Chapter 8

The first parameter is an  id  used to detect memory leaks; unless you're doing 

memory debugging, you can safely use zero. The second parameter can be used to 

preallocate memory inside the SV for strings. If you know you'll be using the SV to 

hold a large string, then you can potentially improve performance by preallocating 

the space.

In practice, NEWSV is rarely used. This is because the Perl API supplies conve 

nience functions to create SVs directly from the available value types:

SV *sv_iv, *sv_uv, *sv_nv, *sv_pv;

sv_iv = newSViv( 10);       // sv_iv contains the signed integer value  10

sv_uv = newSVuv(10);        // sv_uv contains the unsigned integer value 10

sv_nv = newSVnv(10.5);      // sv_nv contains the floating point value 10.5

sv_pv = newSVpv("ten", 0);  // sv_pv contains the string "ten", the second

                            // parameter tells Perl to compute the length with

                            // strlen()

A more efficient version of newSVpv() called newSVpvn() doesn't offer automatic 

strlen() calling:

sv_pv = newSVpvn("ten", 3);  // second parameter gives the length of "ten"

A version that uses sprintf() style format strings, newSVpvf(), is also available:

sv_pv = newSVpvf("%d", 10);  // sv_pv contains the string "10"

NOTE The comments used in the C examples are actually C++ style 

comments (// comment). This was done to improve readability and 

reduce the space required by the comments. Most modern C compilers 

will accept these comments, but if yours doesn't you'll need to change 

them to C style comments (/* comment */) or omit them entirely.

Type Checking

You can test the type of an SV using the SV*OK macros. Specific versions exist for the 

specific types:

if (SvIOK_notUV(sv)) warn("sv contains an IV.");

if (SvIOK_UV(sv))    warn("sv contains a UV.");

if (SvNOK(sv))       warn("sv contains an NV.");

if (SvPOK(sv))       warn("sv contains a PV.");

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