I wrote a function to read the entire content of a file and return a pointer to an allocated bufer containing all the data.

Please could you tell me if there is a better way of doing this ? One solution could be to use stat() to pre-allocate a buffer and do only one read() call but I don't know if it's better, any though ?

#include <stdio.h>
#include <memory.h>

char *file_read(const char *path, size_t *file_size)
    FILE *file = fopen(path, "r");
    char buffer[FILE_READ_BUFFER_SIZE];
    char *tmp = NULL;
    char *data = NULL; // The buffer containing the data of the file
    char *data_cursor = NULL; // The current position in the data buffer
    size_t data_size = 0; // The size of the data buffer
    size_t n = 0; // The number of bytes read by fread()

    if (file == NULL)
        goto error;

    while ((n = fread(buffer, sizeof(char), sizeof(buffer), file)) > 0) {
        if (data == NULL) {
            data = malloc(sizeof(char) * n);
            data_cursor = data;
            data_size = n;
        } else {
            // We compute the position in the buffer as an integer
            // to be able to re-apply after realloc(). Because realloc()
            // can move the pointer we need this position as an integer.
            size_t cursor_pos = data_cursor - data;
            tmp = realloc(data, sizeof(char) * data_size + n);
            if (tmp == NULL)
                goto error;
            data = tmp;
            data_cursor = data + cursor_pos;
            data_size += n;
        memcpy(data_cursor, buffer, n);

    if (ferror(file))
        goto error;

    if (file_size != NULL)
        *file_size = data_size;
    return data;

    if (file != NULL)
    if (data != NULL)
    return NULL;
  • 1
    \$\begingroup\$ Code posted to codereview should compile, it would be great if you could make this a complete file especially could you post the code for error_set_from_errno_ptr() ? \$\endgroup\$ Commented Jan 24, 2018 at 13:45
  • \$\begingroup\$ It's just a utility function which stores internally the last errrno & strerror() and return NULL. I replaced it with return NULL in my example. Thanks ! \$\endgroup\$
    – Thibaut D.
    Commented Jan 24, 2018 at 14:48
  • \$\begingroup\$ Here you find how to find size of a file using fseek+ftell, and then allocating a buffer that exact size: stackoverflow.com/questions/5957845/… After that use rewind and fread. \$\endgroup\$
    – Andreas
    Commented Jan 24, 2018 at 18:46

1 Answer 1

  • I recommend to pass a file pointer rather than a path. This will enable the code to work with streams as well. It would also simplify the awkward error handling.

  • Modern allows, and recommends, to declare variables closer to their use.

  • It is OK to realloc even when data is NULL.

  • sizeof(char) is guaranteed to be 1.

  • Incrementing data_size after memcpy lets you drop data_cursor and cursor_pos, which IMHO obfuscate the code. You'd need to

        memcpy(data + data_size, buffer, n);
        data_size += n;
  • Allocating an array on the stack is usually not recommended. Consider reading directly into data + data_size, and get away without buffer and memcpy.

  • \$\begingroup\$ Thanks a lot for those comments, which totally make sense. \$\endgroup\$
    – Thibaut D.
    Commented Jan 24, 2018 at 20:10

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