strtod(), strtof(), strtold()

Convert a string into a double-precision number

Synopsis:

#include <stdlib.h>

double strtod( const char *ptr, 
               char **endptr );

float strtof( const char *ptr,
              char **endptr );

long double strtold( const char *ptr,
                     char **endptr );

Arguments:

ptr
A pointer to the string to parse.
endptr
If this argument isn't NULL, the function stores in it a pointer to the first unrecognized character found in the string.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The strtod(), strtof(), and strtold() functions convert the string pointed to by ptr into a double-precision representation:

This function: Returns:
strtod() double
strtof() float
strtold() long double

These functions skip any leading white space, and then look for a subject sequence that consists of an optional plus or minus sign followed by one of the following:

  • a non-empty sequence of decimal digits optionally containing a radix character, then an optional exponent part
  • 0x or 0X, then a non-empty sequence of hexadecimal digits optionally containing a radix character, then an optional binary exponent part
  • INF or INFINITY, ignoring case
  • NAN, ignoring case
    POSIX says that these functions can optionally parse an n-char sequence after the NAN. The QNX Neutrino versions don't interpret an n-char sequence, and the result is equivalent to specifying NAN without the n-char sequence.

The conversion ends at the first unrecognized character. If endptr isn't NULL, a pointer to the unrecognized character is stored in the object endptr points to.

Returns:

The converted value. If the correct value would cause overflow, plus or minus HUGE_VAL is returned according to the sign, and errno is set to ERANGE. If the correct value would cause underflow, then zero is returned, and errno is set to ERANGE.

These functions return zero and set errno if the input string can't be converted; they don't change errno if no errors occurred. If you want to check for errors, set errno to 0, call the function, and then check errno again.

Errors:

ERANGE
The value to be returned would cause overflow or underflow.
EINVAL
No conversion could be performed.

Examples:

#include <stdio.h>
#include <stdlib.h>

int main( void )
  {
    double pi;

    pi = strtod( "3.141592653589793", NULL );
    printf( "pi=%17.15f\n",pi );
    return EXIT_SUCCESS;
  }

Classification:

ANSI, POSIX 1003.1

Safety:
Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes

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