libmodbus/docs/modbus_read_registers.md

modbus_read_registers

Name

modbus_read_registers - read many registers

Synopsis

int modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest);

Description

The modbus_read_registers() function shall read the content of the nb holding registers to the address addr of the remote device. The result of reading is stored in dest array as word values (16 bits).

You must take care to allocate enough memory to store the results in dest (at least nb * sizeof(uint16_t)).

The function uses the Modbus function code 0x03 (read holding registers).

Return value

The function shall return the number of read registers if successful. Otherwise it shall return -1 and set errno.

Errors

• EMBMDATA, too many registers requested.

Example

modbus_t *ctx;
uint16_t tab_reg[64];
int rc;
int i;

ctx = modbus_new_tcp("127.0.0.1", 1502);
if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
}

rc = modbus_read_registers(ctx, 0, 10, tab_reg);
if (rc == -1) {
    fprintf(stderr, "%s\n", modbus_strerror(errno));
    return -1;
}

for (i=0; i < rc; i++) {
    printf("reg[%d]=%d (0x%X)\n", i, tab_reg[i], tab_reg[i]);
}

modbus_close(ctx);
modbus_free(ctx);

See also

• modbus_write_register
• modbus_write_registers