Mixing
Weird return value in strcmp
Clash Royale CLAN TAG#URR8PPP
up vote
6
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While checking the return value of strcmp function, I found some strange behavior in gcc. Here's my code:
#include <stdio.h>
#include <string.h>
char str0 = "hello world!";
char str1 = "Hello world!";
int main()
printf("%dn", strcmp("hello world!", "Hello world!"));
printf("%dn", strcmp(str0, str1));
When I compile this with clang, both calls to strcmp return 32. However, when compiling with gcc, the first call returns 1, and the second call returns 32. I don't understand why the first and second calls to strcmp return different values when compiled using gcc.
Below is my test environment.
- Ubuntu 18.04 64bit
- gcc 7.3.0
- clang 6.0.0
c gcc clang
edited 47 mins ago
Eric Schnipke
13213
asked 1 hour ago
fips197
364
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
 |Â
show 1 more comment
up vote
6
down vote
favorite
While checking the return value of strcmp function, I found some strange behavior in gcc. Here's my code:
#include <stdio.h>
#include <string.h>
char str0 = "hello world!";
char str1 = "Hello world!";
int main()
printf("%dn", strcmp("hello world!", "Hello world!"));
printf("%dn", strcmp(str0, str1));
When I compile this with clang, both calls to strcmp return 32. However, when compiling with gcc, the first call returns 1, and the second call returns 32. I don't understand why the first and second calls to strcmp return different values when compiled using gcc.
Below is my test environment.
- Ubuntu 18.04 64bit
- gcc 7.3.0
- clang 6.0.0
c gcc clang
edited 47 mins ago
Eric Schnipke
13213
asked 1 hour ago
fips197
364
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
3
What's "weird" about this?
– melpomene
1 hour ago
3
The standard only specifies the sign of the result; it does not specify the magnitude. If the strings are equal, the result is zero; if the first string sorts before the second, the result is negative; if the second string sorts after the second, the result is positive. In your example, both 1 and 32 are positive; the results are equivalent as far as the standard is concerned, and your code should be written so that it makes no difference to you, either.
– Jonathan Leffler
1 hour ago
Incidentally, the difference between'h'and'H'is 32. Coincidence?
– Christian Gibbons
1 hour ago
1
Thanks for the replies, but i alredy checked man pages and ISO document so I know the exact return value is not specified. I just want to know why there's a difference between literal string and array string in GCC.
– fips197
1 hour ago
1
Likely duplicate of Inconsistent strcmp() return value when passing strings as pointers or as literals ... Tl;DR; both are valid you are seeing the effects of optimization
– Shafik Yaghmour
22 mins ago
 |Â
show 1 more comment
up vote
6
down vote
favorite
up vote
6
down vote
favorite
While checking the return value of strcmp function, I found some strange behavior in gcc. Here's my code:
#include <stdio.h>
#include <string.h>
char str0 = "hello world!";
char str1 = "Hello world!";
int main()
printf("%dn", strcmp("hello world!", "Hello world!"));
printf("%dn", strcmp(str0, str1));
When I compile this with clang, both calls to strcmp return 32. However, when compiling with gcc, the first call returns 1, and the second call returns 32. I don't understand why the first and second calls to strcmp return different values when compiled using gcc.
Below is my test environment.
- Ubuntu 18.04 64bit
- gcc 7.3.0
- clang 6.0.0
c gcc clang
edited 47 mins ago
Eric Schnipke
13213
asked 1 hour ago
fips197
364
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
While checking the return value of strcmp function, I found some strange behavior in gcc. Here's my code:
#include <stdio.h>
#include <string.h>
char str0 = "hello world!";
char str1 = "Hello world!";
int main()
printf("%dn", strcmp("hello world!", "Hello world!"));
printf("%dn", strcmp(str0, str1));
When I compile this with clang, both calls to strcmp return 32. However, when compiling with gcc, the first call returns 1, and the second call returns 32. I don't understand why the first and second calls to strcmp return different values when compiled using gcc.
Below is my test environment.
- Ubuntu 18.04 64bit
- gcc 7.3.0
- clang 6.0.0
c gcc clang
c gcc clang
edited 47 mins ago
Eric Schnipke
13213
asked 1 hour ago
fips197
364
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 47 mins ago
Eric Schnipke
13213
asked 1 hour ago
fips197
364
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
edited 47 mins ago
Eric Schnipke
13213
edited 47 mins ago
Eric Schnipke
13213
edited 47 mins ago
Eric Schnipke
13213
13213
asked 1 hour ago
fips197
364
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
asked 1 hour ago
fips197
364
asked 1 hour ago
fips197
364
364
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
fips197 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
3
What's "weird" about this?
