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Use this page when you need the smallest reliable mental model for how a C program starts, reads input, mutates state, and exits.

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Minimal program shape

#include <thru-sdk/c/tn_sdk.h>

TSDK_ENTRYPOINT_FN void
start( void ) {
  tsdk_txn_t const * txn = tsdk_get_txn( );

  if( tsdk_txn_get_instr_data_sz( txn ) == 0UL ) {
    tsdk_revert( 1UL );
  }

  tsdk_return( TSDK_SUCCESS );
}

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Execution model

StepWhat normally happens
EnterThe VM jumps to a function marked with TSDK_ENTRYPOINT_FN.
Read inputsThe program reads transaction and account context through helpers like tsdk_get_txn, tsdk_get_account_meta, and tsdk_get_account_data_ptr.
DecodeInstruction bytes are read from the transaction body, usually through tsdk_txn_get_instr_data and tsdk_txn_get_instr_data_sz.
ValidatePrograms check instruction size, account indices, ownership, authorization, and any variable-length payload boundaries before mutating state.
Mutate or invokePrograms call syscalls such as tsys_account_resize, tsys_account_transfer, or tsys_invoke.
ExitPrograms terminate with tsdk_return(...) for success or tsdk_revert(...) for failure.
FilePurpose
your_program.hInstruction tags, error codes, packed payload structs, account-data structs, and local constants.
your_program.cEntry point, validation, instruction dispatch, syscall usage, and local helper functions.
This split keeps payload layout definitions easy to reuse without forcing an agent to reread the whole implementation file.

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Typical instruction dispatch pattern

#include <thru-sdk/c/tn_sdk.h>

typedef struct __attribute__(( packed )) {
  uint instruction_type;
} my_instr_hdr_t;

TSDK_ENTRYPOINT_FN void
start( void ) {
  tsdk_txn_t const * txn = tsdk_get_txn( );
  uchar const *      instr = tsdk_txn_get_instr_data( txn );
  ulong              instr_sz = tsdk_txn_get_instr_data_sz( txn );

  if( instr_sz < sizeof( my_instr_hdr_t ) ) tsdk_revert( 0x1000UL );

  my_instr_hdr_t const * hdr = (my_instr_hdr_t const *)instr;

  switch( hdr->instruction_type ) {
  case 0U:
    /* handle create */
    break;
  case 1U:
    /* handle update */
    break;
  default:
    tsdk_revert( 0x1001UL );
  }

  tsdk_return( TSDK_SUCCESS );
}

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Control-flow rules

  • tsdk_return does not come back.
  • tsdk_revert does not come back.
  • tsys_exit underlies both helpers and is also terminal.
  • If you need cleanup, do it before calling either exit helper.

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Common structure choices

GoalCommon choice
Small fixed instructionOne packed struct with a leading tag.
Variable-length instructionFixed header plus byte tail, with explicit size checks before casting or copying.
Stateful programOne or more packed account-data structs plus helper functions to read and write them.
CPI-heavy programEntry point plus small helpers to build invoke payloads and auth descriptors.

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Notes

  • Prefer tsdk_get_txn() as the root of most program reads.
  • Prefer a local header for your instruction/account layouts so the implementation page can stay focused on logic.
  • Keep the entrypoint thin: validate, dispatch, return.