Question

While working on a program which requires frequent memory allocation I came across behaviour I cannot explain. I’ve implemented a work around but I am curious to why my previous implementation didn’t work. Here’s the situation:

Memory reallocation of a pointer works

This may not be best practice (and if so please let me knwow) but I recall that realloc can allocate new memory if the pointer passed in is NULL. Below is an example where I read file data into a temporary buffer, then allocate appropriate size for *data and memcopy content

I have a file structure like so

typedef struct _my_file {
       int size;
       char *data;
}

And the mem reallocation and copy code like so:

// cycle through decompressed file until end is reached
while ((read_size = gzread(fh, buf, sizeof(buf))) != 0 && read_size != -1) {
        // allocate/reallocate memory to fit newly read buffer
        if ((tmp_data = realloc(file->data, sizeof(char *)*(file->size+read_size))) == (char *)NULL) {
                printf("Memory reallocation error for requested size %d.\n", file->size+read_size);
                // if memory was previous allocated but realloc failed this time, free memory!
                if (file->size > 0)
                        free(file->data);
                return FH_REALLOC_ERROR;
        }
        // update pointer to potentially new address (man realloc)
        file->data = tmp_data;
        // copy data from temporary buffer
        memcpy(file->data + file->size, buf, read_size);

        // update total read file size
        file->size += read_size;
}

Memory reallocation of pointer to pointer fails

However, here is where I’m confused. Using the same thought that reallocation of a NULL pointer will allocate new memory, I parse a string of arguments and for each argument I allocate a pointer to a pointer, then allocate a pointer that is pointed by that pointer to a pointer. Maybe code is easier to explain:

This is the structure:

typedef struct _arguments {
        unsigned short int options;    // options bitmap
        char **regexes;                // array of regexes
        unsigned int nregexes;         // number of regexes
        char *logmatch;                // log file match pattern
        unsigned int limit;            // log match limit
        char *argv0;                   // executable name
} arguments;

And the memory allocation code:

int i = 0;
int len;
char **tmp;
while (strcmp(argv[i+regindex], "-logs") != 0) {
        len = strlen(argv[i+regindex]);

        if((tmp = realloc(args->regexes, sizeof(char **)*(i+1))) == (char **)NULL) {
                printf("Cannot allocate memory for regex patterns array.\n");
                return -1;
        }
        args->regexes = tmp;
        tmp = NULL;

        if((args->regexes[i] = (char *)malloc(sizeof(char *)*(len+1))) == (char *)NULL) {
                printf("Cannot allocate memory for regex pattern.\n");
                return -1;
        }

        strcpy(args->regexes[i], argv[i+regindex]);

        i++;
}

When I compile and run this I get a run time error “realloc: invalid pointer “

I must be missing something obvious but after not accomplishing much trying to debug and searching for solutions online for 5 hours now, I just ran two loops, one counts the numbers of arguments and mallocs enough space for it, and the second loop allocates space for the arguments and strcpys it.

Any explanation to this behaviour is much appreciated! I really am curious to know why.

Answer 1

First fragment:

// cycle through decompressed file until end is reached
while (1) {
        char **tmp_data;
        read_size = gzread(fh, buf, sizeof buf);
        if (read_size <= 0) break;

        // allocate/reallocate memory to fit newly read buffer
        tmp_data = realloc(file->data, (file->size+read_size) * sizeof *tmp_data );
        if ( !tmp_data ) {
                printf("Memory reallocation error for requested size %d.\n"
                      , file->size+read_size);

                if (file->data) {
                        free(file->data)
                        file->data = NULL;
                        file->size = 0;
                }
                return FH_REALLOC_ERROR;
        }

        file->data = tmp_data;
        // copy data from temporary buffer
        memcpy(file->data + file->size, buf, read_size);

        // update total read file size
        file->size += read_size;
}

Second fragment:

unsigned i; // BTW this variable is already present as args->nregexes;

for(i =0; strcmp(argv[i+regindex], "-logs"); i++) {
        char **tmp;

        tmp = realloc(args->regexes, (i+1) * sizeof *tmp ); 
        if (!tmp) {
                printf("Cannot allocate memory for regex patterns array.\n");
                return -1;
        }
        args->regexes = tmp;

        args->regexes[i] = strdup( argv[i+regindex] ); 
        if ( !args->regexes[i] ) {
                printf("Cannot allocate memory for regex pattern.\n");
                return -1;
        }
...
return 0;
}

A few notes:

• the syntax ptr = malloc ( CNT * sizeof *ptr); is more robust than the sizeof(type) variant.
• strdup() does exactly the same as your malloc+strcpy()
• the for(;;) loop is less error prone than a while() loop with a loose i++; at the end of the loop body. (it also makes clear that the loopcondition is never checked)
• to me if ( !ptr ) {} is easyer to read than if (ptr != NULL) {}
• the casts are not needed and sometimes unwanted.