Question

Valgrind throws me out this error:

==11204== Syscall param write(buf) points to uninitialised byte(s)
==11204==    at 0x4109033: write (in /lib/libc-2.13.so)
==11204==    by 0x8049654: main (mmboxman.c:289)
==11204==  Address 0xbe92f861 is on thread 1's stack
==11204== 

What’s the problem? I can’t find what uninitialised byte it is yelling about.
Here are the criminal lines of code (the mentioned 289 line is the one which calls the function lockUp):

Request request;            
Response response;              

fillRequest(&request, MANADDUSER, getpid(), argument1, NULL, NULL, 0, 0);
lockUp(&request, &response, NULL);

Here the functions prototype and structs declaration:

void fillRequest(Request *request, char code, pid_t pid, char *name1, char *name2, char   *object, int id, size_t size)
{
    int k;

    request->code = code;
    request->pid = getpid();

    if(name1)    for(k=0; k<strlen(name1)+1; k++)   request->name1[k] = name1[k];
    else         request->name1[0] = '\0';

    if(name2)    for(k=0; k<strlen(name2)+1; k++)   request->name2[k] = name2[k];
    else         request->name2[0] = '\0';  

    if(object)   for(k=0; k<strlen(name2)+1; k++)   request->name2[k] = name2[k];
    else         request->object[0] = '\0'; 

    request->id    = id;
    request->size = size;
}

void lockUp(Request *request, Response *response, void **buffer)
{
    int fifofrom, fifoto, lock;     /* file descriptor delle fifo e del lock */

    /* locko per l'accesso alle FIFO */
    if((lock = open(LOCK, O_RDONLY)) == -1)   logMmboxman("error in opening LOCK\n", 1);
    else                                      logMmboxman("opened LOCK\n", 0);

    if(flock(lock, LOCK_EX) == -1)            logMmboxman("error in acquiring LOCK\n", 1);              
    else                                              logMmboxman("acquired LOCK\n", 0);  

    /* apro la fifoto e scrivo la mia richiesta */
    if((fifoto = open(FIFOTOMMBOXD, O_WRONLY)) == -1)   logMmboxman("error in opening FIFOTO\n", 1); 
    else                                                logMmboxman("opened FIFOTO\n", 0);  

    if((write(fifoto, request, sizeof(Request))) != sizeof(Request))   logMmboxman("error in writing FIFOTO\n", 1);
    else                                                               logMmboxman("written on FIFOTO\n", 0);
    close(fifoto);

    /* rimango in attesa della risposta da mmboxd sulla fifofrom */
    if((fifofrom = open(FIFOFROMMMBOXD, O_RDONLY)) == -1)   logMmboxman("error in opening FIFOFROM\n", 1);
    else                                                    logMmboxman("opened FIFOFROM\n", 0);

    if((read(fifofrom, response, sizeof(Response))) != sizeof(Response))   logMmboxman("error in reading FIFOFROM\n", 1);
    else                                                                   logMmboxman("read from FIFOFROM\n", 0);
    close(fifofrom);

    /* se mi deve comunicare un buffer riapro la fifo e lo leggo */
    if(response->size)
    {
            if((fifofrom = open(FIFOFROMMMBOXD, O_RDONLY)) == -1)   logMmboxman("error in opening FIFOFROM again for the buffer\n", 1);
            else                                                    logMmboxman("opened FIFOFROM again for the buffer\n", 0);

            *buffer = (void*)malloc(response->size);

            if(read(fifofrom, *buffer, response->size) != response->size)   logMmboxman("error in reading FIFOFROM again for the buffer\n", 1);
            else                                                            logMmboxman("read from FIFOFROM again for the buffer\n", 0);
            close(fifofrom);    
    }

    /* letta la risposta rilascio il lock */
    if(flock(lock, LOCK_UN) == -1)            logMmboxman("error in releasing LOCK\n", 1);              
    else                                      logMmboxman("released LOCK\n", 0);  

    return;
}

typedef struct 
{
    char code;          
    pid_t pid;          
    char name1[41];     
    char name2[41];     
    char object[101];   
    int id;             
    size_t size;        
} Request;

typedef struct 
{
    char result;    
    int num;        
    int num2;   
    size_t size;    
} Response;

Answer 1

Your Request structure has arrays name1, name2, etc. which contain null-terminated strings. When you fill them, you don’t write past the null terminator. Later when you write the structure to the file, valgrind complains because these bytes are uninitialized. There may also be other uninitialized bytes (for example, padding inserted by the compiler).

This is not necessarily a problem, other than a small security issue: The previous contents of memory, which may hold sensitive information, will get written to the file.

You can memset the structure to 0 before filling its fields to avoid this error.