'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.TH snprintf 3 2025-12-07 "Linux man-pages (unreleased)"
.SH NAME
snprintf,
vsnprintf
\-
string print formatted
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.P
.BR "int snprintf(" "size_t size;"
.BI "            char " str "[restrict " size "], size_t " size ,
.BI "            const char *restrict " format ", ...);"
.BR "int vsnprintf(" "size_t size;"
.BI "            char " str "[restrict " size "], size_t " size ,
.BI "            const char *restrict " format ", va_list " ap );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR snprintf (),
.BR vsnprintf ():
.nf
    _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE
.fi
.SH DESCRIPTION
These functions are similar to
.BR printf (3),
except that they write to the character string
.I str
instead of a stream.
.P
The functions
.BR snprintf ()
and
.BR vsnprintf ()
write at most
.I size
bytes (including the terminating null byte (\[aq]\[rs]0\[aq])) to
.IR str .
.P
.BR vsnprintf ()
is equivalent to
.BR snprintf (),
except that it is called with a
.I va_list
instead of a variable number of arguments.
This function does not call the
.I va_end
macro.
Because it invokes the
.I va_arg
macro, the value of
.I ap
is undefined after the call.
See
.BR stdarg (3).
.P
C99 and POSIX.1-2001 specify that the results are undefined if a call to
.BR snprintf ()
or
.BR vsnprintf ()
would cause copying to take place between objects that overlap
(e.g., if the target string array and one of the supplied input arguments
refer to the same buffer).
See CAVEATS.
.SS Format of the format string
See
.BR printf (3).
.SH RETURN VALUE
Upon successful return, these functions return the number of bytes
printed (excluding the null byte used to end output to strings).
.P
The functions
.BR snprintf ()
and
.BR vsnprintf ()
do not write more than
.I size
bytes (including the terminating null byte (\[aq]\[rs]0\[aq])).
If the output was truncated due to this limit, then the return value
is the number of characters (excluding the terminating null byte)
which would have been written to the final string if enough space
had been available.
Thus, a return value of
.I size
or more means that the output was truncated.
(See also below under CAVEATS.)
.P
On error,
a negative value is returned,
and
.I errno
is set to indicate the error.
.SH ERRORS
See
.BR printf (3).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR snprintf (),
.BR vsnprintf ()
T}	Thread safety	MT-Safe locale
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
SUSv2, C99, POSIX.1-2001.
.IP
Concerning the return value,
SUSv2 and C99 contradict each other: when
.BR snprintf ()
is called with
.IR size =0
then SUSv2 stipulates an unspecified return value less than 1,
while C99 allows
.I str
to be NULL in this case, and gives the return value (as always)
as the number of characters that would have been written in case
the output string has been large enough.
POSIX.1-2001 and later align their specification of
.BR snprintf ()
with C99.
.SH CAVEATS
Some programs imprudently rely on code such as the following
.P
.in +4n
.EX
snprintf(buf, countof(buf), "%s some further text", buf);
.EE
.in
.P
to append text to
.IR buf .
However, the standards explicitly note that the results are undefined
if source and destination buffers overlap when calling
.BR snprintf ()
and
.BR vsnprintf ().
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=7075
Depending on the version of
.BR gcc (1)
used, and the compiler options employed, calls such as the above will
.B not
produce the expected results.
.P
The glibc implementation of the functions
.BR snprintf ()
and
.BR vsnprintf ()
conforms to the C99 standard, that is, behaves as described above,
since glibc 2.1.
Until glibc 2.0.6, they would return \-1
when the output was truncated.
.\" .SH HISTORY
.\" UNIX V7 defines the three routines
.\" .BR sprintf ().
.\" 2.11BSD has
.\" .BR vsprintf ().
.\" 4.4BSD introduces the functions
.\" .BR snprintf ()
.\" and
.\" .BR vsnprintf ().
.\" FreeBSD also has functions
.\" .BR asprintf ()
.\" and
.\" .BR vasprintf (),
.\" that allocate a buffer large enough for
.\" .BR sprintf ().
.SH BUGS
See
.BR printf (3).
.SH EXAMPLES
To allocate a sufficiently large string and print into it
(code correct for both glibc 2.0 and glibc 2.1):
.P
.EX
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
\&
char *
make_message(const char *fmt, ...)
{
    int n = 0;
    size_t size = 0;
    char *p = NULL;
    va_list ap;
\&
    /* Determine required size.  */
\&
    va_start(ap, fmt);
    n = vsnprintf(p, size, fmt, ap);
    va_end(ap);
\&
    if (n < 0)
        return NULL;
\&
    size = (size_t) n + 1;      /* One extra byte for \[aq]\[rs]0\[aq] */
    p = malloc(size);
    if (p == NULL)
        return NULL;
\&
    va_start(ap, fmt);
    n = vsnprintf(p, size, fmt, ap);
    va_end(ap);
\&
    if (n < 0) {
        free(p);
        return NULL;
    }
\&
    return p;
}
.EE
.P
If truncation occurs in glibc versions prior to glibc 2.0.6,
this is treated as an error instead of being handled gracefully.
.SH SEE ALSO
.BR printf (1),
.BR asprintf (3),
.BR printf (3),
.BR puts (3),
.BR scanf (3),
.BR setlocale (3),
.BR strfromd (3),
.BR wcrtomb (3),
.BR wprintf (3),
.BR locale (5)