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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../../cpp/../c/numeric/math/nan.html below:

nan, nanf, nanl, nand32, nand64, nand128

float nanf( const char* arg );

(1) (since C99)

double nan( const char* arg );

(2) (since C99)

long double nanl( const char* arg );

(3) (since C99)

_Decimal32 nand32( const char* arg );

(4) (since C23)

_Decimal64 nand64( const char* arg );

(5) (since C23)

_Decimal128 nand128( const char* arg );

(6) (since C23)

Converts the implementation-defined character string arg into the corresponding quiet NaN value, as if by calling the appropriate parsing function strtoX, as follows:

The call nan("n-char-sequence"), where n-char-sequence is a sequence of digits, Latin letters, and underscores, is equivalent to the call /*strtoX*/("NAN(n-char-sequence)", (char**)NULL);.
The call nan("") is equivalent to the call /*strtoX*/("NAN()", (char**)NULL);.
The call nan("string"), where string is neither an n-char-sequence nor an empty string, is equivalent to the call /*strtoX*/("NAN", (char**)NULL);.

1)

The parsing function is

strtof

2)

The parsing function is

strtod

3)

The parsing function is

strtold

4) The parsing function is strtod32.

5) The parsing function is strtod64.

6) The parsing function is strtod128.

The functions returning decimal floating point values are declared if and only the implementation predefines __STDC_IEC_60559_DFP__ (i.e. the implementation supports decimal floating point numbers).

(since C23) [edit] Parameters arg - narrow character string identifying the contents of a NaN [edit] Return value

The quiet NaN value that corresponds to the identifying string arg or zero if the implementation does not support quiet NaNs.

If the implementation supports IEEE floating-point arithmetic (IEC 60559), it also supports quiet NaNs.

[edit] Error handling

This function is not subject to any of the error conditions specified in math_errhandling.

[edit] Example

#include <stdio.h>
#include <math.h>
#include <stdint.h>
#include <inttypes.h>
#include <string.h>
 
int main(void)
{
    double f1 = nan("1");
    uint64_t f1n; memcpy(&f1n, &f1, sizeof f1);
    printf("nan(\"1\")   = %f (%" PRIx64 ")\n", f1, f1n);
 
    double f2 = nan("2");
    uint64_t f2n; memcpy(&f2n, &f2, sizeof f2);
    printf("nan(\"2\")   = %f (%" PRIx64 ")\n", f2, f2n);
 
    double f3 = nan("0xF");
    uint64_t f3n; memcpy(&f3n, &f3, sizeof f3);
    printf("nan(\"0xF\") = %f (%" PRIx64 ")\n", f3, f3n);
}

Possible output:

nan("1")   = nan (7ff8000000000001)
nan("2")   = nan (7ff8000000000002)
nan("0xF") = nan (7ff800000000000f)

[edit] References

C17 standard (ISO/IEC 9899:2018):

7.12.11.2 The nan functions (p: 186-187)

F.10.8.2 The nan functions (p: 386)

C11 standard (ISO/IEC 9899:2011):

7.12.11.2 The nan functions (p: 256)

F.10.8.2 The nan functions (p: 529)

C99 standard (ISO/IEC 9899:1999):

7.12.11.2 The nan functions (p: 237)

F.9.8.2 The fabs functions (p: 465)

[edit] See also

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