如何从函数中获取指针的指针

Question

I have a function which open a file, read its content line by line and then push it to an array. I have managed to get the array functionnal inside the right function, but when I want to get it back to my main function, I cannot get any items of my array.

Some code will help you to understand:

My main function:

/* ----------------- MAIN ------------ */
int main(int argc, char *argv[]) {

    /*... some useless code for now ... */

    char **ptrLines = NULL;
    ptrLines = readEventFile(ptrParam, ptrLines);
    outputExecTrace(WAR, "PTRLINES x : %x", ptrLines);
    outputExecTrace(WAR, "PTRLINES char[] : %s", *(ptrLines + 2));

    return EXIT_SUCCESS;
}

My fileReader function:

char** readEventFile(Parameters *parameters, char **arrLine) {

    FILE *fp = fopen(parameters->inputFilePath, "r");
    if (fp == NULL)
        exit(0);

    char line[128];
    int nbCharOfLine = 0;
    while(1) {
        fgets(line, sizeof(line), fp);
        if (feof(fp))
            break;

        nbCharOfLine++;
    }

    fclose(fp);
    arrLine = malloc(sizeof(line) * nbCharOfLine);
    nbCharOfLine = 0;
    fp = fopen(parameters->inputFilePath, "r");
    while(1) {
        fgets(line, sizeof(line), fp);
        if (line[0] != '#') {
            arrLine[nbCharOfLine] = malloc((strlen(line)+1) * sizeof(char));
            strcpy(arrLine[nbCharOfLine], line);
            nbCharOfLine++;
        }
        if (feof(fp))
            break;
    }

    fclose(fp);
    outputExecTrace(WAR, "ARRLINE x : %x", arrLine);
    outputExecTrace(WAR, "ARRLINE char[] : %s", *(arrLine + 2));

    return arrLine;
}

Has it is, my outputs are the followings:

WARNING: ARRLINE int : -2020552688
WARNING: ARRLINE char[] : 1 3 4 //This is the result I am looking for.

WARNING: PTRLINES int : -2020552688 // Same as ARRLINE
Segmentation fault (core dumped) // And this is because ptrLines[2] doesn't contains anything... but why ?!

How can I fix this ?

Answer 1

It's hard to tell exactly what happen, because you have several small problems within code, which could lead to this, try to change your code in following way:

char** readEventFile(char* fileName) {
    char line[128];
    int nbCharOfLine = 0;

    FILE *fp = fopen(fileName, "r");
    if (fp == NULL)
        exit(1);
    while (!feof(fp)) {
        char *r = fgets(line, sizeof(line), fp); // NOTE: proper handling of fgets return value
        if ((r != NULL) && (line[0] != '#'))
            nbCharOfLine++;
    }
    fclose(fp);

    char **arrLine = calloc(nbCharOfLine, sizeof(char*)); // use calloc to be sure
    nbCharOfLine = 0;

    fp = fopen(fileName, "r");
    while (!feof(fp)) {
        char *r = fgets(line, sizeof(line), fp);
        if ((r != NULL) && (line[0] != '#')) {
            arrLine[nbCharOfLine] = calloc(strlen(line) + 1, sizeof(char));
            strcpy(arrLine[nbCharOfLine++], line);
        }
    }
    fclose(fp);

    return arrLine;
}

in your main, you will call this function like:

char **ptrLines = readEventFile(ptrParam->inputFilePath);

Answer 2

Not so sure this is the whole problem, but the posted code:

while(1) {
    fgets(line, sizeof(line), fp);
    if (line[0] != '#') {
        arrLine[nbCharOfLine] = malloc((strlen(line)+1) * sizeof(char));
        strcpy(arrLine[nbCharOfLine], line);
        nbCharOfLine++;
    }
    if (feof(fp))
        break;
}

is not the right approach for several reasons, including that function feof() is only asserted 'true' when the code has tried to read past the end of the file.

suggest:

while( fgets(line, sizeof(line), fp) )
{
    if (line[0] != '#') 
    {
        if( NULL == (arrLine[nbCharOfLine] = malloc((strlen(line)+1) * sizeof(char)) ) )
        { // then, malloc failed
            perror( "malloc failed" );
            // close file and free all malloc'd areas
            exit( EXIT_FAILURE );
        }

        // implied else, malloc successful

        strcpy(arrLine[nbCharOfLine], line);
        nbCharOfLine++;
    }
}