– melpomene
1 hour ago
3
The standard only specifies the sign of the result; it does not specify the magnitude. If the strings are equal, the result is zero; if the first string sorts before the second, the result is negative; if the second string sorts after the second, the result is positive. In your example, both 1 and 32 are positive; the results are equivalent as far as the standard is concerned, and your code should be written so that it makes no difference to you, either.
– Jonathan Leffler
1 hour ago
Incidentally, the difference between'h'and'H'is 32. Coincidence?
– Christian Gibbons
1 hour ago
1
Thanks for the replies, but i alredy checked man pages and ISO document so I know the exact return value is not specified. I just want to know why there's a difference between literal string and array string in GCC.
– fips197
1 hour ago
1
Likely duplicate of Inconsistent strcmp() return value when passing strings as pointers or as literals ... Tl;DR; both are valid you are seeing the effects of optimization
– Shafik Yaghmour
22 mins ago
 |Â
show 1 more comment
3
What's "weird" about this?
– melpomene
1 hour ago
3
The standard only specifies the sign of the result; it does not specify the magnitude. If the strings are equal, the result is zero; if the first string sorts before the second, the result is negative; if the second string sorts after the second, the result is positive. In your example, both 1 and 32 are positive; the results are equivalent as far as the standard is concerned, and your code should be written so that it makes no difference to you, either.
– Jonathan Leffler
1 hour ago
Incidentally, the difference between'h'and'H'is 32. Coincidence?
– Christian Gibbons
1 hour ago
1
Thanks for the replies, but i alredy checked man pages and ISO document so I know the exact return value is not specified. I just want to know why there's a difference between literal string and array string in GCC.
– fips197
1 hour ago
1
Likely duplicate of Inconsistent strcmp() return value when passing strings as pointers or as literals ... Tl;DR; both are valid you are seeing the effects of optimization
– Shafik Yaghmour
22 mins ago
3
3
What's "weird" about this?
– melpomene
1 hour ago
What's "weird" about this?
– melpomene
1 hour ago
3
3
The standard only specifies the sign of the result; it does not specify the magnitude. If the strings are equal, the result is zero; if the first string sorts before the second, the result is negative; if the second string sorts after the second, the result is positive. In your example, both 1 and 32 are positive; the results are equivalent as far as the standard is concerned, and your code should be written so that it makes no difference to you, either.
– Jonathan Leffler
1 hour ago
The standard only specifies the sign of the result; it does not specify the magnitude. If the strings are equal, the result is zero; if the first string sorts before the second, the result is negative; if the second string sorts after the second, the result is positive. In your example, both 1 and 32 are positive; the results are equivalent as far as the standard is concerned, and your code should be written so that it makes no difference to you, either.
– Jonathan Leffler
1 hour ago
Incidentally, the difference between
'h' and 'H' is 32. Coincidence?– Christian Gibbons
1 hour ago
Incidentally, the difference between
'h' and 'H' is 32. Coincidence?– Christian Gibbons
1 hour ago
1
1
Thanks for the replies, but i alredy checked man pages and ISO document so I know the exact return value is not specified. I just want to know why there's a difference between literal string and array string in GCC.
– fips197
1 hour ago
Thanks for the replies, but i alredy checked man pages and ISO document so I know the exact return value is not specified. I just want to know why there's a difference between literal string and array string in GCC.
– fips197
1 hour ago
1
1
Likely duplicate of Inconsistent strcmp() return value when passing strings as pointers or as literals ... Tl;DR; both are valid you are seeing the effects of optimization
– Shafik Yaghmour
22 mins ago
Likely duplicate of Inconsistent strcmp() return value when passing strings as pointers or as literals ... Tl;DR; both are valid you are seeing the effects of optimization
– Shafik Yaghmour
22 mins ago
 |Â
show 1 more comment
4 Answers
4
active
oldest
votes
up vote
6
down vote
accepted
It looks like you didn't enable optimizations (e.g. -O2).
From my tests it looks like gcc always recognizes strcmp with constant arguments and optimizes it, even with -O0 (no optimizations). Clang needs at least -O1 to do so.
That's where the difference comes from: The code produced by clang calls strcmp twice, but the code produced by gcc just does printf("%dn", 1) in the first case because it knows that 'h' > 'H' (ASCIIbetically, that is). It's just constant folding, really.
