File I/O function for C

Question

char *loadTextFile(const char *filename)
{
    FILE *fileh;
    char *text = 0;
    long filelength;
    if((fileh=fopen(filename,"rb"))== 0)
        printf("loadTextFile() - could not open file");
    else
    {
        fseek(fileh, 0, SEEK_END);
        filelength = ftell(fileh);
        rewind(fileh);
        text=(char *) smartmalloc((int)filelength + 1);
        fread(text,(int)filelength, 1, fileh);
        fclose(fileh);
        text[filelength]=0;
    }
    printf(text);
    return(text);
}

This function only returns partial data of a txt file. It is also inconsistent...soemtimes gives me 100 characters of the file some times 20. I don't see anything wrong with it. Thought I might get another pair of eyes on it. Thanks.

Answer 1

Obvious things to check:

What did ftell(fileh) give you?

Can there be embedded NUL characters in the file? That would cause printf(text) to stop prematurely.

Answer 2

Here is a slightly better version of your code. You need more error checking with the IO function calls. Also, there is the annoying long to size_t implicit conversions which I would recommend dealing with properly in production code.

char* loadTextFile(const char *filename) {
    char *text;
    long length;

    FILE *fileh = fopen(filename, "rb");
    if ( !fileh ) {
        return NULL;
    }

    fseek(fileh, 0, SEEK_END);
    length = ftell(fileh);
    rewind(fileh);

    text = malloc(length + 1);

    if ( !text ) {
        return NULL;
    }

    fread(text, 1, length, fileh);
    text[length] = 0;

    fclose(fileh);

    return text;
}

Note that, John R. Strohm is right: If your assessment of what has been read is based on what printf prints, then you are likely being misled by embedded nuls.

Answer 3

fread is not guaranteed to return as many characters as you ask for. You need to check its return value and use it in a loop.

Example loop (not tested):

char *p = text;
do {
    size_t n = fread(p,1,(size_t)filelength, fileh);
    if (n == 0) {
       *p = '\0';
       break;
    }
    filelength -= n;
    p += n;
} while (filelength > 0);

The test for n==0 catches the case where some other process truncates the file as you are trying to read it.