队列算法的分割错误

Question

I made a small queue as shown below which is used to store binary data. queue_read() and queue_del() work on one side of the queue doing reading and deleting operations. queue_add() adds data to the other end of the queue. I am facing segmentation fault issue when I use this code in another code for storing data. But I couldnt find what is causing the segmentation issue even using gdb. It most of the time goes like malloc->memalign issue but I couldnt find any null pointer. Testing this code individually, I couldnt find any issues. It would be great it someone could point out if there is any issue with the implementation. Thanks

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

struct queue_node {
    struct queue_node *next;
    unsigned char data;
};

struct queue {
    struct queue_node *first;
    struct queue_node *last;
};

/* initialize the queue */
void init_queue(struct queue *q) {
    q->first = q->last = NULL;
}

/* check if the queue is empty or not */
int queue_empty_p(const struct queue *q) {
    return q->first == NULL;    
}

/* Reading 'len' bytes data from queue to buffer variable 'value' 
*
*  RETURNS bytes read which can be less than or equal to the requested 
*  value of 'len'
*/
int queue_read(struct queue *q, unsigned char *read_buffer, unsigned int len) {
    unsigned int i;
    struct queue_node *tmp = (struct queue_node *) malloc(
            sizeof(struct queue_node));

    tmp = q->first;
    for (i = 0; i < len; i++) {
        if (tmp == NULL) {
            read_buffer[i] = 0;
            return (i);
        }
        read_buffer[i] = tmp->data;
        tmp = tmp->next;
    }
    return len;
}

/* Deleting specified no of bytes from queue. Usually done after queue_read() */
int queue_del(struct queue *q, unsigned int no_of_bytes) {
    unsigned int i;

    for (i = 0; i < no_of_bytes; i++) {
        if (!q->first) {
            return 1;
        }
        struct queue_node *tmp = q->first;
        if (q->first == q->last)
            q->first = q->last = NULL;
        else
            q->first = q->first->next;

        free(tmp);
    }
    return 0;
}

/* Adding 'len' bytes of data contained in the variable 'value' to the queue */
int queue_add(struct queue *q, unsigned char *value, unsigned int len) {

    struct queue_node *node;
    unsigned int i;

    for (i = 0; i < len; i++) {
        node = (struct queue_node *) malloc(sizeof(struct queue_node));
        if (node == NULL) {
            return 1;
        }
        node->data = *(value + i);
        if (q->first == NULL)
            q->first = q->last = node;
        else {
            q->last->next = node;
            q->last = node;
        }
        node->next = NULL;
    }
    return 0;
}

int main() {
    struct queue left_queue;
    char *buffer = (char *)malloc(100);
    int read_bytes;

    strcpy(buffer, "This is a test");
    init_queue(&left_queue);

    queue_add(&left_queue, (unsigned char *) buffer, strlen(buffer));

    queue_del(&left_queue, 3);

    read_bytes = queue_read(&left_queue, (unsigned char *)buffer, 1500);
    buffer[read_bytes]=0;
    printf("Buffer: %s \nBytes read %d \n", (unsigned char *)buffer, read_bytes);

    return 0;
}

EDIT: gdb backtrace output

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb6ef3b70 (LWP 2678)]
0x0012f4df in memalign () from /usr/lib/libefence.so.0
#0  0x0012f4df in memalign () from /usr/lib/libefence.so.0
#1  0x0012f88b in malloc () from /usr/lib/libefence.so.0
#2  0x08049576 in queue_add (q=0xb7f21fc8, 
    value=0xb7f2da28 "GET /wiki/Special:Search?search=rohc&sourceid=Mozilla-search HTTP/1.1\r\nHost: en.wikipedia.org\r\nUser-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.13) Gecko/20101206 Ubuntu/10.04 (lucid) Fire"..., 
    len=440) at pep_core.h:248
#3  0x0804cc81 in fn_process_tcp () at pep_core.c:584
#4  0x0013b96e in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
#5  0x00228a4e in clone () from /lib/tls/i686/cmov/libc.so.6
#0  0x0012f4df in memalign () from /usr/lib/libefence.so.0
No symbol table info available.
#1  0x0012f88b in malloc () from /usr/lib/libefence.so.0
No symbol table info available.
#2  0x08049576 in queue_add (q=0xb7f21fc8, 
    value=0xb7f2da28 "GET /wiki/Special:Search?search=rohc&sourceid=Mozilla-search HTTP/1.1\r\nHost: en.wikipedia.org\r\nUser-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.13) Gecko/20101206 Ubuntu/10.04 (lucid) Fire"..., 
    len=440) at pep_core.h:248
        node = 0xb661bff8
        i = 245
#3  0x0804cc81 in fn_process_tcp () at pep_core.c:584
        ip_queue_table = 0xb7f2d9fc
        tmp_ip_queue_table = 0x0
        tcp_conn = 0xb7f21fa8
        tmp_tcp_conn = 0x0
        tcphdrs = 0xb7f2da14
        iphdrs = 0xb7f2da00
        packet_data = 0xb7f2da28 "GET /wiki/Special:Search?search=rohc&sourceid=Mozilla-search HTTP/1.1\r\nHost: en.wikipedia.org\r\nUser-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.13) Gecko/20101206 Ubuntu/10.04 (lucid) Fire"...
        read_buffer = 0xb7f17a24 ""
        read_bytes = 0
        payload_size = 440
        ret = 0
        rawfd = 9
        one = 1
#4  0x0013b96e in start_thread () from /lib/tls/i686/cmov/libpthread.so.0
No symbol table info available.
#5  0x00228a4e in clone () from /lib/tls/i686/cmov/libc.so.6
No symbol table info available.

Output of valgrind

I ran valgrind as you suggested and got the following

Syscall param socketcall.sendto(msg) points to uninitialised byte(s)

the above error is seen 4 times and then the below message

==3159== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==3159==  Bad permissions for mapped region at address 0x5AD2FFC
==3159==    at 0x804962F: queue_read (pep_core.h:279)
==3159==    by 0x804D672: fn_thread_pep_left_sctp_client (pep_core.c:856)
==3159==    by 0x403D96D: start_thread (pthread_create.c:300)
==3159==    by 0x412AA4D: clone (clone.S:130)

Answer 1

struct queue_node *tmp = (struct queue_node *) malloc(
        sizeof(struct queue_node));

tmp = q->first;

I think this is the problem, what happens is that you allocate a block of memory at a specific address, but then rather than using that memory you set the pointer to point to something else. Therefore a memory leak. Remember when you assign a value like that to a pointer, you don't copy the values rather your pointer now points to another place in memory.

Answer 2

Not sure if I understand your code, but it seems like queue_read works basically like read , that is, it reads into a buffer passed in.

char *buffer = (char *)malloc(100);
[...]
read_bytes = queue_read(&left_queue, (unsigned char *)buffer, 1500);

Are you simply reading too much data into too small a buffer?