Live example: https://godbolt.org/z/8Hg-gI
As the other answers explain, any positive value will do to indicate that the first string is greater than the second, so the compiler optimizer simply chooses 1. The strcmp library function apparently uses a different value.
answered 1 hour ago
melpomene
50.7k53682
2
Although it's interesting that Clang and GCC can be induced to compile the program either such that their respective results produce the same output or such that they don't, I don't like interpreting that as optimization or lack thereof being the reason for the output to differ. It would be better to generalize that to "implementation details", as optimization is only one reason why the results might differ, whether in this specific case or (even more so) in the general case.
– John Bollinger
1 hour ago
You can also use -fbo-builtin to observe some of these effects
– Shafik Yaghmour
20 mins ago
add a comment |Â
up vote
6
down vote
The standard defines the result of strcmp to be negative, if lhs appears before rhs in lexical order, zero if they are equal, or a positive value if lhs appears lexically after rhs.
It's up to the implementation how to implement that and what exactly to return. You must not depend on a specific value in your programs, or they won't be portable. Simply check with comparisons (<, >, ==).
See https://en.cppreference.com/w/c/string/byte/strcmp
Background
One simple implementation might just calculate the difference of each character c1 - c2 and do that until the result is not zero, or one of the strings ends. The result will then be the numeric difference between the first character, in which the two strings differed.
For example, this GLibC implementation: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=string/strcmp.c;hb=HEAD
answered 1 hour ago
Benjamin Maurer
1,112824
add a comment |Â
up vote
5
down vote
The strcmp function is only specified to return a value larger than zero, zero, or less than zero. There's nothing specified what those positive and negative values have to be.
answered 1 hour ago
Some programmer dude
282k23231385
add a comment |Â
up vote
3
down vote
The exact values returned by strcmp in the case of the strings not being equal are not specified. From the man page:
#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
The strcmp() and strncmp() functions return an integer less than,
equal to, or greater than zero if s1 (or the first n bytes thereof) is
found, respectively, to be less than, to match, or be greater than s2.
Since str1 compares greater than str2, the value must be positive, which it is in both cases.
As for the difference between the two compilers, it appears that clang is returning the difference between the ASCII values for the corresponding characters that mismatched, while gcc is opting for a simple -1, 0, or 1. Both are valid, so your code should only need to check if the value is 0, greater than 0, or less than 0.
answered 1 hour ago
dbush
82.4k1088119
1
The interesting thing is that gcc only gave1when passing in the string literals. I suspect it may have been an optimization knowing that the result would always be the same.
– Christian Gibbons
1 hour ago
add a comment |Â
4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
6
down vote
accepted
It looks like you didn't enable optimizations (e.g. -O2).
From my tests it looks like gcc always recognizes strcmp with constant arguments and optimizes it, even with -O0 (no optimizations). Clang needs at least -O1 to do so.
That's where the difference comes from: The code produced by clang calls strcmp twice, but the code produced by gcc just does printf("%dn", 1) in the first case because it knows that 'h' > 'H' (ASCIIbetically, that is). It's just constant folding, really.
Live example: https://godbolt.org/z/8Hg-gI
As the other answers explain, any positive value will do to indicate that the first string is greater than the second, so the compiler optimizer simply chooses 1. The strcmp library function apparently uses a different value.
answered 1 hour ago
melpomene
50.7k53682
2
Although it's interesting that Clang and GCC can be induced to compile the program either such that their respective results produce the same output or such that they don't, I don't like interpreting that as optimization or lack thereof being the reason for the output to differ. It would be better to generalize that to "implementation details", as optimization is only one reason why the results might differ, whether in this specific case or (even more so) in the general case.
– John Bollinger
1 hour ago
You can also use -fbo-builtin to observe some of these effects
– Shafik Yaghmour
20 mins ago
add a comment |Â
up vote
6
down vote
accepted
It looks like you didn't enable optimizations (e.g. -O2).
From my tests it looks like gcc always recognizes strcmp with constant arguments and optimizes it, even with -O0 (no optimizations). Clang needs at least -O1 to do so.
That's where the difference comes from: The code produced by clang calls strcmp twice, but the code produced by gcc just does printf("%dn", 1) in the first case because it knows that 'h' > 'H' (ASCIIbetically, that is). It's just constant folding, really.
