Xconversions or ll.

The usage of qis not the same as on 4.4BSD, as it may be used in float conversions equivalently to L.  


To use the dynamic allocation conversion specifier, specify mas a length modifier (thus %msor %m[range]). The caller must free(3) the returned string, as in the following example:

char *p; int n;

errno = 0; n = scanf("%m[a-z]", &p); if (n == 1) {
    printf("read: %s\n", p);
    free(p); } else if (errno != 0) {
    perror("scanf"); } else {
    fprintf(stderr, "No matching characters\n"); }

As shown in the above example, it is necessary to call free(3) only if the scanf() call successfully read a string.  


getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)



The 'a' assignment-allocation modifier

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