ben wrote: > > i'm trying to do pretty simple replacement using strtok. > > but it looks like i have missed some subtle difference between the two following > > > > char src[] = "hello world #"; > > char *other = "hello world #"; > > i don't know if it is a compiler feature (storage behavior into the DATA > segment), or a linux kernel feature, or if it is specified in ANSI, but > the second way leads to pointing to a _constant_ string. If someone can > enlighten... ANSI C says that string literals "may" be read-only. On platforms with memory protection they usually are read-only. On Linux, string literals are stored in the "rodata" segment, which is read-only, and thus can be shared between all processes which are using a given executable or shared library. You can list the segments which make up an executable or shared library using "objdump -h", e.g.: $ objdump -h /bin/ls /bin/ls: file format elf32-i386 Sections: Idx Name Size VMA LMA File off Algn 0 .interp 00000013 08048174 08048174 00000174 2**0 CONTENTS, ALLOC, LOAD, READONLY, DATA 1 .note.ABI-tag 00000020 08048188 08048188 00000188 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA 2 .hash 00000298 080481a8 080481a8 000001a8 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA ... 12 .text 0000fbb4 08049880 08049880 00001880 2**4 CONTENTS, ALLOC, LOAD, READONLY, CODE ... 14 .rodata 00003d3c 08059460 08059460 00011460 2**5 CONTENTS, ALLOC, LOAD, READONLY, DATA ... 23 .data 0000024c 0805f160 0805f160 00016160 2**5 CONTENTS, ALLOC, LOAD, DATA 24 .bss 0000046c 0805f3c0 0805f3c0 000163ac 2**5 ALLOC 25 .comment 00000bd0 00000000 00000000 000163ac 2**0 CONTENTS, READONLY [snipped] The main ones are text, rodata, data, and bss. The text segment holds code, and is read-only and executable (CODE flag). The others hold static data: global variables, "static" local variables, string literals, and intialisers for automatic (non-"static" local) arrays. Read-only data (literals, initialisers, "const" variables) goes into the rodata segment, which is read-only. Mutable variables with explicit initialisers go into the data segment. Mutable variables without initialisers (i.e. implicitly initialised to zero) go into the bss segment. As the entire bss segment is initially zero, it doesn't need to be stored in the file (this is indicated by the lack of the CONTENTS, LOAD, and CODE/DATA flags). The other segments tend to be architecture-specific. -- Glynn Clements <glynn@xxxxxxxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