Live example: https://godbolt.org/z/8Hg-gI
As the other answers explain, any positive value will do to indicate that the first string is greater than the second, so the compiler optimizer simply chooses 1. The strcmp library function apparently uses a different value.
answered 1 hour ago
melpomene
50.7k53682
2
Although it's interesting that Clang and GCC can be induced to compile the program either such that their respective results produce the same output or such that they don't, I don't like interpreting that as optimization or lack thereof being the reason for the output to differ. It would be better to generalize that to "implementation details", as optimization is only one reason why the results might differ, whether in this specific case or (even more so) in the general case.
– John Bollinger
1 hour ago
You can also use -fbo-builtin to observe some of these effects
– Shafik Yaghmour
20 mins ago
add a comment |Â
up vote
6
down vote
accepted
up vote
6
down vote
accepted
It looks like you didn't enable optimizations (e.g. -O2).
From my tests it looks like gcc always recognizes strcmp with constant arguments and optimizes it, even with -O0 (no optimizations). Clang needs at least -O1 to do so.
That's where the difference comes from: The code produced by clang calls strcmp twice, but the code produced by gcc just does printf("%dn", 1) in the first case because it knows that 'h' > 'H' (ASCIIbetically, that is). It's just constant folding, really.
Live example: https://godbolt.org/z/8Hg-gI
As the other answers explain, any positive value will do to indicate that the first string is greater than the second, so the compiler optimizer simply chooses 1. The strcmp library function apparently uses a different value.
answered 1 hour ago
melpomene
50.7k53682
It looks like you didn't enable optimizations (e.g. -O2).
From my tests it looks like gcc always recognizes strcmp with constant arguments and optimizes it, even with -O0 (no optimizations). Clang needs at least -O1 to do so.
That's where the difference comes from: The code produced by clang calls strcmp twice, but the code produced by gcc just does printf("%dn", 1) in the first case because it knows that 'h' > 'H' (ASCIIbetically, that is). It's just constant folding, really.
Live example: https://godbolt.org/z/8Hg-gI
As the other answers explain, any positive value will do to indicate that the first string is greater than the second, so the compiler optimizer simply chooses 1. The strcmp library function apparently uses a different value.
answered 1 hour ago
melpomene
50.7k53682
answered 1 hour ago
melpomene
50.7k53682
answered 1 hour ago
melpomene
50.7k53682
answered 1 hour ago
melpomene
50.7k53682
50.7k53682
2
Although it's interesting that Clang and GCC can be induced to compile the program either such that their respective results produce the same output or such that they don't, I don't like interpreting that as optimization or lack thereof being the reason for the output to differ. It would be better to generalize that to "implementation details", as optimization is only one reason why the results might differ, whether in this specific case or (even more so) in the general case.
– John Bollinger
1 hour ago
You can also use -fbo-builtin to observe some of these effects
– Shafik Yaghmour
20 mins ago
add a comment |Â
2
Although it's interesting that Clang and GCC can be induced to compile the program either such that their respective results produce the same output or such that they don't, I don't like interpreting that as optimization or lack thereof being the reason for the output to differ. It would be better to generalize that to "implementation details", as optimization is only one reason why the results might differ, whether in this specific case or (even more so) in the general case.
– John Bollinger
1 hour ago
You can also use -fbo-builtin to observe some of these effects
– Shafik Yaghmour
20 mins ago
2
2
Although it's interesting that Clang and GCC can be induced to compile the program either such that their respective results produce the same output or such that they don't, I don't like interpreting that as optimization or lack thereof being the reason for the output to differ. It would be better to generalize that to "implementation details", as optimization is only one reason why the results might differ, whether in this specific case or (even more so) in the general case.
– John Bollinger
1 hour ago
Although it's interesting that Clang and GCC can be induced to compile the program either such that their respective results produce the same output or such that they don't, I don't like interpreting that as optimization or lack thereof being the reason for the output to differ. It would be better to generalize that to "implementation details", as optimization is only one reason why the results might differ, whether in this specific case or (even more so) in the general case.
– John Bollinger
1 hour ago
You can also use -fbo-builtin to observe some of these effects
– Shafik Yaghmour
20 mins ago
You can also use -fbo-builtin to observe some of these effects
– Shafik Yaghmour
20 mins ago
add a comment |Â
up vote
6
down vote
The standard defines the result of strcmp to be negative, if lhs appears before rhs in lexical order, zero if they are equal, or a positive value if lhs appears lexically after rhs.
It's up to the implementation how to implement that and what exactly to return. You must not depend on a specific value in your programs, or they won't be portable. Simply check with comparisons (<, >, ==).
