C程序-尝试从文件中读取时出现“分段错误”

Question

Please bear with me since I'm still new in programming. I'm trying to read a file and store its context as a variable, here's my code I'm sorry if it's rather long:

#define _BSD_SOURCE
#include <stdio.h>
#include <sys/stat.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>

// CEK ROUTER MODEL
char* router_model;
char* model() {
    char filename[] = "/proc/cpuinfo";
    char* key = "system type";
    char* value;
    FILE *file = fopen(filename, "r");

    if (file != NULL) {
        char line[1000];

        while (fgets(line, sizeof line, file) != NULL) /* read a line from a file */ {
            //fprintf(stdout, "%s", line); //print the file contents on stdout.
            if (strncmp(line, key, strlen(key)) == 0) {
                char* value = strchr(line, ':');
                value += 2;
                router_model = strdup(value);
                break;   // once the key has been found we can stop reading
            }
        }
        fclose(file);
    }
    else {
        perror(filename); //print the error message on stderr.
    }
    return router_model;
}

// TULIS SERIAL NUMBER KE FILE
char tulis(char p[100]) {
    // Write a serial number to a file
    char sn[30];
    char encrypt_sn[50];
    printf("Serial Number:\n");
    scanf("%s", sn);
    FILE *f = fopen("/usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c", "w");
    if (f == NULL) {
        printf("Error opening file!\n");
        exit(1);
    }
    fprintf(f,"Serial Number: %s", sn);
    fclose(f);
    sprintf(encrypt_sn, "ccrypt -e /usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c -K %s", p);
    system(encrypt_sn);
    system("mv /usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c.cpt /usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c");
    printf("Serial number is saved in /usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c\n");
    return 0;
}

// BACA SERIAL NUMBER & SIMPAN DALAM SEBUAH VARIABLE
char baca(char p[100]) {
    // Store the serial number from a file in a variable
    char line[50];
    char decrypt_sn[50];
    char key[30] = "Serial Number";
    char *serial_number;
    if( access( "/usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c", F_OK ) != -1 ) {
        system("cp /usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c /tmp/");
        system("mv /tmp/fsn-55cfc8770b69cc07268fae7f25ee444c /tmp/fsn-55cfc8770b69cc07268fae7f25ee444c.cpt");
        sprintf(decrypt_sn, "ccrypt -d /tmp/fsn-55cfc8770b69cc07268fae7f25ee444c.cpt -K %s", p);
        system(decrypt_sn);
        FILE *file = fopen("/tmp/fsn-55cfc8770b69cc07268fae7f25ee444c", "r");
        if (file == NULL) {
            printf("Error opening file!\n");
            exit(1);
        }
        while (fgets(line, sizeof line, file) != NULL) /* read a line from a file */ {
            //fprintf(stdout, "%s", line); //print the file contents on stdout.
            if (strncmp(line, key, strlen(key)) == 0) {
                char* value = strchr(line, ':');
                value += 2;
                serial_number = strdup(value);
                break;   // once the key has been found we can stop reading
            }
        }
        fclose(file);
        //printf("Your hardware serial number is: (%s)\n", serial_number);
        remove("/tmp/fsn-55cfc8770b69cc07268fae7f25ee444c");
    }
    else {
        printf("fsn not found\n");
        return -1;
    }
    return 0;
}

int main(int argc, char* argv[]) {
    char *r;
    char *del;
    char *decrypt;
    int ret;
    char input[30];
    char *p;
    char *original_sn;
    p = "MmI4MTUxM2FjMjRlMDkzYmRkZGQyMjcwMjQ4OWY3MDAwNGZiYTM0MWNkZGIxNTdlYzAxN2";
    //tulis(p);
    original_sn = baca(p);
    printf("SN: %s", original_sn);
    return 0;
}

The file I'm trying to read is /usr/share/terminfo/f/fsn-55cfc8770b69cc07268fae7f25ee444c and after being decrypted with ccrypt , the content of that file is Serial Number: 1866203214226041 . I want to store 1866203214226041 as a variable but I'm getting Segmentation Fault when running that code, how do I fix it ?

Answer 1

Just look at this snippet extracted from your code:

    while (fgets(line, sizeof line, file) != NULL) /* read a line from a file */ {
        //fprintf(stdout, "%s", line); //print the file contents on stdout.
        if (strncmp(line, key, strlen(key)) == 0) {
            char* value = strchr(line, ':');
            value += 2;
            router_model = strdup(value);
            break;   // once the key has been found we can stop reading
        }
    }

First of all, you don't check the value returned from strchr(line, ':'); as you will get NULL in case the string begins with system type but doesn't have a ':' char on it. You assume it will always find a ':' and increment the value by two (if it doesn't find the char, it will generate a pointer into position 2 of page zero of mem (which normally implies a SIGSEGV signal on later access from strdup(3) call). Also, after incrementing by 2 , you can just skip the last \\0 character, if the ':' character happens to be the last of the line.

Note

A final comment. As it is true that having key initialized at top makes the while loop more efficient, calling strlen(key) inside the loop makes the code to find the end of the string pointed to by key at each loop iteration, to get the string length. Better to precalculate strlen(key) outside the loop (as it is constant) and use that value on the if statement, like this:

    size_t key_length = strlen(key);
    while (fgets(line, sizeof line, file) != NULL) /* read a line from a file */ {
        //fprintf(stdout, "%s", line); //print the file contents on stdout.
        if (strncmp(line, key, key_length) == 0) {
            char* value = strchr(line, ':');
            if (value) { // only if value is != NULL should we continue
                value += 1; // we are not sure if after ':' we'll have a null char `\0` so better do a +1 increment.
                router_model = strdup(value);
                break;   // once the key has been found we can stop reading
            }
        }
    }

Another thing is: Don't use a so short string array for encrypt_sn (only 50 chars space for a max 49 length null terminated string) when you are printing that so large strings (I think only the format is already longer than that [edit] around 75 chars [/edit]) as you are running in a buffer overflow with your own code. This can be another source for SIGSEGV .

No more checks have been done to your code, but ther can be more. I think the same problem happens with decrypt_sn later on.

One more question: Why do you tag your source as _BSD_SOURCE if it is not bsd source anymore? (no procfs in bsd systems, no /proc/cpuinfo in bsd systems) I've tried to check my debian amd64 system's /proc/cpuinfo file and no system type field found there.