如何在 C 中构造和返回一个字符串？ [复制]

Question

我正在尝试用 C 构造并返回一个字符串，但遇到了function returns address of local variable [-Wreturn-local-addr] 编译器警告。我知道返回packet 就像我尝试做的那样不起作用，因为数据包是指向我的大小为packet_size 的字符开头的指针，并且该内存地址在我的函数之外无效。我在 AVR 芯片上运行，不想使用malloc。我该如何解决这个问题？

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

const char* construct_packet(const char* data);
void append(char* str, char c);

int main()
{
    const char* my_packet = construct_packet("QERT");
    printf("%s", my_packet);

    return 0;
}

const char* construct_packet(const char* data)
{
    const char training_chars[] = "___";
    const char start_char = '>';
    uint8_t checksum = 0;
    for(uint16_t i = 0; i < strlen(data); i++) {
        checksum += data[i];
    }
    const char checksum_char = checksum;

    uint8_t packet_size = strlen(training_chars) + strlen(data) + 2; // Plus 2 for start byte and checksum byte
    char packet[packet_size];

    strcat(packet, training_chars);
    append(packet, start_char);
    strcat(packet, data);
    append(packet, checksum_char);

    return packet;
}

void append(char* str, char c)
{
    str[strlen(str) + 1] = c;
}

Answer 1

如果您不想使用动态内存分配，也不想使用static 缓冲区，那么您可以尝试计算并提供堆栈上的内存缓冲区：

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

const char* construct_packet(const char* data, const char* training_chars, const char start_char, char* packet);
void append(char* str, char c);

int main()
{
    const char data[] = "QERT";
    const char training_chars[] = "___";
    const char start_char = '>';
    const uint8_t packet_size = strlen(training_chars) + strlen(data) + 2; // Plus 2 for start byte and checksum byte
    char packet[packet_size];

    const char* my_packet = construct_packet(data, training_chars, start_char, packet);
    printf("%s", my_packet);  // same as printf("%s", packet);

    return 0;
}

const char* construct_packet(const char* data, const char* training_chars, const char start_char, char* packet)
{
    uint8_t checksum = 0;
    for(uint16_t i = 0; i < strlen(data); i++) {
        checksum += data[i];
    }
    const char checksum_char = checksum;

    strcat(packet, training_chars);
    append(packet, start_char);
    strcat(packet, data);
    append(packet, checksum_char);

    return packet;
}

void append(char* str, char c)
{
    str[strlen(str) + 1] = c;
}

Answer 2

Phil Brubaker：如果您不想使用动态内存分配，也不想使用静态缓冲区，那么您可以尝试计算并提供堆栈上的内存缓冲区：

我不会说更好，我的实现：

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

// in some header file
const char *construct_packet(char *packet, const char* data);
size_t size_packet(const char* data);

int main(void)
{
    const char *data = "QERT";
    size_t packet_size = size_packet(data);
    char packet[packet_size];
    construct_packet(packet, data);
    printf("%s", packet);
}

// can be on a other file of course `training_chars` and `start_char` could be give in parameter
// of course cause there are static `size_packet()` and `contruct_packet()` must be in the same file
static const char * const training_chars = "___";
static const char start_char = '>';

size_t size_packet(const char* data)
{
    // you forgot nul terminate byte
    return sizeof training_chars - 1 + 1 + strlen(data) + 1 + 1;
}

const char *construct_packet(char *packet, const char* data)
{
    size_t i = 0;

    // I replace strcat because I supose you want speed note they are better method that this one
    for (size_t j = 0; training_chars[j] != '\0'; j++) {
        packet[i++] = training_chars[j];
    }

    packet[i++] = start_char;

    // maybe this should be a char but you don't give information about checksum so I can't be sure
    uint8_t checksum = 0;
    for (size_t j = 0; data[j] != '\0'; j++) {
        packet[i++] = data[j];
        checksum += data[j];
    }
    packet[i++] = (char)checksum;

    // you forgot nul terminate byte
    packet[i] = '\0';

    return packet;
}

当然，你可以不计算大小并给出最大值：

char packet[256];
construct_packet(packet, data, 256);