在循环中发送数据会导致malloc内存损坏

Question

I'm working on an assignment on socket networking, and have a client and a server that connect and communicate fine, as long as I only send requests for one piece of data at the time. Once I send a request for for example 5 pieces of data, the server crashes due to malloc memory corruption.

I have:

int number_of_jobs;
msg[0] = '0';

read(sd, &number_of_jobs, 4);

for(int i = 0; i < number_of_jobs; i++){
    printf("Total bytes read: %lu\n", total_bytes_read);
    printf("Size: %lu\n", size);
    printf("i: %d\n", i);
    char jobtype;
    unsigned int len;
    fread(&jobtype, sizeof(jobtype), 1, fp);
    DEBUG_PRINT(("\n>>%d<< Jobtype: %c\n", getpid(), jobtype));

    fread(&len, 4, 1, fp);
    DEBUG_PRINT((">>%d<< Len: %d\n", getpid(), len));

    header = make_header(jobtype, len, size);

    read_line = malloc(sizeof(char)*len + 1);
    int n = fread(read_line, sizeof(char), len, fp);
    if(n <= 0){
        printf(">>%d<< ", getpid());
        perror("Fread error");
    }
    *(read_line + strlen(read_line)-1) = '\0';
    total_bytes_read += strlen(read_line);

    package = malloc(sizeof(char)*40 + sizeof(char)* strlen(read_line+1));

    strncpy(package, header, 41);
    strcat(package, read_line);

    printf("%s\n", package);

    if(write(sd, package, strlen(package)) < 0){
        printf(">>%d<< ", getpid());
        perror("Write error");
    }
    printf("Writing %zu bytes\n", strlen(read_line));

    free(package);
    free(header);
    free(read_line);
}

I know it's something to do with overwriting memory (at least I think so?), but I don't know where or why.

The part that works, is essentially everything inside the for i < number_of_jobs part, which I can call as many times as I want.

Answer 1

What makes you think that read_line is null-terminated to call *(read_line + strlen(read_line)-1) = '\\0'; ? If you need to make it null-terminated you better write read_line[n] = '\\0' . Also there is no need to call strlen afterwards since you already know that that you've read n bytes.