Postgres: how to convert a bytea field into c internal string char*

Question

I'm writing an extension for postgres in C.

This function will receive a bytea from hexstring, convert bytea into char* then do some operations on this char*, finally return a number.

In SQL level, I have something like:

select my_function(decode(**hex_string**, 'hex')) AS result;

In C level, I have:

bytea *newval = PG_GETARG_BYTEA_P(0);
unsigned char *byte_array = (unsigned char *) VARDATA(newval);

With following 2056 bytes size hex_string

484c08020000000045422275433435354222a56424534257334242335285345034366432525334333312464268247645b421352562714488235244462423347733333443333764333276343362522524277251533276542454332725235237251552363532232264325822663374483725224343332535227437229446363344422332634444294363815262622b237457236261312655522255533443745345434495535422423538333446152155443333434234632436332479252464522373412835526433584342352523542366227634847374546642323642365338353653a71533345343364234473144362353389232342332322443413532926224952324553425345433643333432344453432542511426513332244536425546335a42744433d324545464526443425415423242423432323433559342336435334953562673627797554634424254244666543493355436341125323452424245373639334354246513531447741247333542743155323233734558844332447322225324132333354523613553151415435271352453365342254232324221414234778332232464534592347534465231314313638453768343683563556541356633572632333364255442632143431635514553373376436143543346262153643342742436465233322525683423123e446544336357342331433443452462425125224476333412446381334464225594443233924433344244313433444426564663554424346733543374552635355543542333416639317251946583222445742341673644642242643473353543461394347765233337733234533255443633522b452149435433424327422665423347553315144237745265223393433343635476442255435554251430424333343324445425342774632a4228363444444443332434220334343273341663652824153342424336644324311235732525324325158343263572565134424214633234532683342322432457543353333232339522354533464533144363252324332654353345754163727552357534532734753232263535342435343363162269235314322422455675262333482233328346333534693356736533423455a45323753135336353323235354345452546435232233665442454533424552745943451623225384362532345324832445434363334542323424321424234332352253435337244524313527454244474444633325347364262534336751353422335243357463127254335634355164463527514872315633739376433422332335a623432624647452044584223643279344345a3425743333744436342463323433641456553235b5645446363465441b33744423675652357e224654555345451725341234471422742335322195244454532243443554332735324352524324325538362123642444343733429345443555132233422658434323318322224812932523225472542222253824567143533934422355564357556544364143582456552a5723633444343373143233232365313457333335c854303223435555376224527522837373432423636175514265729563262724363277553723345655343543644154222243544584235166433234144427432422353223161443225236552236247233344444673254346533332622536836233436524125642424346312831738524337135282253821395545134317236235463527335424225512342442327435876642613664244d514536635365842545133354453224a166632541354457334124224353536547422331543364273124364322734625292685535624235542432444542683b365353344253434363623634111325142362255346447425653656376413463621664322354624552825434226546553711223344382254339694331a345443733528422333233464224343302562636431323453441155432336355556322342624453332737553545436243222c552322231244334321322725226733475682554c22346442473655521543516465465448334224422467733355535543425333132234323422342327836322421563825334624422443373344482523526345348334543148b533426615333354344536242544344743342472843442258644834363462354424363d24c53424536537434343443526324592341423544a3335341325465964443452334322746a336533326413622533422334367451846143234444154323256a61342334334466a9454235284262536232443314514243344433324137222437532175432446364352554334247435442428743735436344235213249423463132b655133627457443533431352332156453564333554547512335657a515425244222640334341643435944324453337235444752332645133633252423164843422335823421322352752122424354212554467474422534143833552b352253232225224153263363334323633a5471673366411342545233727337439482874525135124343341524462335283445c63736631322124546553433242252263a134123146127422735733344265413547565536122239b35403323442335635232234423424424435332a2363343723354333443433462233823434183456b344444444445442942248425434331243444433634647333634233494464242247143252391342a396443523229233536223563992333355a233342343182435324497332744885233732533375454264833433454273236443214223473333264393232272242443522491583234426323266634631344364326

I just got a 4byte char* in c

byte_array content : HL
byte_array length  : 4

The question is what is the problem with my code and how can I get expected char* (2056 byte) ?

clemens · Accepted Answer · 2018-01-21 10:17:18Z

1

Short answer: Your bytes don't contain a printable C string.

Long answer: Your bytes obviously include null characters, which are end markers for C strings. Thus your program prints the first characters until the first 0. Since your hex-string contains 4 consecutive zeros it doesn't seem to be a valid C string as well; regardless of possible encodings.

You can convert the bytes to a Postgres text value with encode(bytea_field, 'escape)`, and read it as text from your C code. But because of the zero bytes this will not print a longer string either.

answered Jan 21, 2018 at 10:17

clemens

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1 Comment

nick Over a year ago

Thanks for you explaination @clemens!

Laurenz Albe · Accepted Answer · 2018-01-21 12:58:14Z

0

You should use VARSIZE(newval) to determine the size of the bytea.

answered Jan 21, 2018 at 12:58

Laurenz Albe

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