使用C的getline（）时出现奇怪的行为

Question

I have encountered some strange behavior in the getline function while writing a small c program. What I want to do:

1. Redirect stdout to a file
2. Redirect stderr to a file
3. Redirect stdin to a file
4. Read stdin line by line into an array of strings ( char *path_array )
5. Print the output

Now, when I run the program, the output looks like this:

retrieving line of size 73.
line1 llllllllllllskdjflaksdlfkalskdddddddffffffffffffffffffffffffffffff

retrieving line of size 6.
line2

retrieving line of size 6.
line3

retrieving line of size 6.
line4

retrieving line of size 6.
line5

retrieving line of size 6.
line6

retrieving line of size 6.
line7

retrieving line of size 6.
line8

retrieving line of size 1.


retrieving line of size 13.
sdkfjlskdfos

retrieving line of size 9.
sldjflsd

retrieving line of size 9.
sdlfkjsd

retrieving line of size 11.
2222222222

retrieving line of size 11.
3333333333




retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 



retrieving line from array: 

At the end of this output, below each retrieving line from array , there should be the corresponding string from the array. As you can see, the array is filled with empty strings.

Using Clion's debug mode, I found out why this is the case:

In the first for loop, every array-entry gets filled with the current line . So instead of

path_array[0] = line1

path_array[1] = line2

path_array[3] = line3 ...

it goes

path_array[0] = line1

path_array[0] = line2, path_array[1] = line2

path_array[0] = line3, path_array[1] = line3, path_array[2] = line3 ...

Why is this the case? And how do I stop this from happening?

Here is the C code:

#include <zconf.h>
#include <dirent.h>
#include <stdlib.h>
#include <fcntl.h>
#include <stdio.h>

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wmissing-noreturn"
int main(void) {

    int out_log;
    int err_log;
    int conf_in;

    pid_t pid = fork();

    DIR *dir;
    struct dirent *entry;

    //for getline()
    char *line = NULL;
    size_t len = 0;
    ssize_t read;

    int array_size = 0;

//    if (pid < 0) {
//        exit(1);
//    }
//    else if (pid > 0) {
//        exit(0);
//    }
//
//    if (setsid() <= 0) {
//        exit(1);
//    }

    if (chdir("/") != 0) {
        exit(1);
    }

//    if ((dir = opendir(".")) == NULL) {
//        exit(1);
//    }

    //redicrect stdout
    if (dup2(out_log = open("PATH_TO_OUT_FILE", O_WRONLY | O_TRUNC), 1) != 1) {
        exit(1);
    }
    if (close(out_log) != 0) {
        exit(1);
    }

    //redirect stderr
    if (dup2(err_log = open("PATH_TO_ERR_FILE", O_WRONLY | O_TRUNC), 2) != 2) {
        exit(1);
    }
    if (close(err_log) != 0) {
        exit(1);
    }

    //redirect stdin
    if (dup2(conf_in = open("PATH_TO_IN_FILE", O_RDONLY), 0) < 0) {
        exit(1);
    }
    if (close(conf_in) != 0) {
        exit(1);
    }

    while ((read = getline(&line, &len, stdin)) != -1) {
        array_size++;

        printf("retrieving line of size %zd.\n", read);

        printf("%s\n", line);
    }

    rewind(stdin);

    char *path_array[array_size];

    for (int i = 0; i < array_size; i++) {
        getline(&line, &len, stdin);

        // HERE IS WHERE IT GOES WRONG
        path_array[i] = line;
    }

    free(line);

    for (int i = 0; i < array_size; i++) {
        printf("\n\n\nretrieving line from array: %s\n", path_array[i]);
    }

//    while (1) {
//        puts("test output");
//        printf("%zd\n", read);
//
//        fflush(stdout);
//
//        sleep(1);
//    }
}
#pragma clang diagnostic pop

Answer 1

On the first call line is null so a new buffer is allocated. After that, line is not null so the same buffer is used. Your array is full of pointers to the same buffer.