See https://en.cppreference.com/w/c/string/byte/strcmp
Background
One simple implementation might just calculate the difference of each character c1 - c2 and do that until the result is not zero, or one of the strings ends. The result will then be the numeric difference between the first character, in which the two strings differed.
For example, this GLibC implementation: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=string/strcmp.c;hb=HEAD
answered 1 hour ago
Benjamin Maurer
1,112824
add a comment |Â
up vote
6
down vote
The standard defines the result of strcmp to be negative, if lhs appears before rhs in lexical order, zero if they are equal, or a positive value if lhs appears lexically after rhs.
It's up to the implementation how to implement that and what exactly to return. You must not depend on a specific value in your programs, or they won't be portable. Simply check with comparisons (<, >, ==).
See https://en.cppreference.com/w/c/string/byte/strcmp
Background
One simple implementation might just calculate the difference of each character c1 - c2 and do that until the result is not zero, or one of the strings ends. The result will then be the numeric difference between the first character, in which the two strings differed.
For example, this GLibC implementation: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=string/strcmp.c;hb=HEAD
answered 1 hour ago
Benjamin Maurer
1,112824
add a comment |Â
up vote
6
down vote
up vote
6
down vote
The standard defines the result of strcmp to be negative, if lhs appears before rhs in lexical order, zero if they are equal, or a positive value if lhs appears lexically after rhs.
It's up to the implementation how to implement that and what exactly to return. You must not depend on a specific value in your programs, or they won't be portable. Simply check with comparisons (<, >, ==).
See https://en.cppreference.com/w/c/string/byte/strcmp
Background
One simple implementation might just calculate the difference of each character c1 - c2 and do that until the result is not zero, or one of the strings ends. The result will then be the numeric difference between the first character, in which the two strings differed.
For example, this GLibC implementation: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=string/strcmp.c;hb=HEAD
answered 1 hour ago
Benjamin Maurer
1,112824
The standard defines the result of strcmp to be negative, if lhs appears before rhs in lexical order, zero if they are equal, or a positive value if lhs appears lexically after rhs.
It's up to the implementation how to implement that and what exactly to return. You must not depend on a specific value in your programs, or they won't be portable. Simply check with comparisons (<, >, ==).
See https://en.cppreference.com/w/c/string/byte/strcmp
Background
One simple implementation might just calculate the difference of each character c1 - c2 and do that until the result is not zero, or one of the strings ends. The result will then be the numeric difference between the first character, in which the two strings differed.
For example, this GLibC implementation: https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=string/strcmp.c;hb=HEAD
answered 1 hour ago
Benjamin Maurer
1,112824
edited 1 hour ago
answered 1 hour ago
Benjamin Maurer
1,112824
answered 1 hour ago
Benjamin Maurer
1,112824
answered 1 hour ago
Benjamin Maurer
1,112824
1,112824
add a comment |Â
add a comment |Â
up vote
5
down vote
The strcmp function is only specified to return a value larger than zero, zero, or less than zero. There's nothing specified what those positive and negative values have to be.
answered 1 hour ago
Some programmer dude
282k23231385
add a comment |Â
up vote
5
down vote
The strcmp function is only specified to return a value larger than zero, zero, or less than zero. There's nothing specified what those positive and negative values have to be.
answered 1 hour ago
Some programmer dude
282k23231385
add a comment |Â
up vote
5
down vote
up vote
5
down vote
The strcmp function is only specified to return a value larger than zero, zero, or less than zero. There's nothing specified what those positive and negative values have to be.
answered 1 hour ago
Some programmer dude
282k23231385
The strcmp function is only specified to return a value larger than zero, zero, or less than zero. There's nothing specified what those positive and negative values have to be.
answered 1 hour ago
Some programmer dude
282k23231385
answered 1 hour ago
Some programmer dude
282k23231385
answered 1 hour ago
Some programmer dude
282k23231385
answered 1 hour ago
Some programmer dude
282k23231385
282k23231385
add a comment |Â
add a comment |Â
up vote
3
down vote
The exact values returned by strcmp in the case of the strings not being equal are not specified. From the man page:
#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
The strcmp() and strncmp() functions return an integer less than,
equal to, or greater than zero if s1 (or the first n bytes thereof) is
found, respectively, to be less than, to match, or be greater than s2.
Since str1 compares greater than str2, the value must be positive, which it is in both cases.
As for the difference between the two compilers, it appears that clang is returning the difference between the ASCII values for the corresponding characters that mismatched, while gcc is opting for a simple -1, 0, or 1. Both are valid, so your code should only need to check if the value is 0, greater than 0, or less than 0.
answered 1 hour ago
dbush
82.4k1088119
1
The interesting thing is that gcc only gave1when passing in the string literals. I suspect it may have been an optimization knowing that the result would always be the same.
