Lære Pack | Compiler Directives and Advanced Control

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.c


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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.c


              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.

Oppgave

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.

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Suggested prompts:

What is the syntax for using #pragma pack with push and pop?

Can you explain when to use #pragma pack(push, 1) in real-world scenarios?

Are there any drawbacks or risks to using 1-byte alignment with #pragma pack?

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Sveip for å vise menyen

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.c


              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.c


              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.

Oppgave

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.

Løsning

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Alt var klart?

Takk for tilbakemeldingene dine!

Seksjon 4. Kapittel 2

single