ICONV(3) GNU ICONV(3)
May 30, 2025
NAME
iconv - perform character set conversion
SYNOPSIS
#include <iconv.h>
size_t iconv (iconv_t cd,
const char* * inbuf, size_t * inbytesleft,
char* * outbuf, size_t * outbytesleft);
or (for POSIX compatibility)
size_t iconv (iconv_t cd,
char* * inbuf, size_t * inbytesleft,
char* * outbuf, size_t * outbytesleft);
DESCRIPTION
The argument cd must be a conversion descriptor created using the
function iconv_open.
The main case is when inbuf is not NULL and *inbuf is not NULL. In
this case, the iconv function converts the multibyte sequence starting
at *inbuf to a multibyte sequence starting at *outbuf. At most
*inbytesleft bytes, starting at *inbuf, will be read. At most
*outbytesleft bytes, starting at *outbuf, will be written.
The iconv function converts one multibyte character at a time, and for
each character conversion it increments *inbuf and decrements
*inbytesleft by the number of converted input bytes, it increments
*outbuf and decrements *outbytesleft by the number of converted output
bytes, and it updates the conversion state contained in cd. If the
character encoding of the input is stateful, the iconv function can
also convert a sequence of input bytes to an update of the conversion
state without producing any output bytes; such input is called a shift
sequence. The conversion can stop for five reasons:
1. An invalid multibyte sequence is encountered in the input. In this
case it sets errno to EILSEQ and returns (size_t)(-1). *inbuf is left
pointing to the beginning of the invalid multibyte sequence.
2. A multibyte sequence is encountered that is valid but that cannot
be translated to the character encoding of the output. This condition
depends on the implementation and on the conversion descriptor. In
the GNU C library and GNU libiconv, if cd was created without the
suffix //TRANSLIT or //IGNORE or //NON_IDENTICAL_DISCARD, the
conversion is strict: lossy conversions produce this condition. If
the suffix //TRANSLIT was specified, transliteration can avoid this
condition in some cases. In the musl C library, this condition cannot
occur because a conversion to [aq]*[aq] is used as a fallback. In the
FreeBSD, NetBSD, and Solaris implementations of iconv, this condition
cannot occur either, because a conversion to [aq]?[aq] is used as a
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fallback. When this condition is met, the iconv function sets errno
to EILSEQ and returns (size_t)(-1). *inbuf is left pointing to the
beginning of the unconvertible multibyte sequence.
3. The input byte sequence has been entirely converted, i.e.
*inbytesleft has gone down to 0. In this case iconv returns the number
of non-reversible conversions performed during this call.
4. An incomplete multibyte sequence is encountered in the input, and
the input byte sequence terminates after it. In this case it sets
errno to EINVAL and returns (size_t)(-1). *inbuf is left pointing to
the beginning of the incomplete multibyte sequence.
5. The output buffer has no more room for the next converted
character. In this case it sets errno to E2BIG and returns
(size_t)(-1).
A different case is when inbuf is NULL or *inbuf is NULL, but outbuf
is not NULL and *outbuf is not NULL. In this case, the iconv function
attempts to set cd's conversion state to the initial state and store a
corresponding shift sequence at *outbuf. At most *outbytesleft bytes,
starting at *outbuf, will be written. If the output buffer has no
more room for this reset sequence, it sets errno to E2BIG and returns
(size_t)(-1). Otherwise it increments *outbuf and decrements
*outbytesleft by the number of bytes written.
A third case is when inbuf is NULL or *inbuf is NULL, and outbuf is
NULL or *outbuf is NULL. In this case, the iconv function sets cd's
conversion state to the initial state.
RETURN VALUE
The iconv function returns the number of characters converted in a
non-reversible way during this call; reversible conversions are not
counted. In case of error, it sets errno and returns (size_t)(-1).
ERRORS
The following errors can occur, among others:
E2BIG
There is not sufficient room at *outbuf.
EILSEQ
An invalid multibyte sequence has been encountered in the input.
EINVAL
An incomplete multibyte sequence has been encountered in the
input.
CONFORMING TO
POSIX:2024
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ICONV(3) GNU ICONV(3)
May 30, 2025
Note: In the functions iconv, execv, execve, execvp, POSIX uses the
type char ** in place of the more correct const char **, thus forcing
programs which use const as usual to add a cast in the second argument
of an iconv invocation. GNU libiconv uses const char ** or char **,
depending on the platform.
Packages that use the GNU Build System can determine which of the two
choices the declaration of iconv in <iconv.h> actually uses. To do
this, the package can use the Autoconf macro AM_ICONV, documented in
the GNU gettext manual and also available from the module iconv in GNU
gnulib, and then use the C preprocessor symbol ICONV_CONST.
NOTES
In each series of calls to the iconv function, the last should be one
with inbuf or *inbuf equal to NULL, in order to complete the
conversion of any partially converted input.
Although inbuf and outbuf are typed as const char ** and char **,
respectively, this does not mean that the objects they point can be
interpreted as C strings or as arrays of characters: the
interpretation of character byte sequences is handled internally by
the conversion functions. In some encodings, a zero byte may be a
valid part of a multibyte character.
The caller of the iconv function must ensure that the pointers passed
to the function are suitable for accessing characters in the
appropriate character set. For the encodings UCS-2-INTERNAL, UCS-4-
INTERNAL, and wchar_t, this includes ensuring correct alignment.
SEE ALSO
iconv_open(3), iconvctl(3), iconv_close(3)
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