How to convert string permission to mode_t type

Question

I am writing a program which changes file permissions. The permissions will be passed through a command line argument as a char * and then converted to mode_t type and given to chmod().

How could I change a string permission/mode to mode_t?

For example given a string 4777 aka rwsrwxrwx how could 4777 be changed to mode_t?

Answer 1

You can parse a string to extract an octal value by calling strtol and providing 8 as the base:

int mode = strtol(str, NULL, 8);

To be safe, you might want to do some additional sanity checks:

// Extract file mode from a string, with sanity checks.
// Returns non-zero on success. 
int string_to_filemode(const char* str, mode_t* mode)
{
    char* end = NULL;
    *mode = (mode_t)strtol(str, &end, 8);
    if (!end) return 0;
    while(isspace(*end)) end++;
    return *end == '\0' && (unsigned)*mode < 010000;
}

Here is an example using this:

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>

#include <sys/stat.h>

int string_to_filemode(const char* str, mode_t* mode)
{
    char* end = NULL;
    *mode = (mode_t)strtol(str, &end, 8);
    if (!end) return 0;
    while(isspace(*end)) end++;
    return *end == '\0' && (unsigned)*mode < 010000;
}

int main(int argc, char *argv[])
{
    for (int i = 1; i < argc; i++)
    {
        mode_t mode;
        if (string_to_filemode(argv[i], &mode))
            printf("Mode: str=%s val=%04o\n", argv[i], mode);
        else
            printf("Invalid mode: %s\n", argv[i]);
    }
    return 0;
}

Output, when invoked with command-line arguments 4777 644 1000 11232 91 abc -1:

Mode: str=4777 val=4777
Mode: str=644 val=0644
Mode: str=1000 val=1000
Invalid mode: 11232
Invalid mode: 91
Invalid mode: abc
Invalid mode: -1

Live demo: https://godbolt.org/z/4nhW9G9jM