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Pack

Using #pragma pack can reduce the overall size of a structure or ensure precise alignment for specific cases, such as network protocols or hardware interfaces.

main


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#include <stdio.h>
#include <stdint.h>

// Without using `#pragma pack`
struct Packet {
    uint8_t  flag;      // 8 bits or 1 bytes
    uint16_t length;   // 16 bits or 2 bytes
    uint8_t  type;    // 8 bits or 1 bytes
};

int main() {
    printf("Size without packing: %zu bytes\n", sizeof(struct Packet));
    return 0;
}

As you may recall, structure alignment is determined by the largest field within the structure.

Using #pragma pack

main


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#include <stdio.h>
#include <stdint.h>

#pragma pack(push, 1)
struct PacketPacked {
    uint8_t flag;       // 8 bits = 1 bytes
    uint16_t length;    // 16 bits == 2 bytes
    uint8_t type;       // 8 bits == 1 bytes
};
#pragma pack(pop)

int main() {
    printf("Size with packing: %zu bytes\n", sizeof(struct PacketPacked));
    return 0;
}

The directive #pragma pack(push, 1) creates a local alignment context for a structure.

You start a new alignment scope with push, apply a specific rule (here, 1-byte alignment), define the structure, then restore the previous alignment with pop.

Using push with 1 ensures that all fields are aligned on 1-byte boundaries, with no padding between them.

Uppgift

Swipe to start coding

You're working with a digital sensor that sends binary data packets over UART. Each packet follows a fixed 4-byte layout. To match this exact layout with no padding, use the #pragma pack directive to enforce 1-byte alignment.

Bytes	Field	Type
1	Command code	`uint8_t`
2	Sensor value	`uint16_t`
1	Checksum	`uint8_t`

Fill in the first ___ to start the packed context with 1-byte alignment.
Fill in the second ___ to restore default alignment after the struct.
Use the correct field access expressions (e.g. packet.cmd) in each printf to output: the command byte, the 2-byte value and the checksum byte.

Lösning

Byt till skrivbordet för praktisk övningFortsätt där du är med ett av alternativen nedan

Var allt tydligt?

Tack för dina kommentarer!

Avsnitt 4. Kapitel 2

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Pack

Using #pragma pack can reduce the overall size of a structure or ensure precise alignment for specific cases, such as network protocols or hardware interfaces.

main


              1234567891011121314
            
#include <stdio.h>
#include <stdint.h>

// Without using `#pragma pack`
struct Packet {
    uint8_t  flag;      // 8 bits or 1 bytes
    uint16_t length;   // 16 bits or 2 bytes
    uint8_t  type;    // 8 bits or 1 bytes
};

int main() {
    printf("Size without packing: %zu bytes\n", sizeof(struct Packet));
    return 0;
}

As you may recall, structure alignment is determined by the largest field within the structure.

Using #pragma pack

main


              123456789101112131415
            
#include <stdio.h>
#include <stdint.h>

#pragma pack(push, 1)
struct PacketPacked {
    uint8_t flag;       // 8 bits = 1 bytes
    uint16_t length;    // 16 bits == 2 bytes
    uint8_t type;       // 8 bits == 1 bytes
};
#pragma pack(pop)

int main() {
    printf("Size with packing: %zu bytes\n", sizeof(struct PacketPacked));
    return 0;
}

The directive #pragma pack(push, 1) creates a local alignment context for a structure.

You start a new alignment scope with push, apply a specific rule (here, 1-byte alignment), define the structure, then restore the previous alignment with pop.

Using push with 1 ensures that all fields are aligned on 1-byte boundaries, with no padding between them.

Uppgift

Swipe to start coding

Bytes	Field	Type
1	Command code	`uint8_t`
2	Sensor value	`uint16_t`
1	Checksum	`uint8_t`

Fill in the first ___ to start the packed context with 1-byte alignment.
Fill in the second ___ to restore default alignment after the struct.
Use the correct field access expressions (e.g. packet.cmd) in each printf to output: the command byte, the 2-byte value and the checksum byte.

Lösning

Byt till skrivbordet för praktisk övningFortsätt där du är med ett av alternativen nedan

Var allt tydligt?

Tack för dina kommentarer!

Avsnitt 4. Kapitel 2

Byt till skrivbordet för praktisk övningFortsätt där du är med ett av alternativen nedan