Manpages - scanf.3

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

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

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

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

      _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

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

If the number of conversion specifications in format exceeds the number of pointer arguments, the results are undefined. If the number of pointer arguments exceeds the number of conversion specifications, then the excess pointer arguments 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 stream using 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 format string consists of a sequence of directives which 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 ’%’). This character must exactly match the next character of input.

·

A conversion specification, which commences with a ’%’ (percent) character. A sequence of characters from the input is converted according to this specification, and the result is placed in the corresponding pointer argument. If the next item of input does not match the conversion specification, the conversion fails—this is a matching failure.

Each conversion specification in format begins with either the character ’%’ or the character sequence “%*/n/*$” (see below for the distinction) followed by:

·

An optional ’*’ assignment-suppression character: *scanf*() reads input as directed by the conversion specification, but discards the input. No corresponding pointer argument is required, and this specification is not included in the count of successful assignments returned by *scanf*().

·

For decimal conversions, an optional quote character (’). This specifies that the input number may include thousands’ separators as defined by the LC_NUMERIC category of the current locale. (See setlocale*(3).) The quote character may precede or follow the ’’ assignment-suppression character.

·

An optional ’m’ 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 pointer argument, 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 (’\0’) to mark the end of the input; the maximum field width does not include this terminator.

·

An optional type modifier character. For example, the l type modifier is used with integer conversions such as %d to specify that the corresponding pointer argument refers to a long rather than a pointer to an int.

·

A conversion specifier that specifies the type of input conversion to be performed.

The conversion specifications in format are of two forms, either beginning with ’%’ or beginning with “%*/n/*$”. The two forms should not be mixed in the same format string, except that a string containing “%*/n/*$” specifications can include %% and %*. If format contains ’%’ specifications, then these correspond in order with successive pointer arguments. In the “%*/n/*$” form (which is specified in POSIX.1-2001, but not C99), n is a decimal integer that specifies that the converted input should be placed in the location referred to by the n-th pointer argument following format.

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

EAGAIN

The file descriptor underlying stream is marked nonblocking, and the read operation would block.

EBADF

The file descriptor underlying stream is invalid, or not open for reading.

EILSEQ

Input byte sequence does not form a valid character.

EINTR

The read operation was interrupted by a signal; see *signal*(7).

EINVAL

Not enough arguments; or format is NULL.

ENOMEM

Out of memory.

ERANGE

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).

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

The q specifier is the 4.4BSD notation for long long, while ll or the usage of L in integer conversions is the GNU notation.

The Linux version of these functions is based on the GNU libio library. Take a look at the info documentation of GNU libc (glibc-1.08) for a more concise description.

All functions are fully C89 conformant, but provide the additional specifiers q and a as well as an additional behavior of the L and l specifiers. 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 q instead of L in combination with d, i, o, u, x, and X conversions or ll.

The usage of q is 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 m as a length modifier (thus %ms or %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)

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