CODEWITHSUNDEEP
clinuxsystem-calls
Álvaro Afonso López
Álvaro Afonso López

Reputation: 3

read() reads characters that do not exist in the file

The system call read() takes characters from a file and it also reads characters that don't exist at the end of each buffer.

The file has string: "AAOOOOOBRRRRRRIIIIIGGGGGGIIIIIINNNNNAAAALLLLLEEEEXXZZZSS"

When it reads, the buffer contains: "AAOOOOOBRRRRRRIIIIIGGGGGGIIIIIINNNNNAAAALLLLLEEEEXXZZZSS??Bf?"

As you see, the last four characters don't exist in the file

My code:

void trOpcionS(int src, int dst, char *cadena)
{
    //BUFFER DE LECTURA
    char buff[100];

    //BUFFER DE ESCRITURA TRAS ANALIZAR EL DE LECTURA
    char buffRes[100];

    //bytes leidos
    ssize_t r = 0;

    //bucle de lectura
    while ((r = read(src, buff, 100)) > 0)
    {
        char *ptrBuf = buff;
        char *ptrBufRes = buffRes;
        //bucle para analizar la lectura
        while (*ptrBuf != '\0')
        { 
            //BUCLE QUE RECORRE EL BUFFER
            int pos = 0;
            while (*(cadena + pos) != '\0')
            { 
                //BUCLE QUE RECORRE LA CADENA A TRANSFORMAR
                if (*(cadena + pos) == *ptrBuf)
                { 
                    //SI ENCUENTRO UNA EQUIVALENCIA, SE ESCRIBE Y SE SALTAN TODAS SUS REPETICIONES
                    *ptrBufRes = *ptrBuf;
                    while (*(ptrBuf + 1) == *ptrBufRes)
                    {
                        ptrBuf++;
                    }
                    ptrBufRes++;
                    break;
                }
                pos++;
            }
            //SI EL VALOR NO SE ENCUENTRA EN LA CADENA SE ESCRIBE SIN MÁS
            if (pos == strlen(cadena))
            {
                *ptrBufRes = *ptrBuf;
                ptrBufRes++;
            }
            ptrBuf++;
        }
        *ptrBufRes = '\0';

        printf("Reading: %s\n", buff);
        printf("%s\n", buffRes);
        ssize_t w = write(dst, buffRes, strlen(buffRes));
    }
}

Upvotes: 0

Views: 122

Answers (2)

user3629249
user3629249

Reputation: 16540

this statement sequence:

while ((r = read(src, buff, 100)) > 0)
{

needs to be followed by:

    buf[ r ] = '\0';

so as to NUL terminate the read string because the function: read() does not do that.

Upvotes: 0

bruno
bruno

Reputation: 32596

while((r = read(src, buff, 100)) > 0){
    char* ptrBuf = buff;
    char* ptrBufRes = buffRes;
    //bucle para analizar la lectura
    while(*ptrBuf != '\0'){ //BUCLE QUE RECORRE EL BUFFER

after you read the buffer you missed to add the final null character the second while expect, so that one continues up to find a null character you never set, of course this is an undefined behavior.

You can add the null character at buff[r] but that supposes you read up to 99 bytes rather than 100 or buff needs to be sized 101, else just replace the second while by

while (ptrBuf != (buff + 100)) {

Warning there is also a problem in the loop 

while(*(ptrBuf + 1) == *ptrBufRes)

where you can go out of the read bytes / out of buff

Upvotes: 1

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