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The old System V C library provided three functions to convert numbers to strings, with unusual and hard-to-use semantics. The GNU C library also provides these functions and some natural extensions.
These functions are only available in glibc and on systems descended
from AT&T Unix. Therefore, unless these functions do precisely what you
need, it is better to use sprintf, which is standard.
All these functions are defined in stdlib.h.
The function
ecvtconverts the floating-point number value to a string with at most ndigit decimal digits. The returned string contains no decimal point or sign. The first digit of the string is non-zero (unless value is actually zero) and the last digit is rounded to nearest.*decpt is set to the index in the string of the first digit after the decimal point.*neg is set to a nonzero value if value is negative, zero otherwise.If ndigit decimal digits would exceed the precision of a
doubleit is reduced to a system-specific value.The returned string is statically allocated and overwritten by each call to
ecvt.If value is zero, it is implementation defined whether
*decpt is0or1.For example:
ecvt (12.3, 5, &d, &n)returns"12300"and sets d to2and n to0.
The function
fcvtis likeecvt, but ndigit specifies the number of digits after the decimal point. If ndigit is less than zero, value is rounded to the ndigit+1'th place to the left of the decimal point. For example, if ndigit is-1, value will be rounded to the nearest 10. If ndigit is negative and larger than the number of digits to the left of the decimal point in value, value will be rounded to one significant digit.If ndigit decimal digits would exceed the precision of a
doubleit is reduced to a system-specific value.The returned string is statically allocated and overwritten by each call to
fcvt.
gcvtis functionally equivalent to `sprintf(buf, "%*g", ndigit, value'. It is provided only for compatibility's sake. It returns buf.If ndigit decimal digits would exceed the precision of a
doubleit is reduced to a system-specific value.
As extensions, the GNU C library provides versions of these three
functions that take long double arguments.
This function is equivalent to
ecvtexcept that it takes along doublefor the first parameter and that ndigit is restricted by the precision of along double.
This function is equivalent to
fcvtexcept that it takes along doublefor the first parameter and that ndigit is restricted by the precision of along double.
This function is equivalent to
gcvtexcept that it takes along doublefor the first parameter and that ndigit is restricted by the precision of along double.
The ecvt and fcvt functions, and their long double
equivalents, all return a string located in a static buffer which is
overwritten by the next call to the function. The GNU C library
provides another set of extended functions which write the converted
string into a user-supplied buffer. These have the conventional
_r suffix.
gcvt_r is not necessary, because gcvt already uses a
user-supplied buffer.
The
ecvt_rfunction is the same asecvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is-1in case of an error and zero otherwise.This function is a GNU extension.
The
fcvt_rfunction is the same asfcvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is-1in case of an error and zero otherwise.This function is a GNU extension.
The
qecvt_rfunction is the same asqecvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is-1in case of an error and zero otherwise.This function is a GNU extension.
The
qfcvt_rfunction is the same asqfcvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is-1in case of an error and zero otherwise.This function is a GNU extension.