– Christian Gibbons
1 hour ago
add a comment |Â
up vote
3
down vote
The exact values returned by strcmp in the case of the strings not being equal are not specified. From the man page:
#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
The strcmp() and strncmp() functions return an integer less than,
equal to, or greater than zero if s1 (or the first n bytes thereof) is
found, respectively, to be less than, to match, or be greater than s2.
Since str1 compares greater than str2, the value must be positive, which it is in both cases.
As for the difference between the two compilers, it appears that clang is returning the difference between the ASCII values for the corresponding characters that mismatched, while gcc is opting for a simple -1, 0, or 1. Both are valid, so your code should only need to check if the value is 0, greater than 0, or less than 0.
answered 1 hour ago
dbush
82.4k1088119
1
The interesting thing is that gcc only gave1when passing in the string literals. I suspect it may have been an optimization knowing that the result would always be the same.
– Christian Gibbons
1 hour ago
add a comment |Â
up vote
3
down vote
up vote
3
down vote
The exact values returned by strcmp in the case of the strings not being equal are not specified. From the man page:
#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
The strcmp() and strncmp() functions return an integer less than,
equal to, or greater than zero if s1 (or the first n bytes thereof) is
found, respectively, to be less than, to match, or be greater than s2.
Since str1 compares greater than str2, the value must be positive, which it is in both cases.
As for the difference between the two compilers, it appears that clang is returning the difference between the ASCII values for the corresponding characters that mismatched, while gcc is opting for a simple -1, 0, or 1. Both are valid, so your code should only need to check if the value is 0, greater than 0, or less than 0.
answered 1 hour ago
dbush
82.4k1088119
The exact values returned by strcmp in the case of the strings not being equal are not specified. From the man page:
#include <string.h>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);
The strcmp() and strncmp() functions return an integer less than,
equal to, or greater than zero if s1 (or the first n bytes thereof) is
found, respectively, to be less than, to match, or be greater than s2.
Since str1 compares greater than str2, the value must be positive, which it is in both cases.
As for the difference between the two compilers, it appears that clang is returning the difference between the ASCII values for the corresponding characters that mismatched, while gcc is opting for a simple -1, 0, or 1. Both are valid, so your code should only need to check if the value is 0, greater than 0, or less than 0.
answered 1 hour ago
dbush
82.4k1088119
answered 1 hour ago
dbush
82.4k1088119
answered 1 hour ago
dbush
82.4k1088119
answered 1 hour ago
dbush
82.4k1088119
82.4k1088119
1
The interesting thing is that gcc only gave1when passing in the string literals. I suspect it may have been an optimization knowing that the result would always be the same.
– Christian Gibbons
1 hour ago
add a comment |Â
1
The interesting thing is that gcc only gave1when passing in the string literals. I suspect it may have been an optimization knowing that the result would always be the same.
– Christian Gibbons
1 hour ago
1
1
The interesting thing is that gcc only gave
1 when passing in the string literals. I suspect it may have been an optimization knowing that the result would always be the same.– Christian Gibbons
1 hour ago
The interesting thing is that gcc only gave
1 when passing in the string literals. I suspect it may have been an optimization knowing that the result would always be the same.– Christian Gibbons
1 hour ago
add a comment |Â
fips197 is a new contributor. Be nice, and check out our Code of Conduct.
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fips197 is a new contributor. Be nice, and check out our Code of Conduct.
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3
What's "weird" about this?
– melpomene
1 hour ago
3
The standard only specifies the sign of the result; it does not specify the magnitude. If the strings are equal, the result is zero; if the first string sorts before the second, the result is negative; if the second string sorts after the second, the result is positive. In your example, both 1 and 32 are positive; the results are equivalent as far as the standard is concerned, and your code should be written so that it makes no difference to you, either.
– Jonathan Leffler
1 hour ago
Incidentally, the difference between
'h'and'H'is 32. Coincidence?– Christian Gibbons
1 hour ago
1
Thanks for the replies, but i alredy checked man pages and ISO document so I know the exact return value is not specified. I just want to know why there's a difference between literal string and array string in GCC.
– fips197
1 hour ago
1
Likely duplicate of Inconsistent strcmp() return value when passing strings as pointers or as literals ... Tl;DR; both are valid you are seeing the effects of optimization
– Shafik Yaghmour
22 mins ago