CODEWITHSUNDEEP
cunixsystem
user2252999
user2252999

Reputation: 183

strlen in c returning a negative result

Program snippet :-

if((fd = open(buf_serv, O_RDONLY)) < 0) {
    char c[MAXLEN+1];
    snprintf(c, sizeof(c), ": can't open, %s\n", strerror(errno));
    n = strlen(c);

...

gdb :-
102     if((fd = open(buf_serv, O_RDONLY)) < 0) {
(gdb) n
104        snprintf(c, sizeof(c), ": can't open, %s\n", strerror(errno));
(gdb) p sizeof(c)
$1 = 4097
(gdb) n
105        n = strlen(c);
(gdb) p strlen(c)
$2 = -1428323296
(gdb) n
...

Can anyone please tell me how come strlen is returning a negative number? (which is crashing my program with a sigsegv signal)

Thanks in advance all!

(gdb) l
101
102     if((fd = open(buf_serv, O_RDONLY)) < 0) {
103        char ch[MAXLEN+1];
104        write(STDOUT_FILENO, "foo", 3);
105        sprintf(ch, ": can't open, %s\n", strerror(errno));
106        n = strlen(ch);
107        write(writefd, ch, 100);
108     } else {
109        while((n = read(fd, buf_serv, MAXLEN)) > 0) {
110             write(writefd, buf_serv, n);
(gdb) p ch
$1 = ": can't open, No such file or directory\n", '\000' <repeats 4056 times>
(gdb) p strlen(ch)
$2 = -1428323296

Upvotes: 2

Views: 4220

Answers (3)

aisbaa
aisbaa

Reputation: 10633

You dont need to use strlen since snprintf returns number of characters printed, if buffer was large enough to fit all characters including terminating \0. Source provided by @Casey in comments (probably).

So make sure to check if returned value is not negative and is less then buffer size. Also check function documentation provided by your compiler, since implementations may vary.

Upvotes: -1

Bhavesh
Bhavesh

Reputation: 1

strlen() returns the difference between strings on some compilers. so if for example strlen("a","b") is there then this will return 1; while strlen("b"... Note: original answer truncated

Upvotes: -3

user529758
user529758

Reputation:

It's just that you can't use GDB :P

(gdb) n
105        n = strlen(c);
(gdb) p strlen(c)

GDB shows the next line to be executed. So at the time you are printing n, it's yet uninitialized. Write n <enter> one more time before printing n.

Upvotes: 8

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